Construction Calculator
Concrete Footing Calculator
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Work out the concrete a footing takes — cubic feet, cubic yards and cubic metres from length, width, depth and the number of footings, with a waste allowance and the bag count if you are mixing on site.
Concrete Footing Calculator
Volume in cubic feet, yards and metres
Footing dimensions
Each dimension has its own unit, so a run that is 40 ft long but 24 in wide can be entered as measured.
Results
Cubic yards
2.20
2.00 before waste
Cubic feet
59.4
1.68 m³
Bagged Mix — 59.40 cu ft to Cover
| Bag size | Yield | Bags | Weight |
|---|---|---|---|
| 40 lb * | 0.3 ft³ | 198 | 7,920 lb |
| 60 lb * | 0.45 ft³ | 132 | 7,920 lb |
| 80 lb | 0.6 ft³ | 99 | 7,920 lb |
Counts round up, since a bag cannot be split. The 80 lb yield of 0.60 cu ft is confirmed on manufacturer product data; figures marked * are widely published but were not traceable to a data sheet, so use the yield printed on the bag.
One cubic yard is 27 cubic feet, so the yard figure is always the cubic-feet figure divided by 27 — here 54.00 ÷ 27 = 2.00 cu yd before waste.
Volumes are reported as calculated rather than rounded up, because concrete is a continuous material and the fractional yard is real. Ready-mix is normally sold in quarter-yard increments, so the order quantity will be the next quarter yard above 2.20.
Bag counts are rounded up, since a bag cannot be split. The 80 lb yield of 0.60 cubic feet is confirmed on the manufacturer product data; the 40 lb and 60 lb figures of 0.30 and 0.45 are the widely published values but were not traceable to a manufacturer data sheet, so the yield printed on the bag should take precedence.
All 6 footings are treated as identical. Footings of differing sizes should be calculated separately and added, since the volume of a group is not the volume of an average one times the count unless they genuinely match.
This calculates volume only. It does not size a footing: width, depth and reinforcement follow from the soil bearing capacity, the load carried and the local frost depth, which are engineering decisions rather than geometry.
Reinforcement displaces a little concrete and formwork can bulge outward, so the poured volume rarely matches the nominal one exactly. The waste allowance is what covers the difference, along with spillage and what stays in the chute.
Step-by-Step Calculation
Step 1 — Convert dimensions
Length = 3 ft (already in ft)
Width = 3 ft (already in ft)
Depth = 1 ft (already in ft)
Step 2 — Volume of one footing
3 × 3 × 1 = 9.0000 cu ft
Step 3 — Total for all footings
9.0000 × 6 footings = 54.0000 cu ft
Step 4 — Convert to cubic yards
54.0000 ÷ 27 = 2.0000 cu yd
Step 5 — Add 10% waste
54.0000 × 1.1 = 59.4000 cu ft
which adds 5.4000 cu ft of allowance
Step 6 — Bag counts from the with-waste volume
59.4000 ÷ 0.3 = 198 × 40 lb bags
59.4000 ÷ 0.45 = 132 × 60 lb bags
59.4000 ÷ 0.6 = 99 × 80 lb bags
Result — 2.20 cu yd (59.40 cu ft / 1.682 m³) including waste
Footing Dimensions
A footing spreads the load of a wall or column over the soil. Its volume is simply length × width × depth, but footings are usually specified in mixed units (feet of length, inches of width and depth), so convert everything to feet first.
Volume, Then Divide by 27
Footing volume is the simplest calculation in concrete work: length × width × depth. A 4 ft × 2 ft × 1 ft pad is 8 cubic feet. Several identical footings multiply out, so ten footings at 13.5 cubic feet each come to 135.
The step that trips people up is the unit. Concrete is sold by the cubic yard, and a cubic yard is 27 cubic feet — a yard is three feet, so a cubic yard is a three-foot cube and 3 × 3 × 3 = 27. That 8 cubic foot footing is 8 ÷ 27 = 0.2963 cubic yards.
27 rarely divides evenly, which is why footing volumes so often come out as awkward decimals. This calculator reports them as calculated rather than rounding up, because the fractional yard is real concrete — and ready-mix is sold in quarter-yard increments, so the order quantity is the next quarter yard above what is shown.
Concrete Footing Formulas
1. Volume in Cubic Feet
2. Convert to Cubic Yards
3. Total for All Footings
4. Add the Waste Factor
Common Pad Footing Sizes
Volume for one footing at each size, before any waste allowance:
| Footing size | Cubic feet | Cubic yards | 80 lb bags |
|---|---|---|---|
| 24 × 24 × 8 in | 2.67 | 0.099 | 5 |
| 2 × 2 × 1 ft | 4.00 | 0.148 | 7 |
| 4 × 2 × 1 ft | 8.00 | 0.296 | 14 |
| 3 × 3 × 1 ft | 9.00 | 0.333 | 15 |
| 3 × 3 × 18 in | 13.50 | 0.500 | 23 |
| 4 × 4 × 1 ft | 16.00 | 0.593 | 27 |
Note that a 3 × 3 × 18 in footing is exactly half a cubic yard, since 13.5 is half of 27. Sizes that land on clean fractions of a yard are the exception rather than the rule.
Continuous Footings: One Run, Not Many Segments
A strip or continuous footing under a wall is entered as a single footing whose length is the total run. At 1 ft deep:
| Total run | 16 in wide | 24 in wide |
|---|---|---|
| 20 ft | 26.7 ft³ · 0.99 yd³ | 40.0 ft³ · 1.48 yd³ |
| 40 ft | 53.3 ft³ · 1.98 yd³ | 80.0 ft³ · 2.96 yd³ |
| 60 ft | 80.0 ft³ · 2.96 yd³ | 120.0 ft³ · 4.44 yd³ |
| 80 ft | 106.7 ft³ · 3.95 yd³ | 160.0 ft³ · 5.93 yd³ |
| 100 ft | 133.3 ft³ · 4.94 yd³ | 200.0 ft³ · 7.41 yd³ |
Widening the footing from 16 in to 24 in raises the volume by half, which is the single largest lever on the concrete bill — and it is not a free choice, since footing width comes from the soil bearing capacity and the load above. The calculator warns if a footing is many times longer than it is wide while the count is above one, because that usually means a continuous run has been entered as segments by mistake.
Mixed Units Are the Norm, Not the Exception
Footings are rarely specified in one unit. A continuous footing is 40 ft long but 24 in wide and 12 in deep, because the run is a building dimension while the section comes off a detail drawing in inches.
Each dimension here carries its own unit, so that can be entered exactly as measured. 40 ft × 24 in × 12 in gives 80 cubic feet, identical to entering 40 × 2 × 1 in feet — the conversion happens before the multiplication rather than after.
Metric mode works the same way, accepting metres, centimetres or millimetres, and reports cubic metres alongside cubic feet and cubic yards. The conversion is exact: 1 foot is defined as 0.3048 metres, so a cubic yard is 0.764555 m³.
Bags or Ready-Mix?
Bag counts per cubic yard, which is where the decision becomes obvious:
| Bag size | Yield | Per cubic yard | Weight |
|---|---|---|---|
| 40 lb * | 0.30 ft³ | 90 bags | 3,600 lb |
| 60 lb * | 0.45 ft³ | 60 bags | 3,600 lb |
| 80 lb | 0.60 ft³ | 45 bags | 3,600 lb |
The three yields are internally consistent: every bag size comes to 3,600 lb — 1.8 US tons — of mix per cubic yard, so the published figures agree with one another regardless of which size you buy. The 80 lb yield of 0.60 cubic feet is confirmed on the manufacturer's product data; the 40 lb and 60 lb figures marked * are the widely published values but were not traceable to a manufacturer data sheet, so the yield printed on the bag should take precedence.
That weight figure settles most decisions. Below roughly a cubic yard, ready-mix suppliers generally bill a minimum load and may add a short-load charge, so bagging is often cheaper. Above two or three yards it stops being realistic: the 5.5 cubic yard pour in Example 2 is 248 bags, nearly 10 US tons to carry, open and mix. Minimum loads and short-load fees vary by supplier, so both are worth a phone call before ordering.
What the Waste Factor Covers
On the 135 cubic foot pour from Example 2:
| Waste | Cubic feet | Cubic yards | 80 lb bags |
|---|---|---|---|
| 0% | 135.0 | 5.000 | 225 |
| 5% | 141.8 | 5.250 | 237 |
| 10% | 148.5 | 5.500 | 248 |
| 15% | 155.3 | 5.750 | 259 |
The allowance covers spillage, what stays in the chute or the mixer drum, formwork that bulges slightly outward under the weight of the pour, and over-excavation where the trench came out wider than specified. Reinforcement works the other way, displacing a little concrete. Footings sit at the low end of the usual range — 5–10% rather than the larger allowance a slab wants — because a trench is a contained pour. Running short mid-pour is the expensive failure here, so the allowance is worth having.
Example Calculations
A single pad, a group of ten, and a continuous run:
Example 1 — A Single Pad Footing
4 ft × 2 ft × 1 ft, one footing, no waste allowance.
Volume = 4 × 2 × 1 = 8 cubic feet
Cubic yards = 8 ÷ 27 = 0.2963 yd³
Rounded for display that is 0.30 yd³
The exact value is 8/27, which never divides evenly
At 0.30 yd³ this is far below any ready-mix minimum load
14 × 80 lb bags covers it — a bag job, not a truck job
Example 2 — Ten Identical Footings
3 ft × 3 ft × 1.5 ft, ten footings, 10% waste.
Per footing = 3 × 3 × 1.5 = 13.5 cubic feet
Total = 13.5 × 10 = 135 cubic feet
Cubic yards = 135 ÷ 27 = 5.0 yd³ exactly
With 10% waste = 5.0 × 1.10 = 5.5 yd³
Both figures are exact — 135 is a whole multiple of 27
Ready-mix territory: 248 × 80 lb bags would be 9.9 US tons
Example 3 — A Continuous Strip Footing
40 ft total run × 2 ft × 1 ft, entered as one length.
Volume = 40 × 2 × 1 = 80 cubic feet
Cubic yards = 80 ÷ 27 = 2.9630 yd³
Often quoted as 3.0 yd³, which overstates it by 1.25%
That round-up is smaller than any waste factor
Same roof entered as 40 ft × 24 in × 12 in gives 80 cu ft too
A continuous footing is one run, not several segments
Before You Pour
Take the footing dimensions from the plans rather than from the trench. Width, depth and reinforcement follow from the soil bearing capacity, the load carried from above and the local frost depth, and none of those is a geometry question a calculator can answer — if the plans do not specify a footing, that is the gap to close first, not a number to estimate. Then measure the trench as actually dug, because over-excavation is poured as concrete whether it was intended or not, and a trench that came out two inches wide over a 40 ft run is most of a quarter yard. Footings of different sizes need working out separately and adding, since an average size times a count is not the same volume. Order against the with-waste figure, rounded up to the next quarter yard, and confirm the supplier's minimum load and short-load charge beforehand — on small pours those can dominate the concrete cost. If you are mixing from bags, check the yield printed on the bag against the figures used here, and bear in mind that the practical limit arrives earlier than the arithmetic suggests: a couple of cubic yards is already several tons of material to move by hand. This calculates volume only, and is not a substitute for a footing design.
Frequently Asked Questions
- How do you calculate concrete for footings?
- Multiply length by width by depth to get cubic feet, then divide by 27 for cubic yards. A 4 ft × 2 ft × 1 ft footing is 8 cubic feet, which is 8 ÷ 27 = 0.2963 cubic yards. For several identical footings, multiply the volume of one by the count, then add a waste allowance of 5–10%.
- How many cubic feet are in a cubic yard?
- Exactly 27. A yard is three feet, and a cubic yard is a three-foot cube, so 3 × 3 × 3 = 27 cubic feet. That is why every footing volume in cubic feet gets divided by 27 to reach the figure concrete is actually sold by.
- How much concrete do I need for 10 footings?
- Work out one footing and multiply. Ten footings at 3 ft × 3 ft × 1.5 ft are 13.5 cubic feet each, so 135 cubic feet in total, which is exactly 5.0 cubic yards. With a 10% waste allowance that comes to 5.5 cubic yards. This only holds if the footings are genuinely identical — mixed sizes must be calculated separately and added.
- What waste factor should I use for concrete footings?
- 5–10% is the usual range, and 10% is a safe default. Footings waste less than slabs because they are a contained pour, but the allowance still covers spillage, what stays in the chute or mixer, formwork that bulges slightly under the pour, and the concrete displaced by reinforcement. On a 5.0 cubic yard pour, 10% adds half a yard.
- How many bags of concrete do I need for a footing?
- Divide the volume in cubic feet by the bag yield and round up. An 80 lb bag yields 0.60 cubic feet, so an 8 cubic foot footing needs 14 bags. A useful check: every bag size works out to 3,600 lb — 1.8 US tons — of mix per cubic yard, which is 45 × 80 lb, 60 × 60 lb or 90 × 40 lb bags.
- Should I use bagged concrete or ready-mix for footings?
- It turns on volume. Below about a cubic yard, ready-mix suppliers generally bill a minimum load and may add a short-load charge, so bagged mix often wins. Above two or three cubic yards, bagging becomes impractical — a 5.5 cubic yard pour is 248 × 80 lb bags, nearly 10 US tons to move and mix by hand. Get the minimum load and any short-load fee from your local supplier, since both vary.
- Why is my footing volume not a round number of yards?
- Because 27 rarely divides evenly into a footing volume. An 8 cubic foot footing is 0.2963 cubic yards and an 80 cubic foot run is 2.9630 — neither terminates. Volumes here are reported as calculated rather than rounded up, since the fractional yard is real concrete. Ready-mix is normally sold in quarter-yard increments, so the order quantity will be the next quarter yard above the figure shown.
- Does this calculator size the footing for me?
- No — it calculates volume only. Footing width, depth and reinforcement follow from the bearing capacity of the soil, the load carried from the structure above and the local frost depth. Those are engineering decisions rather than geometry, and they belong on the plans before any volume is worked out.