Concrete Footing Calculator
Calculate cubic yards of concrete for strip footings from length, width and depth, plus how many 80 lb, 60 lb, 50 lb or 40 lb bags to buy and the cost.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Concrete needed
- 1.45 cu yd
- 39.1 cu ft including 10% waste
- Without waste
- 1.32 cu yd
- 35.6 cu ft for 40 ft of footing at 16 in x 8 in
- 80 lb bags to buy
- 66 bags
- Approximate weight
- 2.67 tons
- At 150 lb per cubic foot
Concrete Footing formula
Cubic feet = Length (ft) x Width (in) / 12 x Depth (in) / 12 x number of footings
Cubic yards = Cubic feet / 27
Order quantity = Cubic yards x (1 + waste %)
Bags = Cubic feet with waste / bag yield (80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft)Worked example
A 60 ft perimeter footing 20 in wide and 10 in deep: 60 x (20 / 12) x (10 / 12) = 83.3 cubic feet. 83.3 / 27 = 3.09 cubic yards. Add 10% waste for 3.40 yards, so order 3.5 cubic yards of ready-mix.
In bags: 83.3 x 1.1 = 91.7 cubic feet. 91.7 / 0.60 = 153 bags of 80 lb mix, which is a job for a truck rather than a mixer.
Quick reference
| Footing size | Cubic feet | Cubic yards | 80 lb bags |
|---|---|---|---|
| 12 x 6 in | 5.0 | 0.19 | 9 |
| 16 x 8 in | 8.9 | 0.33 | 15 |
| 18 x 8 in | 10.0 | 0.37 | 17 |
| 20 x 8 in | 11.1 | 0.41 | 19 |
| 24 x 10 in | 16.7 | 0.62 | 28 |
| 24 x 12 in | 20.0 | 0.74 | 34 |
How to size a footing
Under the International Residential Code, the minimum continuous footing for a one-story wood-frame house on soil rated at 1,500 psf is 12 inches wide and 6 inches thick. Two stories need 15 inches and three stories 23 inches on the same soil, and softer clay soils call for wider footings. Most contractors pour 16 to 24 inches wide and 8 to 12 inches thick, which gives room for rebar and covers minor soil variation.
The bottom of the footing must sit below the frost line so freezing ground cannot heave it. Frost depth is set by your local building department: it is essentially zero along the Gulf Coast, 12 to 30 inches across much of the middle of the country, and 42 to 60 inches in the northern states. Deck and fence post footings follow the same rule.
Ordering the concrete
Footings are trenched into soil rather than formed on all sides, so the actual volume is always more than the drawing. Trench walls slough, corners get overdug, and the bottom is never perfectly flat. Add 10 percent for a clean trench and 15 to 20 percent for rocky or sandy ground.
Anything over about half a cubic yard is easier and usually cheaper by ready-mix truck. Bagged mix works for a few pier footings or a short addition: an 80 lb bag yields 0.60 cubic feet, so a 16 by 8 inch footing takes about 1.5 bags per linear foot. Place two #4 rebar continuous in most footings and keep 3 inches of cover from the soil.
Key facts
- The IRC minimum footing for a one-story house on average soil is 12 in wide and 6 in thick.
- Footings must extend below the frost line, which ranges from 0 in the Gulf states to 4 ft or more in the northern Midwest.
- A 16 x 8 in footing uses 0.89 cubic feet of concrete per linear foot, about 0.33 cubic yards per 10 ft.
- A footing should be at least as thick as the distance it projects past the wall on each side.
- One cubic yard of concrete weighs about 4,050 lb, so a 40 ft footing at 16 x 8 in weighs roughly 2.7 tons.
Frequently asked questions
How much concrete do I need for a 12 x 12 footing?
A 12 in wide, 12 in deep footing uses exactly 1 cubic foot per linear foot. A 40 ft run is 40 cubic feet, or 1.48 cubic yards, before waste.
How wide and deep should a house footing be?
The IRC minimum is 12 in wide and 6 in thick for a one-story house, but 16 to 24 in wide and 8 to 12 in thick is typical. The bottom must be below the local frost line.
How many bags of concrete per foot of footing?
For a 16 x 8 in footing, about 1.5 bags of 80 lb mix per linear foot. For a 12 x 6 in footing, about 0.85 bags per foot.
Do footings need rebar?
The IRC does not require it in most residential footings on stable soil, but two continuous #4 bars are standard practice and are required in many jurisdictions and seismic zones.
Sources and references
Next steps for this project
The usual order for a concrete & masonry project. See all concrete & masonry calculators.
Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.