Concrete footing calculator
Ver el procedimiento — con sus números
This works arithmetic on the trench size you give it and nothing more. How wide and how deep a footing must be — and whether it needs steel or a wider spread — is set by your plans, your frost line and your ground, not by this page. Dig and pour to a detail you trust, and confirm the load with your supplier.
Enter the trench length, its width and its depth, and the tool returns the concrete volume in cubic yards and cubic metres along with a bag count. A footing is just a long shallow box, so the volume is those three dimensions multiplied, with the waste factor added for a trench that never digs perfectly square.
Ejemplo resuelto
A strip footing 30 ft long, 16 inches wide, 8 inches deep: sixteen inches is 1.33 ft and eight is 0.667 ft, so 30 × 1.33 × 0.667 ≈ 26.6 cubic feet, which is 26.6 ÷ 27 ≈ 0.99 yd³. Add 10% for the rough trench and you are ordering about 1.1 yd³ — round that up to the supplier's minimum increment when you book the truck.
In metric: a 9 m run, 400 mm wide and 200 mm deep — type the width as 0.4 m and the depth as 20 cm — is 9 × 0.4 × 0.2 = 0.72 m³. With 10% that is 0.79 m³, and the order line says 0.8 m³.
En la obra
A full perimeter, walked with a wheel. A 28 ft × 40 ft foundation has a centreline perimeter of 2 × (28 + 40) = 136 ft — walk the trench centre with the measuring wheel and it should agree. At 20 in wide and 10 in deep that is 136 × 1.667 × 0.833 = 189 ft³ = 7.0 yd³, and 7.7 with waste, so the ticket reads 8 yd³ once the plant rounds to the half. Measure the outside of the trench instead and the corners get counted twice — nearly 7 ft of footing you paid for and never poured. In metric: an 8.5 m × 12 m foundation has 2 × (8.5 + 12) = 41 m of centreline; at 500 × 250 mm that is 41 × 0.5 × 0.25 = 5.13 m³, and 5.64 m³ with waste.
The digger wandered. The drawing says 16 in wide. You drop a tape in every 10 ft and get 16, 18, 17, 20, 16, 19. Average 17.7 in, and the concrete fills what is there, not what was drawn. On a 100 ft footing 8 in deep that is 98 ft³ instead of 89 — an extra third of a yard before waste. Type the average width you measured, and keep the waste for the spots you did not tape.
Pad footings under posts. Six pads, each 2 ft × 2 ft × 10 in: run one at 2 × 2 × 0.833 = 3.33 ft³ and multiply by six — 20 ft³, 0.74 yd³. Under a yard and scattered across the site is a bagged job: 20 ÷ 0.6 = 34 bags, buy 37 with waste. (Metric pads of 600 × 600 × 250 mm are 0.09 m³ each — 0.54 m³ for six, about 39 bags of 30 kg before waste.) For a round pier use the column tool; a sonotube is a cylinder, not a box.
What went wrong?
Quick referencefor a footing
- A cubic yard
- 27 ft³ — about 45 bags at 0.6 ft³
- Inches as feet
- 8 in = 0.667 · 10 in = 0.833 · 12 in = 1 · 16 in = 1.333 · 20 in = 1.667 · 24 in = 2
- ft³ per foot of footing
- width (ft) × depth (ft) — a 16 × 8 in footing is 0.889 ft³ per foot
- Yards per 100 ft, 16 × 8 in
- 3.3 yd³ (12 × 8 in: 2.5 · 20 × 10 in: 5.1 · 24 × 12 in: 7.4)
- Centreline perimeter
- 2 × (length + width) of the drawing — corners once
- Métrico
- 1 m³ = 1.31 yd³ · 400 × 200 mm = 0.08 m³ per metre
- m³ per 10 m of footing
- 400 × 200 mm 0.8 · 500 × 250 mm 1.25 · 600 × 300 mm 1.8
Waste & overagewhat to add, and why
- Trench footings over-run more than any other pour — the sides slough, the bottom is uneven, and every inch of extra width along a long run adds up; crews carry 10–15% here, more in soft ground. The waste field starts at 10%.
- A footing usually runs with the slab or wall pour behind it; check whether the plant's minimum load makes it cheaper to pour both in one order.
Mistakes to catchbefore you order
- Measuring the trench outside-to-outside on a rectangle: that counts the corners twice. Use the centreline length, which is the perimeter at the middle of the footing.
- Typing the footing depth in inches when the field is in feet, so a 10 in footing becomes 10 ft.
- Ignoring a step in the footing — each step adds a short vertical block that a plain length × width × depth never sees.
What an inspector looks forthe building inspector, at the footing call
On this part of the job
- Width and depth of the trench against the drawing, and that the bottom is on the bearing material the design assumed — an engineer may have to confirm the soil before the pour proceeds.
- Depth below finished grade for frost, taken from the level the site will actually finish at, not from today's rough grade.
- Steps in a footing on sloping ground: where the step falls, how the bars run through it, and that the step is formed rather than dug by eye.
- Reinforcement: size, count, spacing, cover to the soil and the lap at corners and splices, all against the detail.
- Keyways, dowels and anchor-bolt positions set before the concrete goes in, not pushed in wet.
Have this ready
- The foundation plan with the footing schedule.
- The engineer's letter if the soil was reviewed or the design was changed on site.
- Clean, dry trench and a way down into it — an inspector who cannot get to the bottom of the footing will not approve it from the edge.
Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.
La fórmula
volume = length × width × depth
Put every dimension into the same unit before multiplying — inches to feet over 12, then cubic feet to cubic yards over 27. Metric depth in centimetres divides by 100 to metres and the result is already cubic metres. The bag figure is the volume divided by the yield you set per bag.
What this tool assumes
Nothing you cannot change. Width, depth, waste and bag yield are all editable, since they are the numbers that shift between one footing and the next. The only fixed figure is that 27 cubic feet make a cubic yard, which is a unit definition.
Preguntas
- How do I calculate concrete for a footing?
- A strip footing is a long, shallow box, so its volume is length × width × depth. A run 30 ft long, 16 inches wide and 8 inches deep is 30 × 1.33 × 0.667 ≈ 26.6 cubic feet, or just under a cubic yard. In metric, 9 m long, 400 mm wide and 200 mm deep is 9 × 0.4 × 0.2 = 0.72 m³. Enter the trench dimensions above and both cubic yards and cubic metres appear as you type.
- How deep and wide should a footing be?
- That comes from your plans, your frost depth and your soil, not from a calculator — this tool pours the number for the dimensions you were told to dig. Width and depth are yours to set here; the arithmetic only turns them into a volume to order.
- Should I over-order concrete for footings?
- Almost always. A hand-dug or machine-cut trench is never a tidy rectangle — the sides slump, the bottom undulates, and every extra inch of width over the run adds up fast. The waste field starts at 10%; on a rough trench in soft ground, 15% is more honest. A short pour means a cold joint or a second delivery charge.
- Can I use this for a pad footing under a post?
- Yes. A square or rectangular pad footing is the same box maths — enter its plan size as length and width and its thickness as the depth. For a round pier or sonotube, switch to the column tool instead, because a cylinder holds less concrete than the box around it.
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