Concrete Footing Calculator
Concrete for a strip footing or trench, where the run is long and the section small — the shape in which a per-yard rounding error costs the most.
A footing is long and thin, which is why a rounding error hurts here more than under a slab: the same 10% miscalculation spread down a 40 ft trench is a barrow-load, and there is nowhere to put the excess once the forms are struck.
With trench length 40 ft, trench width 1.5 ft, depth 12 inch, waste allowance 10 percent and 1 more field, concrete comes to 2.44 cu yd — concrete needed. It is reached in 8 steps, the last of which is 66 / 27, and each one is printed on the page with its numbers filled in. The formula is the one published by American Concrete Institute, not an approximation fitted to it.
Formula and sources checked · How we check
- Volume
60 cu ft- Volume with waste
60 * (1 + 10 / 100)66 cu ft- Concrete needed
66 / 272.444 cu yd- Cubic meters
2.4444444 * 0.76455491.869 m3- 60 lb bags (0.45 cu ft each)
ceil(66 / 0.45)147 bags- 80 lb bags (0.60 cu ft each)
ceil(66 / 0.6)110 bags- Weight of the pour
2.4444444 * 40509,900 lbs- Ready-mix cost at that price
2.4444444 * 145354.444
Several at once
A job is rarely one of anything. Work out the first, add it, change the figures and add the next — the list keeps what each line was, and totals them.
How deep does a footing have to go?
Below the local frost line, which the building department publishes and which runs from nothing on the Gulf coast to 48 in across the northern plains. The calculator uses whatever depth you enter and knows nothing about your county. A footing set above the frost line will heave whatever its volume says.
Why trenches take more than they measure
A trench is dug rather than formed, and a dug trench is wider at the top than the string line claims — more so in sand, less in clay. Measure the real width at three points along the run and use the largest, or add ten per cent and accept the overage. A footing short of concrete gets poured twice.
The formula
- Volume
shape_slab * (length * width * thickness / 12) + shape_circle * (pi * pow(diameter / 2, 2) * thickness / 12) + shape_footing * (footing_length * footing_width * depth / 12) + shape_column * (pi * pow(hole_diameter / 2, 2) * height) + shape_wall * (wall_length * wall_height * wall_thickness / 12) + shape_stairs * (stair_width * rise / 12 * run / 12 * steps_count * (steps_count + 1) / 2) - Volume with waste
cubic_ft * (1 + waste / 100) - Concrete needed
cubic_ft_waste / 27 - Cubic meters
cubic_yards * 0.764554857984 - 60 lb bags (0.45 cu ft each)
ceil(cubic_ft_waste / 0.45) - 80 lb bags (0.60 cu ft each)
ceil(cubic_ft_waste / 0.60) - Weight of the pour
cubic_yards * 4050 - Ready-mix cost at that price
cubic_yards * price
Formula from American Concrete Institute, Portland Cement Association.
The same calculator, other cases
Undecided? The Concrete Calculator carries every case behind one control.