How to calculate concrete for a footing
A continuous footing is a long rectangular beam, so its volume is length × width × thickness. Measure the total run in feet — for a closed rectangle that is the perimeter — and the width and thickness in inches. Multiply width × thickness and divide by 144 to get cubic feet per foot of footing, then multiply by the length. Divide by 27 for cubic yards. Pad (spread) footings are the same math for each pad, times the number of pads.
Worked example — a 20×20 ft garage on a 16 × 8 in footing: the perimeter is 80 ft. 16 × 8 = 128 sq in ÷ 144 = 0.889 cu ft per foot × 80 ft = 71.1 cu ft = 2.63 yd³. With 10% extra that is 78.2 cu ft ÷ 0.60 cu ft per 80-lb bag = 131 bags — 10,480 lb. A 3 yd³ ready-mix order costs about $825 against $777 in bags, and saves a day of mixing.
Concrete per foot of footing
| Width × thickness | Cu ft per foot | Feet per cubic yard | 80-lb bags per 10 ft |
|---|---|---|---|
| 12 × 6 in (IRC minimum) | 0.5 | 54 | 9 |
| 12 × 8 in | 0.667 | 40.5 | 12 |
| 16 × 8 in | 0.889 | 30.4 | 15 |
| 20 × 10 in | 1.389 | 19.4 | 24 |
| 24 × 12 in | 2 | 13.5 | 34 |
How big does a footing have to be?
The International Residential Code sets a floor of 12 in wide and 6 in thick (R403.1.1), and the footing must stick out at least 2 in past each face of the wall but no further than its own thickness. Table R403.1(1) widens it for heavier buildings and weaker soil. An excerpt for light-frame construction:
| Building | 1,500 psf soil | 2,000 psf soil | 2,500 psf + soil |
|---|---|---|---|
| 1 story, slab-on-grade | 12 × 6 | 12 × 6 | 12 × 6 |
| 1 story, plus basement | 16 × 6 | 12 × 6 | 12 × 6 |
| 2 story, crawl space | 15 × 6 | 12 × 6 | 12 × 6 |
| 2 story, plus basement | 19 × 6 | 14 × 6 | 12 × 6 |
| Building | 1,500 psf soil | 2,000 psf soil | 2,500 psf + soil |
|---|---|---|---|
| 1 story, slab-on-grade | 12 × 6 | 12 × 6 | 12 × 6 |
| 1 story, plus basement | 18 × 6 | 13 × 6 | 12 × 6 |
| 2 story, crawl space | 17 × 6 | 13 × 6 | 12 × 6 |
| 2 story, plus basement | 21 × 7 | 15 × 6 | 12 × 6 |
The table assumes a 32 ft wide building; wider buildings add 2 in of width and 1 in of thickness per extra 4 ft. Your local code, soil report or engineer has the final say, and many local codes go beyond the model minimums.
Depth, frost and soil
- Exterior footings sit at least 12 in below undisturbed ground (R403.1.4); deck footings follow the same 12 in rule (R507.3.2).
- They must also reach below the local frost depth unless you build a frost-protected shallow foundation. Your building department publishes the number.
- Pour on firm, undisturbed soil or engineered fill. Loose spoil in the trench bottom settles — dig it out.
- Deck footing sizes come from IRC Table R507.3.1, which depends on the load, the deck area each post carries and the soil bearing value.
Rebar in footings
The IRC only mandates footing reinforcement in Seismic Design Categories D0–D2 — for example one #4 bar near the bottom of a stem-wall footing, or a #4 top and bottom in a turned-down slab edge. Elsewhere, reinforcement is up to your plans or engineer; the calculator starts at two continuous #4 bars, which you can change. Keep 3 in of concrete between the steel and the soil (R403.1.3.5.3) by setting bars on chairs or concrete blocks, and overlap the bars at splices; the calculator adds 10% to the bar length for laps.
Bags or ready-mix?
A footing is one continuous pour, and stopping halfway leaves a cold joint. Anything past about a cubic yard usually goes faster and cheaper with ready-mix — plants sell in quarter-yard steps and charge a short-load fee under 4 yd³ or so. Small pads and short repairs are fine in bags. The calculator compares both with prices you can edit.
Footings for posts in tubes? Use the Sonotube calculator. Setting fence posts? Try the post hole calculator. The garage floor itself is a slab.