How Many Bags of Concrete Do I Need?
The number of bags you need depends on bag size and project volume:
- 40 lb bag = 0.011 yd³ → ~90 bags per cubic yard
- 60 lb bag = 0.017 yd³ → ~60 bags per cubic yard
- 80 lb bag = 0.022 yd³ → ~45 bags per cubic yard
- 90 lb bag = 0.025 yd³ → ~40 bags per cubic yard
Most common deck-pier question: a 12-inch Sonotube at 4 feet deep needs about 6 bags of 80 lb mix.
→ Use the free calculator below for your exact count.
Whether you are setting fence posts, pouring a patio slab, or filling tube forms for a deck, the question is always the same: how many bags of concrete do I need? The answer depends on two variables - the volume of your pour and the bag size you plan to use. This guide brings the math, the ready-reference tables, and the practical buying advice into one place. Use it to compare 40, 50, 60, 80, and 90 lb bags, check the most common post-hole and slab sizes, and decide when a project has grown large enough that ready-mix makes more sense than hauling bags.
In This Article
- 1. Quick Answer
- 2. Free Bag Calculator
- 3. Bags Per Cubic Yard - All Bag Sizes
- 4. How Many Bags for a Post Hole?
- 5. How Many Bags for a Concrete Slab?
- 6. How Many Bags for a Footing?
- 7. How Many Bags for a Round Column?
- 8. How to Calculate Bags for Any Pour
- 9. Quikrete and Sakrete Reference
- 10. Common Mistakes
- 11. FAQ
Free Concrete Bag Calculator
Enter your project volume or dimensions below to get an exact bag count for all standard sizes, updated in real time. This embedded calculator uses the same core logic as the full Concrete Bag Calculator, but keeps the article workflow focused on quick comparison rather than export and history tools.
💡 Tip: For post holes and tube forms, use the Concrete Tube Calculator. For flatwork, use the Concrete Slab Calculator. For wider project takeoffs, the full concrete calculator lets you compare slabs, walls, columns, and stairs in one place.
How Many Bags of Concrete Per Cubic Yard?
One cubic yard of concrete equals 27 cubic feet. The number of bags you need per cubic yard depends on the yield of the product in your cart, not just the number printed on the front of the bag. The table below compares the common retail sizes, including the fast-setting 50 lb option many people use for posts.
Bag Count and Cost by Standard Bag Size
| Bag Size | Yield per Bag | Bags per Cubic Yard | With 10% Overage | Approx. Cost/Bag | Cost per yd³ |
|---|---|---|---|---|---|
| 40 lb | 0.011 yd³ (0.30 ft³) | 90 bags | 99 bags | ~$4.50 | ~$405 |
| 50 lb fast-set | 0.014 yd³ (0.375 ft³) | 72 bags | 80 bags | ~$7.00 | ~$504 |
| 60 lb | 0.017 yd³ (0.45 ft³) | 60 bags | 66 bags | ~$5.50 | ~$330 |
| 80 lb | 0.022 yd³ (0.60 ft³) | 45 bags | 50 bags | ~$6.50 | ~$293 |
| 90 lb | 0.025 yd³ (0.675 ft³) | 40 bags | 44 bags | ~$7.50 | ~$300 |
Yields are based on standard Quikrete and Sakrete concrete mix products. Fast-setting yield is based on the common 50 lb post-setting mix. Costs are national planning averages and should be checked against local store pricing before purchase.
The 80 lb bag is usually the most cost-efficient standard choice because it combines a strong yield per bag with a bag price that is still widely competitive at big-box stores. The 60 lb bag is a useful compromise when one-person handling matters more than pure material efficiency, and the 40 lb bag makes sense for repairs where you only need a small fraction of a yard.
Fast-setting 50 lb mix earns its place for post work because it trades some yield for speed. If you are setting a few fence posts and want the convenience of pouring dry mix into the hole and adding water in place, the lower yield is often worth it. If you are filling a footing or patio, standard bagged mix is usually the better choice.
How Many Bags of Concrete for Post Holes?
Fence posts, mailbox posts, and deck piers are the most common reason homeowners buy bagged concrete. The number of bags depends on the hole diameter, the depth, and whether you are treating the hole as a solid cylinder or pouring around a wood post that displaces some volume. The table below uses solid-fill geometry and 80 lb bags, which gives you the conservative buying number most people want before they head to the store.
Post Hole Bag Count (80 lb bags)
| Hole Diameter | 2 ft Deep | 3 ft Deep | 4 ft Deep | Notes |
|---|---|---|---|---|
| 6 in | 1 bag | 2 bags | 2 bags | Light fence or sign post |
| 8 in | 2 bags | 2 bags | 3 bags | Standard residential fence post |
| 10 in | 2 bags | 3 bags | 4 bags | Heavy fence, gate, or mailbox post |
| 12 in | 3 bags | 5 bags | 6 bags | Deck pier and the most common Sonotube search |
| 14 in | 4 bags | 6 bags | 8 bags | Heavy deck pier |
| 16 in | 6 bags | 8 bags | 11 bags | Pergola, pavilion, or carport post |
Counts include a 10% overage and assume a solid cylindrical fill. If you are pouring around a wood post rather than filling a tube solid, you can often subtract roughly 15% to 20% for the displaced post volume.
The most common deck-pier question online is still the 12-inch Sonotube at 4 feet deep. The planning answer is about 6 bags of 80 lb concrete with 10% overage included. That makes it a good sanity check whenever you are comparing store calculators, product labels, or someone else's rule of thumb.
If you are pouring around a centered wood post instead of filling a cardboard form solid, the actual concrete volume will be a little lower because the post displaces part of the cylinder. In practice, many DIY buyers still round up to the same whole-bag number because the cost of one extra bag is small compared with a second trip.
How Many Bags of Concrete for a Slab?
Small slabs for patios, walkways, shed bases, and equipment pads are often the biggest flatwork job that still looks tempting to do with bags. For most residential slabs, 4 inches is the standard thickness, while 3.5 inches appears on lighter patios and 6 inches shows up on heavier-duty slabs. The table below uses 80 lb bags, includes 10% overage, and gives you a fast way to spot when the bag count is getting large enough that a ready-mix quote should be part of the decision.
Slab Bag Count (80 lb bags)
| Slab Size | 3.5 in thick | 4 in thick | 6 in thick | Volume (4 in) |
|---|---|---|---|---|
| 4 × 4 ft | 9 bags | 10 bags | 15 bags | 0.2 yd³ |
| 4 × 8 ft | 18 bags | 20 bags | 30 bags | 0.4 yd³ |
| 6 × 6 ft | 20 bags | 23 bags | 33 bags | 0.44 yd³ |
| 8 × 8 ft | 35 bags | 40 bags | 59 bags | 0.79 yd³ |
| 10 × 10 ft | 54 bags | 62 bags | 92 bags | 1.23 yd³ |
| 10 × 12 ft | 65 bags | 74 bags | 110 bags | 1.48 yd³ |
| 12 × 12 ft | 78 bags | 89 bags | 132 bags | 1.78 yd³ |
| 12 × 20 ft | 129 bags | 147 bags | 220 bags | 2.96 yd³ |
| 20 × 20 ft | 214 bags | 245 bags | 367 bags | 4.94 yd³ |
| 20 × 30 ft | 321 bags | 367 bags | 550 bags | 7.41 yd³ |
All counts include a 10% overage and assume an 80 lb bag yield of 0.60 cubic feet. Once a slab gets close to or above 1 cubic yard, a ready-mix quote is usually worth requesting before you commit to bagged concrete.
A 10 × 10 foot slab at 4 inches thick is already about 62 bags of 80 lb mix once you include overage. That is why the bag-versus-ready-mix conversation starts earlier than many DIY buyers expect. The material math is simple, but the labor of moving, opening, mixing, and placing dozens of bags adds up fast.
Once you move beyond roughly 1 cubic yard, ready-mix concrete is frequently cheaper per cubic yard and far easier to place. That threshold happens well before a large driveway. Even a 12 × 12 foot slab at 4 inches thick is close to 1.8 cubic yards net volume and almost 90 bags of 80 lb mix with overage included.
How Many Bags of Concrete for a Footing?
Footings come in two common residential forms: strip footings, which run as long rectangular sections under walls, and pad footings, which act as isolated blocks under posts or columns. Because footing dimensions often mix feet with inches, they are a classic place for unit mistakes. The table below keeps everything on the same basis and shows what the bag count looks like when you translate the drawing dimensions into overage-adjusted material.
Footing Bag Count (80 lb bags)
| Footing Size | Length | Volume | 80 lb Bags | Notes |
|---|---|---|---|---|
| 12 in × 6 in strip | 10 ft | 0.19 yd³ | 10 bags | Small shed footing |
| 12 in × 6 in strip | 20 ft | 0.37 yd³ | 19 bags | Small addition or porch |
| 12 in × 8 in strip | 20 ft | 0.49 yd³ | 25 bags | Standard residential footing |
| 16 in × 8 in strip | 20 ft | 0.66 yd³ | 33 bags | Heavier load or frost footing |
| 24 in × 24 in × 12 in pad | - | 0.15 yd³ | 8 bags | Column or pier pad |
| 36 in × 36 in × 12 in pad | - | 0.33 yd³ | 17 bags | Heavy column pad |
All counts include a 10% overage. Strip footings are calculated as long rectangular sections, while pad footings are shown as square pads with the full length, width, and thickness included in the row label.
Strip footings reach ready-mix territory quickly because length drives the volume up even when the width and depth look modest on paper. Pad footings are different: they can stay well within a bagged-concrete workflow if you only have one or two isolated placements, especially for small decks, sheds, and light columns.
If you also need formwork area or want to compare imperial and metric dimensions without recalculating by hand, it is faster to move from this article into the dedicated estimator than to keep working off a spreadsheet.
How Many Bags of Concrete for a Round Column?
Round columns cast in Sonotube or Quik-Tube forms are one of the most common residential uses for bagged concrete. A quick bag-count chart saves time here because the cylindrical geometry is simple but easy to misjudge in the field. Even small diameter increases have a noticeable effect on the total volume once the height climbs.
Round Column and Tube Form Bag Count (80 lb bags)
| Tube Diameter | 2 ft tall | 4 ft tall | 8 ft tall | 12 ft tall |
|---|---|---|---|---|
| 8 in | 2 bags | 3 bags | 6 bags | 8 bags |
| 10 in | 2 bags | 4 bags | 8 bags | 12 bags |
| 12 in | 3 bags | 6 bags | 12 bags | 18 bags |
| 14 in | 4 bags | 8 bags | 16 bags | 24 bags |
| 16 in | 6 bags | 11 bags | 21 bags | 31 bags |
| 18 in | 7 bags | 13 bags | 26 bags | 39 bags |
All counts include a 10% overage and assume a solid fill inside the form. For a hollow form, rebar cage, or embedded hardware, the actual concrete quantity may be slightly lower.
The 12-inch diameter row is highlighted because it is the size that shows up most often in deck and pier searches. It is also the point where the bag count starts to become meaningfully different from a small fence-post hole. At 12 feet tall, even a 12-inch round column already calls for around 18 bags of 80 lb mix when you include overage.
If your round column uses rebar cages, anchor bolts, or a partial hollow core, the solid-fill value becomes slightly conservative, which is usually fine for purchase planning. Use the more detailed calculator when the project has multiple columns or a different diameter at each location.
How to Calculate How Many Bags of Concrete You Need
The hand calculation has four steps. Once you do it once or twice, it takes less than a minute. The real value is understanding which number matters at each stage: net volume, order volume, and bag yield.
Step 1 - Calculate Your Volume in Cubic Feet
Start by finding the raw volume of the pour. Multiply length by width by thickness when the project is rectangular, or use the cylinder formula for holes and Sonotubes. If the thickness is listed in inches, divide by 12 first so the math stays in feet. That single conversion step prevents the most common bag-ordering mistake.
10 ft × 12 ft slab at 4 in thick Thickness = 4 ÷ 12 = 0.333 ft Volume = 10 × 12 × 0.333 = 40 ft³
Step 2 - Convert Cubic Feet to Cubic Yards
Bag tables and ready-mix quotes are easier to compare once the project is in cubic yards. Divide the cubic-foot result by 27 because there are exactly 27 cubic feet in one cubic yard. This gives you the net concrete volume before waste, spillage, and form variation are considered.
40 ft³ ÷ 27 = 1.48 yd³ That 1.48 yd³ is the net concrete requirement.
Step 3 - Add 10% Overage
Concrete jobs almost always consume a little more material than the clean geometry suggests. Form deflection, uneven subgrade, small leaks, and shovel waste all add up. Multiply the net cubic-yard figure by 1.1 to build in the standard field overage that keeps you from running short halfway through placement.
1.48 yd³ × 1.1 = 1.63 yd³ 1.63 yd³ is the recommended order volume.
Step 4 - Divide by Yield Per Bag
Take the overage-adjusted volume and divide by the yield of the bag you plan to buy. An 80 lb bag yields about 0.60 cubic feet, or 0.022 cubic yards. Always round up to the next whole bag. A partial leftover bag is manageable; a pour that stops because you ran out of mix is not.
For 1.63 yd³: 40 lb bag → 1.63 ÷ 0.011 ≈ 148 bags 60 lb bag → 1.63 ÷ 0.017 ≈ 96 bags 80 lb bag → 1.63 ÷ 0.022 ≈ 74 bags 90 lb bag → 1.63 ÷ 0.025 ≈ 65 bags
Quikrete and Sakrete Bag Sizes and Yields
Quikrete and Sakrete are the two brands most buyers compare when they are standing in front of a pallet at Home Depot, Lowe's, or a local yard. The bag sizes and standard yields are very similar, but the product names and specialty mixes differ. This reference table keeps the usual retail options in one place so you can translate the bag count from this guide into a shopping list faster.
Quikrete and Sakrete Product Reference
| Product | Brand | Bag Size | Yield | Best For |
|---|---|---|---|---|
| Concrete Mix 5000 | Quikrete | 80 lb | 0.60 ft³ | Slabs, footings, and columns |
| Concrete Mix | Quikrete | 60 lb | 0.45 ft³ | General-purpose pours |
| Concrete Mix | Quikrete | 40 lb | 0.30 ft³ | Small repairs and patches |
| Fast-Setting Concrete Mix | Quikrete | 50 lb | 0.375 ft³ | Posts with no mixing tub |
| Crack-Resistant Concrete | Quikrete | 80 lb | 0.60 ft³ | Slabs with reinforcement |
| Concrete Mix | Sakrete | 80 lb | 0.60 ft³ | General-purpose residential pours |
| Concrete Mix | Sakrete | 60 lb | 0.45 ft³ | General-purpose pours |
| Fast-Setting Concrete | Sakrete | 50 lb | 0.375 ft³ | Fence and mailbox posts |
| 5000 Plus Concrete | Sakrete | 80 lb | 0.60 ft³ | Higher-strength pours |
Yields are approximate planning numbers. Always check the printed label on the bag in your store because specialty mixes, fiber blends, and regional products can vary slightly from the standard yield shown here.
For most residential projects, Quikrete Concrete Mix 5000 in the 80 lb size and Sakrete Concrete Mix in the 80 lb size are effectively interchangeable from a bag-count standpoint because both produce roughly 0.60 cubic feet per bag. The decision usually comes down to price, availability, and whether you want a standard or fast-setting formulation.
Fast-setting products from both brands are the obvious exception. They trade yield efficiency and working time for speed, which is why they are the go-to option for fence and mailbox posts but not the default answer for slabs or other large pours.
Common Mistakes When Buying Bagged Concrete
❌ Mistake 1: Forgetting to convert inches to feet
This is the most common bag-calculation error. A slab that is 4 inches thick must be entered as 0.333 feet in a volume formula. If you enter 4 instead of 0.333, the result becomes roughly twelve times too large and the bag count explodes.
❌ Mistake 2: Skipping the 10% overage
Ordering exactly the theoretical volume is risky. Slightly loose subgrade, irregular forms, shovel loss, and cleanup all consume more material than the clean drawing dimension suggests. The standard 10% buffer is cheap insurance.
❌ Mistake 3: Rounding down on bag counts
Always round up to the next full bag. A half bag left over can be used for a patch or discarded. Stopping a pour to drive back to the store is the much more expensive outcome.
❌ Mistake 4: Using fast-set bags for large pours
Fast-setting mix is designed for posts and small placements where speed matters. On slabs, walls, or larger footings, it can stiffen before you finish placing and screeding the surface. Use standard concrete mix unless the application specifically benefits from short set time.
❌ Mistake 5: Not checking the bag yield
Specialty mixes, crack-resistant blends, fiber mixes, and high-strength products sometimes have slightly different yields from the standard numbers. The label on the bag is the final authority. Use it when you make the purchase list.
❌ Mistake 6: Buying bags when ready-mix is already cheaper
For pours that reach or exceed about 1 cubic yard, ready-mix is often cheaper per cubic yard and substantially easier to place. Before you load 50 or 60 bags into a cart, get a local ready-mix price and compare total labor.
Frequently Asked Questions
These answers cover the questions people search most often before buying concrete bags, including cubic-yard conversions, 10 × 10 slab counts, Sonotube bag counts, and when ready-mix beats retail bag pricing.
How many 80 lb bags of concrete do I need per cubic yard?+
You need about 45 bags of 80 lb concrete per cubic yard of net volume. If you add the standard 10% overage that most jobs need, the working rule becomes about 50 bags per cubic yard ordered. That estimate is based on the standard 80 lb yield of roughly 0.60 cubic feet, or about 0.022 cubic yards, per bag. If your project volume is already known, divide the overage-adjusted cubic-yard total by 0.022 and round up to the next whole bag.
How many bags of concrete do I need for a 10x10 slab?+
For a 10 × 10 foot slab at the standard 4-inch thickness, you need about 62 bags of 80 lb concrete when you include the recommended 10% overage. The net volume is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, which equals 1.23 cubic yards. Adding 10% raises the order volume to about 1.36 cubic yards. Divide that by the 80 lb yield and round up. At this size, you should also compare the labor of mixing more than 60 bags against a local ready-mix quote.
How many bags of concrete for a 4x4 post hole?+
For a standard 4×4 fence post in an 8-inch diameter hole, most homeowners need about 1 to 2 bags depending on the hole depth and whether they use standard mix or fast-setting mix. A 2-foot-deep, 8-inch hole is around 2 bags of standard 80 lb mix if you treat it as a solid cylinder with overage included. If the post displaces some of the volume or you use a lighter fast-setting product, the practical buying answer is still usually two bags per post so you are not short.
How many bags of concrete do I need for a 12-inch Sonotube?+
A 12-inch Sonotube is the most searched deck-pier size, and it typically needs about 3 bags of 80 lb mix at 2 feet deep, 5 bags at 3 feet deep, 6 bags at 4 feet deep, and 9 bags at 6 feet deep when you include 10% overage and round up. The 12-inch by 4-foot case is the one most homeowners ask about, and the planning answer is 6 bags of 80 lb concrete. If you are pouring several piers, calculate each depth the same way and then total the order.
Is it cheaper to use bags or ready-mix concrete?+
Below about 0.5 to 1 cubic yard, bagged concrete is often more practical because ready-mix suppliers usually charge a delivery minimum or short-load fee. Once the pour moves above about 1 cubic yard, ready-mix is almost always cheaper per cubic yard and dramatically easier on labor. A single cubic yard equals about 45 bags of 80 lb mix. That is more than 3,600 pounds of material to move, mix, and place by hand, so the labor cost becomes the real deciding factor even before the material price difference shows up.
How many bags of concrete do I need for a fence post?+
A standard wood fence post usually needs 1 to 2 bags of 80 lb concrete per post, depending on the hole diameter and the frost depth in your area. Small residential fence holes around 6 to 8 inches in diameter and 2 feet deep are often a one- to two-bag job. Deeper holes, gate posts, and 6×6 posts trend higher. The safest rule when shopping is to calculate the hole size, round up, then buy one extra bag for every handful of posts so a minor hole-size variation does not force a return trip.
How long does a bag of concrete take to set?+
Standard bagged concrete mix usually reaches an initial set in about 2 to 4 hours and can often handle light traffic after 24 hours, but full design strength still develops over 28 days. Fast-setting post mix is different: it often stiffens in 20 to 40 minutes after water is added, which is why it is so common for fence and mailbox posts. Fast-set products are useful when speed matters, but they are not ideal for large pours where you need more working time to place, screed, and finish the surface.
How do I calculate bags of concrete for an irregular shape?+
Break the project into simple shapes you can measure accurately, such as rectangles, triangles, circles, or cylinders. Calculate the volume of each part separately, add them together, convert to cubic yards, and then divide by the yield of the bag size you plan to use. That approach works for L-shaped slabs, stepped footings, curved pads, and segmented pours. If you would rather skip the arithmetic, use the ConcreteCalc Pro slab, tube, footing, or full concrete calculator for each section and total the results.
Related Guides and Calculators
If you want to go deeper on slab math, bag pricing, or Sonotube sizing, these guides and calculators are the next places to go after you finish the bag-count comparison above.
How to Calculate Concrete for a Slab
Step-by-step guidance on measuring slab dimensions, converting thickness, and ordering the right amount of concrete for patios, walkways, and shed bases.
Read guideConcrete Cost Per Yard
Current ready-mix prices by state and pour type, with delivery fees, labor benchmarks, and installed cost planning.
Read guideSonotube Size Guide: How Many Bags Per Tube?
Quick reference guidance for standard Sonotube and Quik-Tube diameters, with bag-count logic for deck piers, posts, and round columns.
Read guideConcrete Bag Calculator
Enter cubic yards, cubic feet, or dimensions to get bag counts for all standard sizes at the same time.
Open CalculatorConcrete Tube Calculator
Calculate bags for Sonotube and Quik-Tube forms by diameter and depth across the standard residential sizes.
Open CalculatorConcrete Slab Calculator
Calculate slab volume, bag counts, overage, and cost for rectangular and circular slabs without doing the unit math by hand.
Open CalculatorReady to calculate your exact bag count?
Use the free ConcreteCalc Pro bag calculator to get an instant count for 40, 60, 80, and 90 lb bags. If your project uses Sonotube or Quik-Tube forms, jump straight to the diameter-based estimator instead.