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Concrete Footing Calculator

Two shapes cover most small jobs: a continuous strip under a wall or a thickened edge, and square pads under posts or piers.

Calculator

Footing type
Your call — we add exactly what you enter. QUIKRETE says its yields include no allowance for waste; Sakrete suggests adding 10%.
Cubic feet
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Cubic yards
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Bags, shared
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Bags, whole bags per pad
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Bag yield: QUIKRETE Concrete Mix data sheet, checked 2026-09-24.

How the number is worked out

strip = length (ft) × width (in) ÷ 12 × depth (in) ÷ 12
square pad = (width ÷ 12)² × depth ÷ 12, times the number of pads

For a strip that runs around a rectangle, measure the length along the centre line of the trench. Adding the four outside edges counts each corner twice.

Worked examples

Strip: 40 ft long, 16 in wide, 8 in deep. 40 × 16 ÷ 12 × 8 ÷ 12 = 35.56 ft³ (1.32 yd³) → 35.56 ÷ 0.60 = 59.3 → 60 bags of 80 lb.

Pads: four pads, 24 in square, 12 in deep. 2 × 2 × 1 = 4 ft³ each, 16 ft³ in all → 27 bags of 80 lb shared, or 7 × 4 = 28 if each pad gets whole bags.

What this calculator is not for

Footing width and depth are set by the load, the soil and your local code and frost depth. This page only turns the sizes you already have into a quantity of concrete. It does not account for rebar, stepped footings or trenches with sloping sides.

Sources

  1. QUIKRETE, QUIKRETE® Concrete Mix (No. 1101) — Product Data Sheet. Checked 2026-09-24.
    “A 40 lb (18.1 kg) bag yields approximately 0.30 ft3 (8.5 L), a 50 lb (22.7 kg) bag yields approximately 0.375 ft3 (10.6 L), a 60 lb (27.2 kg) bag yields approximately 0.45 ft3 (12.7 L), an 80 lb (36.3 kg) bag yields approximately 0.60 ft3 (17 L) and a 90 lb (40.8 kg) bag yields approximately 0.675 ft3 (19.1 L).”
  2. QUIKRETE, QUIKRETE® 5000 High Early Strength Concrete Mix (No. 1007) — Product Data Sheet. Checked 2026-09-24.
    “YIELD • A 50 lb (22.6 kg) bag yields approximately 0.375 ft3 (10.6 L) • A 60 lb (27.2 kg) bag yields approximately 0.45 ft3 (12.7 L) • An 80 lb (36.2 kg) bag yields approximately 0.60 ft3 (17 L)”
  3. QUIKRETE, QUIKRETE® Fast-Setting Concrete Mix (No. 1004) — Product Data Sheet (revised November 2025). Checked 2026-09-24.
    “YIELD • Each 50 lb (22.6 kg) bag yields approximately 0.375 cu ft (10.6 L) • Each 60 lb (27.2 kg) bag yields approximately 0.45 cu ft (12.7 L) … Start by digging the post hole about 3 times the diameter of the post. Hole depth should be 1/3 the overall post height.”
  4. Sakrete, Sakrete 5000 Plus High Early Strength Concrete Mix — Technical Data Sheet. Checked 2026-09-24.
    “PACKAGE SIZE YIELD WATER REQUIRED SLUMP RANGE 50 lb 22.7 kg 0.38 ft3 0.011 m3 2.1 qts 2.0 L 2 – 3 in 50 – 75 mm 60 lb 27.2 kg 0.47 ft3 0.013 m3 2.5 qts 2.4 L 2 – 3 in 50 – 75 mm 80 lb 36.3 kg 0.6 ft3 0.017 m3 3.5 qts 3.3 L 2 – 3 in 50 – 75 mm … A general rule of thumb is that the depth should be 1/3 the length of the post or pole and the diameter should be 3 times that of the pole or post”
  5. Sakrete, Sakrete Fast Setting Concrete Mix — Technical Data Sheet. Checked 2026-09-24.
    “Post Setting - Based on Average 10” Diameter Hole: … Bag Size Yield Water 50 lb (22.6 kg) 0.38 ft3 (.01 m3) 2.5 to 3 qts (2.4 - 2.9 L) 12” (305 mm) deep 18” (457 mm) deep 1.25 bags 2 bags 24” (610 mm) deep 30” (762 mm) deep 2.5 bags 3 bags”
  6. QUIKRETE, QUIKRETE® Quantity Calculator. Checked 2026-09-24.
    “All yields are approximate and do not include allowance for uneven subgrade, waste, etc.”
  7. Sakrete, Sakrete Calculators. Checked 2026-09-24.
    “PRO TIP: Remember to add 10% to your estimates for safety!”
  8. NIST, NIST Handbook 44 (2026), Appendix C — General Tables of Units of Measurement. Checked 2026-09-24.
    “1 728 cubic inches (in3) = 1 cubic foot (ft3) 27 cubic feet = 1 cubic yard (yd3)”