Concrete Slab Calculator
Cubic yards and bag count for a rectangular slab, from its length, width and thickness, with the waste allowance kept separate from the figure it is added to.
A slab is the one pour where the waste allowance decides whether you make a second trip. Ready-mix arrives in fixed increments, so 1.36 cubic yards means ordering 1.5 and paying for the rest; bags round the other way and 82 sixty-pound bags is a pallet you have to move by hand.
With length 10 ft, width 10 ft, thickness 4 inch, waste allowance 10 percent and 1 more field, concrete comes to 1.36 cu yd — concrete needed. It is reached in 8 steps, the last of which is 36.666667 / 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
33.333 cu ft- Volume with waste
33.333333 * (1 + 10 / 100)36.667 cu ft- Concrete needed
36.666667 / 271.358 cu yd- Cubic meters
1.3580247 * 0.76455491.038 m3- 60 lb bags (0.45 cu ft each)
ceil(36.666667 / 0.45)82 bags- 80 lb bags (0.60 cu ft each)
ceil(36.666667 / 0.6)62 bags- Weight of the pour
1.3580247 * 40505,500 lbs- Ready-mix cost at that price
1.3580247 * 145196.914
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 much extra should you order?
Five to ten per cent over the computed volume is the trade figure. Forms bow, subgrade is never as flat as the string line, and the last barrow always comes up short. On a 10 by 10 ft slab at 4 in that allowance is about a tenth of a yard. Add it on top of the figure above rather than inside it, so you can see what you are paying for.
Ready-mix or bags?
The crossover is around one cubic yard. Below it bags win on price and on not having a truck idling; above it, mixing forty-five bags by hand is a day of work and a cold joint wherever you stopped. A yard is 45 eighty-pound bags, and that is the number to weigh against a short-load fee.
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.
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