Concrete · updated 2026
Deck Footing Concrete Calculator
Deck footings carry the entire weight of your deck, so getting the concrete math right matters more here than anywhere else in the backyard. This deck footing concrete calculator sizes standard round sonotube footings: enter the tube diameter, how deep you're digging, and your footing count, and it returns total concrete volume plus the number of 60-lb and 80-lb bags to buy, with current material costs. The math assumes round holes; if your footings flare wider at the base (a good idea in soft soil), add about 10%.
12 in is standard for most residential decks; 16–18 in for heavy loads or soft soil
Must go below your local frost line — 36–48 in in most northern states; check local code
Show the math
Radius = 12 ÷ 24 = 0.50 ft
Volume per footing = π × 0.50² × (48 ÷ 12) = 3.14 ft³
Total volume = 3.14 × 8 footings = 25.13 ft³
80-lb bags = 25.13 ÷ 0.6 = 42 (rounded up — an 80-lb bag yields 0.6 cu ft)
What the numbers mean
The total volume is the space inside all of your round holes. Because holes are cylindrical, a small change in diameter can change the concrete quantity quickly.
Bag counts are rounded up because concrete is sold in whole bags. The 80-lb estimate uses a 0.6 cu ft yield; the 60-lb estimate uses a 0.45 cu ft yield.
The price range is for the seeded 80-lb bag price multiplied by the rounded bag count. Add posts, delivery, and any local tax separately.
2026 material prices
| Material | 2026 average price | Note |
|---|---|---|
| 80-lb concrete mix bag | $6.47 – $7.58 / bag | Quikrete #110180 at Home Depot, Sep 2026 |
| 60-lb concrete mix bag | $5.00 – $6.00 / bag | Same source |
| 12-in × 48-in sonotube | $12.00 – $18.00 / tube | Per tube, retail |
Material prices: Home Depot, Sep 2026. National averages; local prices vary. 2026 U.S. national averages. Prices vary by region and retailer — confirm locally before buying.
Pro tips
- Call 811 before you dig — utility locates are free and required in most states.
- Set the sonotube 2–6 in above grade so the post base stays out of standing water.
- Bell out the bottom of the hole wider than the tube in soft soil; it doubles as frost protection.
- Drop a J-bolt or post-base anchor into the wet concrete — never bury the wood post itself.
- Past about 40 bags (roughly 1 cubic yard), price ready-mix delivery — it is usually cheaper and far less labor.
- Pour all footings in one day so they cure evenly before you set posts.
Common questions
Frequently asked questions
How many bags of concrete per deck footing?
A standard 12-in diameter footing at 48 in deep needs about 6 eighty-lb bags (3.14 cu ft ÷ 0.6). Enter your exact tube size and depth above for your number.
How deep do deck footings need to be?
Below the local frost line — 36–48 in across most northern states, shallower in the South. Your local building code sets the minimum; the inspector will check.
Can I use deck blocks instead of poured footings?
For small, low, freestanding decks, precast deck blocks on compacted gravel are acceptable in many areas. Attached or elevated decks almost always require poured footings below the frost line.
How many footings does a 12×16 deck need?
Typically 6–8, depending on beam spans and joist layout — a common layout is two beams with 3–4 footings each. Your span tables decide the final count.
Should the sonotube stick up above the ground?
Yes — 2 to 6 in above grade keeps the post base dry and is required by most codes.
Ready-mix or bags for deck footings?
Bags win under about 1 cubic yard (roughly 40 eighty-lb bags). Above that, a ready-mix truck is usually cheaper per yard and saves a brutal day of mixing.
Related calculators
Planning estimates only. Prices are 2026 U.S. national averages and vary by region — confirm with your local supplier before buying.