Concrete Footing Calculator
Find how many bags or yards of concrete your footing needs.
Measure your project
Your answer
That is 22.0 cubic feet. We added 10% extra.
If you buy bags
One 80 lb bag makes 0.6 cubic feet of concrete.
If you order a truck (ready-mix)
Ask for 1.00 cubic yards. We round up to the next quarter yard.
This answer is a guide. Footing size depends on your soil, your house and your frost depth. Check your local building rules or ask a builder or engineer.
Which costs less?
But you must mix 37 bags by hand. That is 1.5 tons. For a big job, a truck saves a lot of hard work.
Prices are US middle prices from October 2026. Ask your store for their price.
Quick answers
- Smallest footing (IRC)
- 12 in wide and 6 in thick. Yours may need more; see the table.
- A 40 ft footing, 12 × 6 in
- 0.81 cubic yards with 10% extra. That is 37 bags of 80 lb.
- How deep?
- An outside footing sits at least 12 in below undisturbed ground. It also goes below your frost line, or uses another frost method the code allows.
- What strength?
- 2,500 psi or more (IRC).
- Rebar?
- Only in earthquake areas rated D0, D1 and D2, says the IRC.
How to work it out
- Measure how long the footing runs, in feet. Add up every side.
- Measure its width and thickness in inches. Divide each one by 12 to get feet.
- Multiply the length, width and thickness together. That gives cubic feet.
- Add 10% extra. The total ÷ 27 gives cubic yards.
- One 80 lb bag fills 0.6 cubic feet. Divide by that, and round up.
12 in = 1 ft wide. 6 in = 0.5 ft thick.
40 × 1 × 0.5 = 20 cubic feet
With 10% extra: 22 cubic feet, or 0.81 cubic yards
22 ÷ 0.6 = 36.7, so buy 37 bags of 80 lb
Footing sizes from the IRC
The IRC is the model building code for US homes. States and towns can change it. Its Table R403.1(1) sets the smallest footing for light-frame houses, such as wood-framed homes. Brick-veneer, masonry and concrete houses use other IRC tables, not shown here.
| House | 1,500 | 2,000 | 2,500 | 3,000 | 3,500 | 4,000 |
|---|---|---|---|---|---|---|
| 1 story, slab | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 |
| 1 story, crawl space | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 |
| 1 story, basement | 16 × 6 | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 |
| 2 stories, slab | 13 × 6 | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 |
| 2 stories, crawl space | 15 × 6 | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 |
| 2 stories, basement | 19 × 6 | 14 × 6 | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 |
| 3 stories, slab | 16 × 6 | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 |
| 3 stories, crawl space | 18 × 6 | 14 × 6 | 12 × 6 | 12 × 6 | 12 × 6 | 12 × 6 |
| 3 stories, basement | 22 × 7 | 16 × 6 | 13 × 6 | 12 × 6 | 12 × 6 | 12 × 6 |
Columns are soil strength in psf (pounds per square foot). Based on: Building 32 ft wide, walls 9 ft high, basement walls 8 ft high.
The table has 3 more blocks, for 30, 50 and 70 psf of snow. Heavier snow makes some footings wider. Take a 1-story house on a slab, on clay. It needs 12 × 6 in here, but 14 × 6 in under 70 psf of snow.
Is your house wider than 32 ft? For each 4 ft more, add 2 in to the width and 1 in to the thickness.
The footing must also stick out past the wall by at least 2 in. It may not stick out more than its own thickness.
What kind of soil do you have?
The IRC gives a safe load for each kind of soil. Softer soil needs a wider footing.
| Soil | psf |
|---|---|
| Clay, sandy or silty clay, or silt | 1,500 |
| Sand, silty or clayey sand, or silty or clayey gravel | 2,000 |
| Sandy gravel or gravel | 3,000 |
| Sedimentary and foliated rock | 4,000 |
Not sure? The clay column gives the widest footing, so it is the safest pick from this table. If the soil may be weaker than 1,500 psf, the building office can ask for a soil test.
Concrete for each foot of footing
| Size (width × thickness) | Cubic feet per foot | Feet from 1 cubic yard | 80 lb bags per 10 ft |
|---|---|---|---|
| 12 × 6 in | 0.5 | 54 | 10 |
| 14 × 6 in | 0.58 | 46.3 | 11 |
| 16 × 6 in | 0.67 | 40.5 | 13 |
| 19 × 6 in | 0.79 | 34.1 | 15 |
| 22 × 7 in | 1.07 | 25.2 | 20 |
| 24 × 8 in | 1.33 | 20.3 | 25 |
Bags include 10% extra. The other columns have none.
Multiply the total length of your footing by the cubic feet per foot.
How deep to dig
- The bottom of an outside footing must be at least 12 in below undisturbed ground. That is soil no one has dug or filled.
- Footings must go below the local frost line, or use another method the code lists.
In Minnesota, for example, the state rule is 42 in in the south and 60 in in the north. Your building office can tell you yours.
Small sheds can skip the frost rule. They must stand alone, be light-frame, cover up to 600 sq ft and have eaves up to 10 ft high.
Strength and rebar
Strength is given in psi (pounds per square inch). The IRC asks for at least 2,500 psi concrete in footings.
The IRC only requires footing bars where the earthquake rating (Seismic Design Category) is D0, D1 or D2. There, a footing under a concrete stem wall (a short wall on the footing) needs:
- A #4 bar near the bottom, set 3–4 in up.
- Standing #4 bars, spaced no wider than 4 ft.
Footings at the edge of a slab follow a different bar rule. Any bars poured against soil need 3 in of concrete around them. For bar lengths, try the rebar calculator.
Need post pads or round piers too? Add them in step 1 with "+ Add another part". For tube forms, see the Sonotube calculator.
Footings carry your whole house. Check your local building rules or ask a builder or engineer.
Questions people ask
How big should a concrete footing be?
At least 12 in wide and 6 in thick, says the IRC. Soft soil, more stories and heavy snow make it bigger.
How do you calculate concrete for footings?
Change the width and thickness to feet, then multiply by the length. Divide the cubic feet by 27, and order about 10% more.
How do I work out the footing size?
For a light-frame (wood-framed) house, find your soil, stories, foundation type and snow load. Then read the size from IRC Table R403.1(1).
How much concrete per foot of footing?
A 12 × 6 in footing takes 0.5 cubic feet for each foot. So one cubic yard fills about 54 ft of it.
How deep should a footing be?
An outside footing goes at least 12 in below undisturbed ground. It also goes below the frost line, unless the code allows another frost method.
What psi concrete do footings need?
At least 2,500 psi, says IRC Table R402.2. That is in every weather area.
Do concrete footings need rebar?
Only in Seismic Design Categories D0, D1 and D2, says the IRC. Your plans or local rules may ask for more.
Where our numbers come from
- Bag sizes and weight: Quikrete Concrete Mix data sheet
- Extra for spills: Sakrete slab calculator (10%)
- Footings, frost and footing rebar: US home building code (IRC R403)
- Soil strength for footings: US home building code (IRC Table R401.4.1)
- Concrete strength by use: US home building code (IRC Table R402.2)
- Frost depth example: Minnesota building rules
- Bag price: the middle price of 9 US stores (Oct 2026)
- Truck price: the middle price of 5 US concrete sellers (Oct 2026)
- Small-order fee: the middle of 4 US cost guides (2026)
Made by BuildersCount. Last checked 3 Oct 2026.
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