Deck footing calculator
Footing concrete is the cheapest part of a deck and the most annoying to get wrong — a short pour means a cold joint or a second trip. Bags are sold in fixed yields, so the useful number is not cubic yards, it is how many bags to put in the cart.
Your footings
How this is calculated
- Volume per hole = π × (diameter ÷ 2)² × depth, in cubic inches.
- Cubic feet = cubic inches ÷ 1728.
- Cubic yards = cubic feet ÷ 27.
- Bags = total cubic feet ÷ yield per bag, rounded up. 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³.
Worked example
Six holes, 12″ diameter, 36″ deep:
- Volume each = π × 6² × 36 = 4071.5 in³
- = 4071.5 ÷ 1728 = 2.36 ft³ per hole
- Total = 2.36 × 6 = 14.1 ft³ = 0.52 yd³
- 80 lb bags = ⌈14.1 ÷ 0.60⌉ = 24 bags
Twenty-four bags is 1,920 lb of concrete — which is why this is worth calculating before you go to the store, and why an order of ready-mix starts to make sense above about 30 bags.
A note on what actually determines footing size
A 12-inch hole is the code minimum diameter under IRC R403.1.4 for most cases, but the depth is set by frost, not by the deck. In cold climates the footing must extend below the frost line — often 36 to 48 inches, and deeper in parts of the upper Midwest and mountain states. In warm climates it can be shallower, but it still has to bear on undisturbed soil.
Soil matters too. A footing sized for sandy gravel will be undersized in soft clay, because bearing capacity drops. This tool multiplies a cylinder; it does not know your soil.
What this calculator will not set
- It does not set your frost depth. That is a local number, published by your building department. Ask them, or look at the footing detail on a permit drawing for a neighbouring deck.
- It does not size the footing. Diameter and depth come from load, soil bearing and frost — not from the deck's square footage.
- It does not model a flared base. A bell-bottom footing holds considerably more concrete than a straight cylinder; add it separately.
- It assumes clean cylindrical holes. Augered holes in rocky soil are often wider than the auger, which quietly increases volume by 10–20%.
Questions people actually ask
How many bags of concrete for a deck footing?
A 12″ × 36″ hole takes about 2.36 ft³, which is 4 bags of 80 lb mix or 6 bags of 60 lb. For six footings that is 24 bags of 80 lb — just under half a cubic yard.
How deep should deck footings be?
Below the frost line, which is a local number — commonly 36″ in moderate climates and 48″ or more in cold ones. IRC also requires the footing bottom to bear on undisturbed soil. Your building department publishes the number for your area.
Can I pour concrete directly in the hole without a tube?
Yes for straight-sided holes in firm soil. Sonotube is used where the hole is unstable or where you want a clean above-grade pier. The concrete volume is the same; the tube just holds the shape.
Do I need rebar in deck footings?
Not always, but it is cheap and it helps with cracking. A single #4 bar set vertically, or a short L into the post base, is common practice. Check your local requirement — some jurisdictions mandate it.
Why buy a spare bag?
Because a partial bag is not enough to finish a pour, and a cold joint between two pours is weak. One extra bag per ten costs a few dollars and removes the failure mode entirely.