Footing concrete calculator
A continuous strip footing. Trench width, depth and total run.
Decimals fine. Bag counts round up.
At 20 ft of 16 × 8 in footing you need
0.66 cubic yards
17.8 cubic feet · 2,490 lb of concrete · order 0.72 yd³ with the 10% waste allowance
Bag it. 30 bags of 80 lb is one trip in a pickup and an afternoon with a rented mixer. You pay for what you open, and no plant charges you a short-load fee.
Two rules, each where it applies. Bag counts carry no waste allowance — a partial bag is not a thing you can buy, and an unopened one goes back — so the headline count is the volume and the figure beside it is the same count with waste. The ready-mix figure carries its allowance inside because a truck will not come back for a quarter yard.
The arithmetic, in the open
How this page gets its numbers
- Volume, cubic feet
- (16 ÷ 12) × (8 ÷ 12) × 20 = 17.8 ft³
- Cubic yards
- 17.8 ft³ ÷ 27 = 0.66 yd³
- 80 lb bags
- ceiling(17.8 ÷ 0.60 ft³ a bag) = 30 bags, no waste allowance; with 10% waste, 33
- 60 lb bags
- ceiling(17.8 ÷ 0.45 ft³ a bag) = 40 bags, no waste allowance; with 10% waste, 44
- Weight
- 17.8 ft³ × 140 lb per ft³ = 2,490 lb
- Order allowance
- 0.66 yd³ plus 10% = 0.72 yd³ ordered
- Truck loads
- ceiling(0.72 yd³ ordered ÷ 10 yd³ a load) = 1 — a full truckload is typically about 10 cubic yards
Bag counts round up. A partial bag is not a thing you can buy, so a calculated 222.2 bags is 223 bags.
Footing presets
The 6 sets people ask for most
| Footing | Cubic feet | Cubic yards | 80 lb bags→ with 10% waste | Buy as |
|---|---|---|---|---|
| 16 × 8 in × 20 ft | 17.8 | 0.66 | 30→33 | Bags |
| 16 × 8 in × 40 ft | 35.6 | 1.32 | 60→66 | Bags |
| 20 × 10 in × 30 ft | 41.7 | 1.54 | 70→77 | Ready-mix |
| 24 × 12 in × 20 ft | 40.0 | 1.48 | 67→74 | Ready-mix |
| 24 × 12 in × 40 ft | 80.0 | 2.96 | 134→147 | Ready-mix |
| 12 × 6 in × 20 ft | 10.0 | 0.37 | 17→19 | Bags |
What a bag holds
Every bag size, and what it yields
| Bag size | Yield ft³ | Bags a yard |
|---|---|---|
| 40 lb bag | 0.300 | 90 |
| 50 lb bag | 0.375 | 72 |
| 60 lb bag | 0.450 | 60 |
| 80 lb bag | 0.600 | 45 |
| 90 lb bag | 0.675 | 40 |
A different shape
The other 7 calculators
Asked about this shape
Questions
How much concrete does 20 ft of 16 × 8 in footing take?
0.66 cubic yards, which is 17.8 cubic feet before any allowance. Order 0.72 yd³ to carry the 10% waste.
How many bags is that?
30 bags of 80 lb, or 40 of 60 lb, both rounded up and both stated without a waste allowance. With 10% waste they are 33 and 44.
Do I measure the trench or the footing?
The footing. A trench dug wider than the form still only holds concrete where the form is; an unformed trench, poured against soil, holds whatever it was dug to, and that is what to measure. Over-dig is the usual reason a footing takes more than the arithmetic said.
How much extra should I order?
10 percent, and where it appears depends on what you are buying. Ready-mix carries it inside the ordered figure, because a truck will not come back for a quarter yard. Bag counts do not: the headline count is the volume, and the plus figure sits beside it, because bags are bought one at a time and an unopened one goes back. Either way the allowance covers spillage, a base that is not perfectly flat and the thickness you gain at the edges of the form.
Bags or ready-mix — where is the line?
Around 60 bags of 80 lb, which is 36 cubic feet or 1.33 cubic yards. Under that you can mix it in a day with a rented mixer and one helper, and you pay only for what you open. Over it you are buying several pallets, handling two tons or more by hand, and racing the set on the first batch you placed. Some plants charge a short-load fee below about 3 yards, so ask when you book.
What is a mixer-truck load?
A full truckload is typically about 10 cubic yards, and that is the figure this site plans with. Ask when you book: drums vary, and a plant will tell you what it is sending. The load count here is worked from the ordered volume — the one that already includes the 10% allowance — because that is what actually has to fit on the truck.
Does this include rebar, forms or labour?
No. The price band is concrete delivered: your own price per yard on the ordered volume, taken 15% either side of the middle to cover what a quote actually varies by. Forms, gravel, mesh or bar, a pump if the truck cannot reach, and finishing are all separate. Get the plant to quote your postcode before you budget.
Bag yields: QUIKRETE Concrete Mix data sheet No. 1101. Price basis: Concrete Financial Insights weighted average selling price for 2025, escalated with U.S. Bureau of Labor Statistics PPI series PCU327320327320 (2026-07). Prices vary by region — enter your own quote above.
Updated 2026-08-25