gizmobench

Concrete Slab Calculator

Type each dimension in the unit you actually measured it in, feet, inches, yards, metres, centimetres or millimetres, mixed freely across the three fields, and read the slab in cubic yards, cubic metres and cubic feet at once. A waste allowance of 0, 5 or 10 percent is applied to the figure you order, with the geometric volume kept beside it, and a bag count comes with it for 40, 60 and 80 lb bags, rounded up. Thickness is a measurement you type and the page reads back to you: it never suggests one.

12 ft10 ft5 in
To orderincl. 10% waste
Cubic yards2.04yd³1.56 m³ · 55.00 ft³ · 1.85 yd³ before waste
80 lb bags59
Length
Width
Thickness
Waste
Bag size
Lead with

Mix the units freely: a length in feet, a width in metres and a thickness in inches all land in the same volume. The bag count is for the 80 lb bag selected above and rounds up, because half a bag is not something you can buy.

  • 12 ft x 10 ft x 5 inone-car pad, 10% waste
    2.04 yd³ · 1.56 m³ · 59 bags
  • 20 ft x 20 ft x 0.5 ftdouble garage, all in feet, no waste
    7.41 yd³ · 5.66 m³ · 213 bags
  • 3 m x 3 m x 100 mmmetric entry, 5% waste
    1.24 yd³ · 0.95 m³ · 36 bags
Volume, never structure. Exact arithmetic on the dimensions you enter, in the units you enter them, plus the waste you choose. It cannot know your subgrade, your local price of concrete, or what thickness a structure needs. Thickness is a measurement you type and this page reads back to you: it never proposes one, and it prints no rebar size and no mix strength, because choosing those for a load is structural work. There is no price field either, since a rate per cubic yard is set regionally and moves.

Common questions

How much concrete does a one-car parking pad take?
A one-car pad is commonly about 10 feet by 20 feet. At 4 inches thick that is 66.67 cubic feet, which the page shows as 2.47 cubic yards or 1.89 cubic metres. Add the 10 percent waste allowance and the figure to order reads 2.72 cubic yards, with 2.47 still shown underneath as the volume before waste. The bag row for the same pad reads 79 bags of 80 lb, 104 of 60 lb or 157 of 40 lb. Your pad is probably not exactly 10 by 20, so type the length and width you measured and the numbers follow.
Why is the answer bigger than length times width times thickness?
Because the waste allowance is added on top, and the page shows both figures rather than one. A slab 12 feet by 10 feet by 5 inches is exactly 50 cubic feet, which is 1.85 cubic yards; with the allowance set to 10 percent the large figure becomes 2.04 cubic yards and the line under it still reads 1.85 before waste. Set the allowance to 0 percent and the two are the same number. The allowance exists because a subgrade is never perfectly flat, forms flex, and some concrete stays in the barrow.
Which bag does the bag count assume?
The one you pick. Three sizes are offered with the yield of mixed concrete they make: 40 lb at 13.3 litres, 60 lb at 20 litres and 80 lb at 26.6 litres. The page divides your volume, waste included, by that yield and rounds up, because half a bag is not something you can buy. A slab of one cubic metre is 38 bags of 80 lb, 50 of 60 lb or 76 of 40 lb. If the bag in your hand prints a different yield, that number is the one that counts, and your count will differ.
Can this page tell me how thick a slab should be?
No, and it is built not to. Thickness is an input here: you type the thickness, the page reports the volume it implies, and nothing on the page prints a thickness, a rebar size or a mix strength. Choosing a thickness for a load, a vehicle or a footing is structural work, and a number we estimated is not something anyone should build on. Get the thickness from an engineer, your local building code or the person specifying the job, then come back and type it in.
Is anything uploaded?
No. The arithmetic runs in your browser, there is no account and nothing is sent anywhere. The only things kept are your three dimensions with their units, the waste allowance, the bag size and which unit the answer leads with, saved in your browser's own storage so the page opens where you left it. Clearing your browser data clears that too.
Can I mix feet and inches, or feet and metres?
Yes, and that is the point of the three unit selects. A slab measured as 12 feet by 10 feet with a 5 inch thickness is one entry, no conversion first. So is 3.6576 metres by 3.048 metres by 127 millimetres, which is the same slab: both read 50.00 cubic feet and 1.85 cubic yards. The conversions use the exact definitions, an inch is 0.0254 metres and a foot 0.3048 metres, so nothing is lost on the way in.

Exact arithmetic on the dimensions you enter, in the units you enter them, plus the waste you choose. It cannot know your subgrade, your local price of concrete, or what thickness a structure needs.