Standard Pier (Sonotube): Volume (ft3) = pi x (Diameter / 2)^2 x Depth Bell Footing - Shaft: Volume (ft3) = pi x (Shaft Diameter / 2)^2 x Shaft Height Bell Footing - Bell Base (Frustum): Volume (ft3) = (pi x Bell Height / 3) x (R^2 + R x r + r^2) where R = bell base radius, r = shaft radius Total Bell Footing Volume = Shaft Volume + Bell Base Volume Example 6 footings, 10" Sonotube, 42-inch depth, 80 lb bags, 10% waste Radius = 10 / 2 / 12 = 0.417 ft Volume/pier = pi x 0.417^2 x 3.5 ft = 1.909 ft3 Check: pi x (5/12)^2 x (42/12) = 3.1416 x 0.1736 x 3.5 = 1.909 ft3 6 footings = 1.909 x 6 = 11.45 ft3 With 10% waste = 12.60 ft3 = 0.467 yd3 80 lb bags = ceil(12.60 / 0.60) = 21 bags -> 4 bags per footing Note: Most online calculators use the same cylinder formula but skip the bell base entirely, underestimating concrete for bell footings by 15-25%.
Sonotube Calculator for Deck
Concrete Calculator for Deck Footings
The only deck footing calculator that handles both standard Sonotube piers and bell footings, recommends footing count and spacing based on your deck size, and flags frost line depth for your climate — all in one tool. Get exact bag counts in seconds.
Sonotube Size Selector — Pick 8", 10", 12", 14", or 16" diameter directly; no manual diameter entry needed.
Bell Footing Mode — Switch to bell footing to calculate the flared base volume separately from the pier shaft — the step most calculators skip.
Footing Count Estimator — Enter your deck dimensions and the calculator recommends how many footings you need and at what spacing.
Frost Line Alert — Flags automatically if your entered depth is above the frost line for northern climates.
50 / 60 / 80 lb Bag Support — Switch between all standard bag sizes including Quikrete Fast-Setting.
Ready-Mix Threshold — Tells you when ordering a truck beats mixing bags by hand.
Deck Footing Concrete Calculator
Calculate deck footing bag counts
Use this concrete calculator for deck footings to estimate standard Sonotube piers or bell footings. For adjacent work, compare post holes with the fence post concrete calculator, size round piers with the Sonotube concrete calculator, compare columns with the concrete column calculator, convert volume with the concrete bag calculator, and price the order with the concrete cost estimator.
Input
Deck footing inputs
Footing Count Estimator
Starting point: 6 footings at about 8 ft spacing for a 192 ft2 deck. Confirm spans and loads with local code.
Result
21
bags of 80 lb concrete total
4 bags per footing x 6 footings
Volume per footing
1.91 ft3
0.071 yd3
Total volume
0.467 yd3
Includes 10% waste.
Frost line check
Frost line check
Depth meets minimum requirements for most U.S. climates. Verify against your local frost line.
Ready-mix threshold
Bagged concrete is the right call for this project. Total: 0.467 yd3.
How the Deck Footing Calculator Works
Standard mode uses the cylinder formula for a round pier. Bell Footing mode adds the shaft cylinder and the flared base frustum before applying waste and rounding bag counts up.
How Many Footings Does My Deck Need?
Footing count depends on deck size, joist span, and beam span. The table below gives starting-point recommendations for standard residential decks. Always confirm with your local building department.
| Deck Size | Typical Footing Count | Recommended Spacing | Min. Sonotube Size | Notes |
|---|---|---|---|---|
| 8x10 ft | 4-6 | 6-8 ft | 10" | Simple rectangle, single beam |
| 10x12 ft | 6-8 | 6-8 ft | 10" | Most common starter deck |
| 12x16 ft | 8-10 | 6-8 ft | 10-12" | Double beam typical |
| 16x20 ft | 10-12 | 6-8 ft | 12" | Consider 3 beam lines |
| 20x24 ft | 12-16 | 6 ft | 12-14" | Engineer review recommended |
| Multi-level | Varies | 6 ft max | 12-14" | Always get engineered drawings |
| Hot tub support | 4 dedicated | Per tub spec | 14-16" | Verify tub weight with manufacturer |
Spacing over 8 ft typically requires larger beams or additional footings. Corner and beam-end footings are always required regardless of spacing. Enter your footing count in the calculator above to get your bag total instantly.
Frost Line Depth by U.S. Region - How Deep Should Deck Footings Be?
Deck footings must extend below the local frost line to prevent heaving. A footing above the frost line will shift every winter, cracking the concrete and eventually destabilizing the deck structure.
| Region / States | Frost Line Depth | Min. Footing Depth |
|---|---|---|
| Deep South (FL, LA, southern TX) | 0-6 in | 18-24 in (structural min) |
| Southeast (GA, AL, MS, SC, NC) | 6-12 in | 24-30 in |
| Mid-Atlantic (VA, MD, DE, NJ) | 12-20 in | 30-36 in |
| Midwest / Mid-South (TN, KY, MO, KS) | 20-30 in | 36-42 in |
| Great Lakes / Northeast (OH, PA, NY, CT) | 30-42 in | 42-48 in |
| Northern States (MN, WI, MI, ME, VT) | 42-60 in | 54-66 in |
| Mountain West (CO, UT, WY, MT, ID) | 36-60 in | 48-66 in (varies) |
| Pacific Northwest (WA, OR) | 12-24 in | 30-36 in |
| California (most areas) | 0-12 in | 24-30 in |
Important: Many jurisdictions require deck footings to extend a minimum of 12 inches below undisturbed soil regardless of frost depth. Always check local building codes before digging.
How Many Bags of Concrete Per Deck Footing - Quick Reference
Values include 10% waste. Standard pier only, no bell base.
| Sonotube Size | Depth | Volume (ft3) | 80 lb Bags | 60 lb Bags | Fast-Set 50 lb |
|---|---|---|---|---|---|
| 8" | 24 in | 0.70 | 2 | 2 | 3 |
| 8" | 36 in | 1.05 | 2 | 3 | 4 |
| 8" | 48 in | 1.40 | 3 | 4 | 5 |
| 10" | 36 in | 1.64 | 3 | 4 | 5 |
| 10" | 42 in | 1.91 | 4 | 5 | 6 |
| 10" | 48 in | 2.18 | 4 | 6 | 7 |
| 12" | 42 in | 2.75 | 6 | 7 | 9 |
| 12" | 48 in | 3.14 | 6 | 8 | 10 |
| 14" | 48 in | 4.28 | 8 | 11 | 13 |
| 16" | 48 in | 5.59 | 11 | 14 | 17 |
For bell footings, add 1-3 bags per footing depending on bell diameter. Use Bell Footing mode in the calculator above for exact figures.
Step-by-Step Deck Footing Installation Guide
1
Determine footing count and layout before you dig
Count your footings first. Use the spacing table above as a starting point: most residential decks need footings every 6 to 8 feet along each beam line, plus corner footings and ledger footings if attaching to the house. Mark all footing centers with stakes before renting an auger.
2
Dig to the correct depth (frost line + 6 inches)
The bottom of your footing must sit below the local frost line. Find your region in the frost line table above, then add 6 inches of clearance. In Minnesota, that means digging 54 to 66 inches deep. In Georgia, 24 to 30 inches is typically sufficient. Enter your actual depth in the calculator; it will flag a warning if you are above the frost line for your climate.
3
Choose your Sonotube size based on load, not just convenience
Use 10-inch tubes for standard line posts on decks up to 16 feet wide. Use 12-inch tubes for corner posts, beam-end posts, and decks over 16 feet. Use 14-inch or 16-inch tubes for hot tub supports or any point load exceeding the standard residential design load.
4
Consider a bell footing in soft or sandy soil
A bell footing flares out at the base to distribute the load over a wider bearing area. In soft soil, clay, or sandy conditions, a bell base can significantly improve stability without requiring a larger-diameter tube for the full depth. Switch to Bell Footing mode in the calculator above to get the correct concrete volume.
5
Pour, set post base hardware, and cure properly
Pour concrete in lifts, tapping the tube to remove air pockets. Set your post base hardware while the concrete is still wet. Check alignment carefully; you have about 20-30 minutes with standard mix. Do not set posts or load the footings until concrete has cured: 24-48 hours for Quikrete Fast-Setting, 3-7 days for standard 80 lb mix.
Deck Footing Concrete Scenarios
Standard Residential Deck (10x12 to 12x16 ft)
Recommended: 10" Sonotube · 42-inch depth · 4 bags per footing (80 lb)
The most common deck build in the U.S. A 12x16 ft deck typically needs 8 to 10 footings. At 4 bags of 80 lb concrete per footing, budget for 32 to 40 bags total — a manageable one-day pour for two people. Use standard 80 lb mix for best economy, or Quikrete Fast-Setting if you need to set posts the same day.
Large Deck or Multi-Level (16x20 ft and above)
Recommended: 12" Sonotube · 48-inch depth · 6 bags per footing (80 lb)
Larger decks need 12-inch tubes at beam-end and corner positions to handle the increased tributary load. At this scale, total concrete volume often approaches or exceeds 1 cubic yard. Use the ready-mix threshold indicator in the calculator to decide whether truck delivery makes more sense than mixing 70+ bags by hand.
Hot Tub Deck Support
Recommended: 14-16" Sonotube · 48-inch depth · 10-12 bags per footing (80 lb)
A filled hot tub weighs 3,000 to 6,000 lbs. The four footings directly beneath it must be engineered for point loads far beyond standard residential deck design. Use 14-inch or 16-inch Sonotubes, confirm depth with a structural engineer, and verify footing diameter with the hot tub manufacturer's installation manual.
Cold Climate Install (Frost Zone 4-5, 42+ in Depth)
Recommended: 10-12" Sonotube · 54-66-inch depth · 5-7 bags per footing (80 lb)
In Minnesota, Wisconsin, Maine, and similar climates, frost lines run 42 to 60 inches deep. Every extra inch of depth adds concrete cost, but a footing that heaves in spring costs far more to repair than the extra bags you saved. Use the frost line table above to confirm minimum depth for your state.
FAQ
Frequently Asked Questions
How many bags of concrete do I need for deck footings?+
Most deck footings need 3 to 6 bags of 80 lb concrete per footing, depending on Sonotube diameter and depth. A 10-inch tube at 42 inches deep takes about 4 bags of 80 lb mix. A 12-inch tube at 48 inches deep takes about 6 bags. Enter your exact dimensions in the calculator above; it includes a 10% waste factor automatically.
How deep should deck footings be?+
Deck footings must extend below the local frost line to prevent heaving. Minimum depths range from 18-24 inches in warm southern states to 54-66 inches in northern states like Minnesota and Maine. Always check your local building code because many jurisdictions require a permit for deck footings and specify minimum depth by ordinance.
What size Sonotube do I need for a deck?+
Use 10-inch Sonotubes for standard line posts on decks up to 16 feet wide. Use 12-inch tubes for corner posts, beam-end posts, and larger decks. Use 14-inch or larger for hot tub supports or any engineered point load. The Sonotube size selector in the calculator covers 6" through 16" standard sizes.
What is a bell footing and do I need one?+
A bell footing has a flared base that spreads the load over a wider bearing area than the pier shaft alone. They are most useful in soft soil, clay, or sandy conditions where a standard cylindrical pier might punch through the bearing layer under load. In firm, undisturbed soil, a standard cylindrical Sonotube pier is usually sufficient.
Why does my bell footing need more concrete than the calculator shows?+
Most online calculators only compute the cylindrical shaft volume and ignore the bell base entirely. The bell base is a frustum, or truncated cone, and requires a different formula: V = (pi x h / 3) x (R^2 + R x r + r^2). For a 10-inch shaft with a 20-inch bell base at 8 inches tall, the bell base adds about 0.85 ft3 per footing.
Can I use Quikrete Fast-Setting Concrete for deck footings?+
Yes, for line posts and standard footings. Pour the dry mix into the tube around the post, add water on top, and the footing is structurally set in 20-40 minutes. Do not use Fast-Setting for footings that require extended time to adjust post base hardware alignment; use standard 80 lb mix instead.
When should I order ready-mix instead of bags for deck footings?+
Once your total footing volume reaches 1 cubic yard or more, roughly 10 to 12 standard footings, a ready-mix delivery is worth pricing out. Mixing 45+ bags of 80 lb concrete by hand is exhausting and time-consuming. The calculator flags the ready-mix threshold automatically based on your inputs.
Related Calculators
More calculators for deck footing work
Sonotube Calculator
Estimate concrete for round piers, tubes, and cylindrical forms.
Open CalculatorFence Post Concrete Calc
Calculate concrete for post holes with post displacement deducted.
Open CalculatorConcrete Column Calculator
Estimate concrete for round and square structural columns.
Open CalculatorConcrete Cost Estimator
Turn bag counts or yardage into a material budget.
Open Calculator