Free Estimator
Calculate exactly how many bags of concrete you need for Sonotube, Quik-Tube, and standard cardboard tube forms - fence posts, deck piers, and round columns.
Use this free concrete tube calculator to find out exactly how many bags of concrete you need to fill a tube form. It works with common Sonotube and Quik-Tube diameters from 6 inches to 24 inches. Enter the tube diameter, fill depth, and number of tubes to instantly get bag counts for 40 lb, 60 lb, 80 lb, and 90 lb bags, cubic yardage, a 10% overage recommendation, and an optional cost estimate, all updated in real time with no field math required.
All Standard Tube Diameters Covered - Supports 6", 8", 10", 12", 14", 16", 18", 20", and 24" tube forms with one-click presets.
Bag Count as the Primary Output - Bag count comes first because that is what you need at the hardware store.
Multi-Tube Projects in One Pass - Enter the number of tubes to get combined bag counts and total volume for the whole project.
10% Overage Built In - Recommended waste buffer is automatically included in every estimate.
PDF Export & Share Link - Save or share your tube estimate with your crew or supplier before heading to the store.
Calculator Section
Enter your tube diameter, fill depth, and quantity below to get an instant bag count and volume estimate. If you need to calculate the footing beneath the tube, use the concrete footing calculator. For square posts or mixed column shapes, see the concrete column calculator. For a full project cost breakdown, use the concrete cost calculator. To compare bag yields across brands and bag sizes, open the concrete bag calculator. If you want the general method first, read how to calculate concrete.
Filling a Sonotube or Quik-Tube is one of the most common DIY concrete tasks, and the math is simple once you convert tube diameter into feet before applying the cylinder formula. The sections below show the core formula, a step-by-step workflow, and a quick lookup table for standard tube sizes and fill depths so you can verify the calculator output before heading to the store.
Volume (yd3) = pi x (D / 2)^2 x H / 27 Bags needed = Volume x 1.1 / Yield per bag Where: D = tube inside diameter in feet (inches / 12) H = fill depth in feet Yield per bag: 40 lb = 0.011 yd3 60 lb = 0.017 yd3 80 lb = 0.022 yd3 90 lb = 0.025 yd3 Example (12 in tube, 4 ft deep, 6 tubes) D = 12 / 12 = 1.0 ft, r = 0.5 ft Single tube = 3.1416 x 0.25 x 4 / 27 = 0.116 yd3 6 tubes = 0.116 x 6 = 0.698 yd3 With 10% = 0.698 x 1.1 = 0.768 yd3 80 lb bags = 0.768 / 0.022 = 35 bags
Check the Sonotube or Quik-Tube label for the inside diameter. Standard sizes typically run from 6 inches to 24 inches in 2 inch increments. The printed tube size is the dimension that controls how much concrete fits inside the form, so that is the number you enter in the calculator.
Fill depth is the actual length of the tube that will be filled with concrete. For deck piers and fence posts this is usually the full structural embedment, sometimes with the top kept slightly below the rim for finishing. For tube-formed columns above grade, fill depth often matches the full tube length.
Enter the total number of identical tube forms in the quantity field. The calculator multiplies the single-tube volume by the quantity and then applies the built-in 10% overage to the total. If your project uses different diameters or different depths, calculate each group separately and use Pin Baseline to compare them.
80 lb bags are the most common reference because they are widely available and deliver the best yield per bag for many residential tube pours. If lifting weight is a concern, switch focus to 60 lb bags. If you are comparing store inventory or crew handling, the results table shows 40 lb, 60 lb, 80 lb, and 90 lb counts side by side.
Always round up to whole bags and keep the built-in overage in the final shopping list. Running short mid-pour is the fastest way to end up with a cold joint or an incomplete tube fill. On multi-tube projects it is still smart to carry one or two extra bags in the truck, especially when the store is not nearby.
| Tube Diameter | 2 ft Deep | 3 ft Deep | 4 ft Deep | 6 ft Deep |
|---|---|---|---|---|
| 6 in | 1 bag | 2 bags | 2 bags | 3 bags |
| 8 in | 2 bags | 2 bags | 3 bags | 4 bags |
| 10 in | 3 bags | 4 bags | 5 bags | 7 bags |
| 12 in | 3 bags | 5 bags | 6 bags | 9 bags |
| 14 in | 4 bags | 6 bags | 8 bags | 12 bags |
| 16 in | 6 bags | 8 bags | 11 bags | 16 bags |
| 18 in | 7 bags | 10 bags | 14 bags | 20 bags |
| 20 in | 9 bags | 13 bags | 17 bags | 25 bags |
| 24 in | 12 bags | 18 bags | 24 bags | 35 bags |
80 lb bags per tube, including 10% overage. Values are rounded up to whole bags using the standard cylinder formula and an 0.022 yd3 yield per 80 lb bag.
Cardboard tube forms sold as Sonotube, Quik-Tube, and similar brands are available in standard diameters from 6 to 24 inches at most home improvement stores. Each diameter lines up with common use cases from fence posts to larger flagpole bases. Use these examples as a sizing guide before you calculate your final bag count.
Bags per 3 ft deep: 2 bags (80 lb)
Six and eight inch tube forms are common for setting wood and metal fence posts. The tube is set in the post hole, the post is braced plumb, and concrete is poured around the post inside the cardboard form. One 6 inch tube is a very small pour, but 8 inch forms at greater depths can still add up quickly when a fence line has many posts.
Bags per 4 ft deep: 5-6 bags (80 lb)
Ten and twelve inch tube forms are standard sizes for residential deck piers. The tube is placed in a hole that extends below frost depth, filled with concrete, and topped with a post base or anchor while the mix is still wet. A 12 inch tube at 4 feet deep is one of the most common tube-form calculations and usually takes about 6 bags of 80 lb mix per pier.
Bags per 4 ft deep: 8-11 bags (80 lb)
Larger backyard structures often need 14 or 16 inch piers to spread load over more soil area. These diameters are common for heavy deck frames, pergolas, shade structures, and carports. Once a project has several piers at this size, the total volume can move beyond the point where bagged concrete is the easiest or lowest-cost option.
Bags per 4 ft deep: 14-17 bags (80 lb)
Eighteen and twenty inch tube forms are used for larger round columns and heavier structural supports in residential additions and light commercial work. At these diameters, each tube already represents a meaningful volume of concrete, so ready-mix often becomes more practical when the job includes more than a few columns.
Bags per 2 ft deep: 2-3 bags (80 lb)
Small tube forms are a clean way to build mailbox posts, sign posts, and light pole bases with a smooth cylindrical finish above grade. For these light-duty applications, fast-setting bagged concrete can be a convenient choice when you want to set posts quickly without mixing a full truckload or renting extra equipment.
Bags per 3-4 ft deep: 5-24 bags (80 lb)
Flagpole bases are often built with larger tube forms to resist overturning from wind. Residential poles commonly use 12 to 18 inch diameters, while taller poles and sign structures can push into 20 or 24 inch territory. Depth and diameter should always match the pole manufacturer's recommendation and local loading requirements.
These tube calculator FAQs cover Sonotube bag counts, Quik-Tube sizing, fast-setting mixes, frost depth, and when tube-form projects are large enough to justify ready-mix delivery. The first two answers are expanded by default.
Bag count depends on tube diameter, fill depth, and how many identical forms you are filling. A very common example is a 12 inch tube at 4 feet deep, which takes about 6 bags of 80 lb mix per tube before you add any reserve. Smaller 6 inch and 8 inch forms use far fewer bags, while larger deck piers and structural columns can climb quickly. The calculator above converts those dimensions directly into bag counts without requiring any manual cylinder math.
A 12 inch tube form uses about 3 bags of 80 lb concrete at 2 feet deep, about 5 bags at 3 feet deep, about 6 bags at 4 feet deep, and about 9 bags at 6 feet deep when you include the built-in 10% overage. That 12 inch by 4 foot case is one of the most common deck pier takeoffs, which is why it shows up so often in Sonotube bag-count searches and store planning.
Sonotube and Quik-Tube are both cardboard concrete form products sold in standard nominal diameters. From a volume standpoint, the bag count is determined by the inside diameter and the fill depth, not the logo printed on the form. In practice, the differences are store availability, surface coating, and price. If the nominal size is the same, the concrete quantity estimate is effectively the same as well.
Fast-setting concrete is a strong option for small post installations such as fence posts, mailbox bases, and sign posts where speed matters more than extended working time. It is less convenient for heavier deck piers and structural columns because those pours may need placement time for bracing, anchors, and vibration. Standard 80 lb concrete mix usually makes more sense for larger tube diameters or any pour where alignment and finishing take longer.
Deck pier depth is usually controlled by local frost depth and structural load. In warm climates, relatively shallow piers may be accepted, but in colder regions the pier must often extend below the local frost line to avoid heave. That can mean 36 to 48 inches or more depending on location. The tube form often extends a little above grade so hardware sits clear of soil and standing water.
Tube forms are most commonly used inside a dug or drilled hole, where the surrounding soil helps support the cardboard during placement. They can also be used above grade for decorative or structural round columns, but then they must be braced so the form stays plumb and does not shift under the weight of wet concrete. Either way, the form needs stable support until the concrete reaches initial set.
Bagged concrete is practical for a handful of small tubes, especially fence posts and common deck piers. Once the total project volume starts pushing past about 1 cubic yard, ready-mix is often easier and cheaper when you factor in labor, mixing time, and consistency. Several 16 inch piers or a cluster of large structural tubes can hit that threshold quickly, so it is worth comparing a local ready-mix quote before buying a large pallet of bags.
Below grade, many installers simply leave the cardboard form in place because it will break down over time without affecting the structural concrete. Above grade, stripping the exposed portion after curing usually gives a cleaner finish and avoids trapping moisture against the form surface. Some premium tube forms are designed to strip more cleanly than basic cardboard, which matters if the finished concrete will remain visible.
Related Tools
Use these related calculators to compare bag sizes and brands, estimate the footing beneath your tube forms, or handle mixed round and square post work on the same project.
Compare 40 lb, 60 lb, 80 lb, and 90 lb bag counts side by side for any project volume. Useful when you want to compare yields and store options before buying.
Open CalculatorEstimate concrete for round and square columns in one tool. Best when your project includes both tube-formed round shapes and square post bases.
Open CalculatorCalculate concrete for the footing or pad beneath your tube-formed pier or column. Includes bag counts and formwork guidance for standard footing types.
Open Calculator