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 10, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Concrete needed
- 1.45 cu yd
- 39.1 cu ft including 10% waste. Plants batch in 1/4 yard steps, so the ticket reads 1.5 cu yd
- 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
- Over about 20 bags, batches set up between mixes and leave cold joints in a footing; order ready-mix
- 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
IRC Table R403.1(1) sets the minimum continuous footing for a one-story wood-frame house on 1,500 psf soil at 12 inches wide, and R403.1.1 sets the thickness at 6 inches. Two stories need 15 inches and three stories 23 inches on the same soil; the footing may not project past the wall more than its own thickness (R403.1.1). Most contractors pour 16 to 24 inches wide and 8 to 12 inches thick, which fits two #4 bars with 3 inches of cover 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 building department under IRC R403.1.4.1. It is essentially zero along the Gulf Coast, 12 to 30 inches across the middle of the country, and 42 to 60 inches in the northern states. Deck footings follow the same rule; fence posts are not regulated but heave the same way.
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 in clay and 15 to 20 percent for rocky or sandy ground. Call the footing inspection before you call the plant: the inspector wants to see the open trench, the depth to undisturbed soil and the rebar on chairs.
A footing is one continuous structural element, and bag mixing more than about 20 bags leaves cold joints between batches, so most footings over half a yard get a truck. Plants batch in 1/4 yard steps: 1.45 yards is ordered as 1.5, and most will not dispatch under 2 to 3 yards without a short-load fee. Bagged mix works for a few pier pads: an 80 lb bag yields 0.60 cubic feet, so a 16 by 8 inch footing takes about 1.5 bags per linear foot. Set two continuous #4 bars on 3 inch chairs, lap them 20 inches, and bend corner bars instead of butting them. Order 3,000 psi at a 4 to 5 inch slump and do not add water at the chute to make it flow down the trench; a wet mix segregates and loses strength. Footings that stand exposed above grade in freeze-thaw country get air entrainment (IRC Table R402.2).
What this calculator leaves out
The result is concrete only. Add #4 rebar and chairs, vertical dowels for the stem wall or block, and a keyway or dowels at the top. Form boards and stakes are needed where the soil will not hold a trench wall. The ticket will also carry a short-load fee under about 4 to 6 yards and a fuel or environmental surcharge. Standby is billed after the free unload window of about 5 to 7 minutes per yard.
A loaded mixer weighs 60,000 to 70,000 lb and its chute reaches about 16 feet. A footing around the back of a house often needs a line pump, typically $600 to $1,000 with a half-day minimum. Permit and inspection fees, excavation, and hauling spoil are separate as well.
Code limits that apply
- IRC R403.1.1checked
Footing at least 6 in thick, projecting past the wall no more than its own thickness
If you miss it: A 4 in footing shears at the edge of the wall; the inspector probes the trench and refuses the pour
- IRC Table R403.1(1)checked
12 in wide for one story on 1,500 psf soil, 15 in for two stories, 23 in for three
If you miss it: A narrow footing on soft clay settles unevenly and the block wall above cracks in a stair-step
- IRC R403.1.4
Footing bottom at least 12 in below undisturbed ground, on undisturbed soil or engineered fill
If you miss it: Loose fill left in the trench bottom compresses under the wall and the sill line drops at that corner
- IRC R403.1.4.1
Footing bottom below the local frost depth set by the building department
If you miss it: Footing poured above frost depth heaves the first winter and cracks the wall above it
- IRC R403.1.5
Top of the footing level; bottom sloped no more than 1 in 10, stepped on steeper grades
If you miss it: A footing poured on a sloping trench bottom creeps downhill and opens a crack at the step
- IRC Table R402.2
At least 2,500 psi; 3,000 psi air-entrained where exposed to weather in freeze-thaw areas
If you miss it: An exposed stem wall scales and spalls at grade after a few winters of freeze-thaw and salt
- IRC R109.1.1
Foundation inspection with the trench open and rebar in place, before any concrete goes in
If you miss it: Pour ahead of the inspector and the footing gets dug out or cored to prove its depth, at your expense
Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.
Mistakes that cost money
- Pouring before the footing inspection: the inspector wants the open trench, rebar on chairs and undisturbed soil; pour early and you dig it out.
- Rebar laid on the trench bottom has no cover and rusts; set bars on 3 in chairs or dobies and lap #4 bars 20 in.
- Ordering 1.32 yards for a 1.45 yard footing: plants batch in 1/4 yard steps, and a second truck pays the full short-load fee.
- Frost depth is set locally (IRC R403.1.4.1): about 12 in in north Georgia, 42 in in Chicago, 60 in in Minnesota. Guess and it heaves.
- Loose fill or mud left in the trench bottom settles under the wall; IRC R403.1.4 wants undisturbed soil or engineered fill, so shovel it clean.
- A sloped lot needs stepped footings: the top stays level and the bottom may not slope more than 1 in 10 (IRC R403.1.5).
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.
Does the inspector have to see the footing before I pour?
Yes, in almost every jurisdiction. Schedule the footing inspection with the trench open, rebar set on chairs and the bottom cleaned to undisturbed soil. Book the truck for the day after the inspection passes, not the same morning.
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
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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.