Guide
How to Calculate Concrete for a Foundation
A plain-English concrete estimator for slabs, footings, and walls — the same math our calculators use, walked through step by step.
The basic formula
Concrete is sold by the cubic yard. Every shape boils down to:
cubic yards = (length × width × thickness in feet) ÷ 27
One cubic yard = 27 cubic feet. Convert inches to feet by dividing by 12.
Step 1 — Slabs
A 20 ft × 30 ft slab poured 4" thick: 4" ÷ 12 = 0.333 ft. Then 20 × 30 × 0.333 = 200 cubic feet → 200 ÷ 27 ≈ 7.4 cubic yards.
Add 10% waste → order ~8.2 yards.
Step 2 — Footings
Footings are measured by perimeter length × width × depth. A 16" wide × 8" deep footing around a 30 ft × 40 ft house has 140 ft of perimeter. (140 × 1.333 × 0.667) ÷ 27 ≈ 4.6 cubic yards.
Step 3 — Walls
Stem walls: length × height × thickness. 140 ft of 8 ft tall, 8" thick wall = 140 × 8 × 0.667 = 747 ft³ → ~27.7 cubic yards.
Step 4 — Bag counts (small pours)
For pier holes or repairs, skip the truck:
- 60 lb bag yields ≈ 0.45 ft³
- 80 lb bag yields ≈ 0.60 ft³
Divide your cubic-foot total by the yield to get bag count.
Step 5 — Don't forget rebar
Most residential footings spec #4 rebar in a grid at 16" on-center. Use the Rebar & Forms calculator to get stick counts and tie-wire weight.
Skip the math
Plug your dimensions into the built-in tools — both apply the 10% waste factor:
Quick FAQ
How much waste should I order?
10% is standard. Bump to 15% on uneven ground or hand-dug footings.
How thick should a foundation slab be?
Residential slabs are typically 4". Garages and shops 5–6".
Bags or ready-mix?
Above ~1 cubic yard, ready-mix is almost always cheaper and faster.