How Much Concrete for Deck Footings?

Updated 2026-09-12 · Yardly

A typical deck footing is a cylinder — not a slab — so the volume is π × radius² × depth. An 8-inch tube 4 feet deep takes about 1.4 cubic feet of concrete (roughly 3 bags of 80-lb mix); a 12-inch tube the same depth takes about 3.1 cubic feet (about 6 bags). Multiply by the number of footings, add 10%, and you have your order.

The cylinder formula

Deck piers are poured into round cardboard forms (sonotube), so use the cylinder volume:

π × (Diameter ÷ 2)² × Depth × 1.10 = cubic feet

π is about 3.14. Diameter and depth go in feet. The ”× 1.10” is a 10% waste margin for spillage and slight overpour. If you also pour a wider bell or a buried block at the bottom, add that separately.

The concrete slab calculator handles slabs and pads; footings use the cylinder math above.

Bags vs ready-mix

An 80-lb bag of premixed concrete yields about 0.6 cubic feet when mixed, so one cubic yard (27 cu ft) is roughly 45 bags — plan for about 50 with waste. That conversion comes straight from the bag yield printed on products like QUIKRETE mixes.

Rule of thumb: under about one cubic yard total, bags are fine. Beyond that, a ready-mix truck is cheaper and saves your back — mixing 45+ bags by hand is a full day of labor.

How deep?

Footing depth is set by your local frost line and soil, not by guesswork — check your building department or the International Code Council adoption of the IRC for the number in your area. Common buried depths run 12–48 inches; deeper, colder climates go lower. The diameter typically matches the post or beam load: 8, 10, or 12 inches.

Worked examples

8-inch tube, 4 ft deep (one footing):

• Radius = 0.33 ft → π × 0.33² = 0.349 sq ft
• × 4 ft = 1.396 cu ft → × 1.10 = 1.54 cu ft
• Bags: 1.54 ÷ 0.6 = ~3 bags (80-lb)

12-inch tube, 4 ft deep (one footing):

• Radius = 0.5 ft → π × 0.25 = 0.785 sq ft
• × 4 ft = 3.14 cu ft → × 1.10 = 3.45 cu ft
• Bags: 3.45 ÷ 0.6 = ~6 bags (80-lb)

A 12×16 deck on nine 8-inch footings at 4 ft:

• 9 × 1.54 = 13.9 cu ft → ÷ 27 = 0.51 cubic yards → about 23 bags (with waste). Still under a yard, so bags are reasonable — but it’s a lot of mixing.

Same deck on nine 12-inch footings:

• 9 × 3.45 = 31 cu ft → ÷ 27 = 1.15 cubic yards → about 52 bags, which is exactly where most people switch to a short ready-mix load.

Don’t forget the bell or block

Many decks sit a wider footing or a concrete block at the bottom of the tube for lateral hold. Add that volume: a 16-inch square block 8 inches thick is 16×16×0.667 = 1.4 cu ft per footing — nearly a full extra bag’s worth per pier. Count it or you’ll come up short mid-pour.

Avoiding a short pour

Always round up. A 10% buffer covers the usual surprises, and cold joints from running short are a headache to fix. If your dimensions are irregular, estimate each footing separately and add them up.

Keep reading

• Estimating a slab or pad instead? The concrete slab calculator totals the cubic yards.
• How bag math works in detail? The concrete bags and yield guide breaks down bag sizes and coverage.
• Pouring footings for a fence instead? The fence post concrete guide uses the same cylinder math at post scale.

Frequently asked questions