Concrete Calculator

How to measure

Measure the finished dimensions, not the forms. A 24 ft form board sits outside the pour, so if you measure form-to-form you will be over by the thickness of two boards — about an inch and a half on each side.

For a rectangle, take the length and width at both ends and use the smaller of each. Forms bow outward under the weight of wet concrete, and the number you want is the one you would have got if they had not.

Thickness is the one people get wrong. Measure down from a string line to the compacted subgrade in at least four places, and use the deepest reading. Subgrade is never flat. A slab you called 4 inches will pour closer to 4½ over a soft spot, and that half inch across 700 square feet is most of a cubic yard.

For a circle, measure the diameter — straight across the widest point, through the centre. If you measure the radius by eye from the middle, you will be short, because finding the true centre of a marked-out circle is harder than it looks.

A worked example

A detached garage pad, 24 ft by 30 ft, 4 inches thick, poured over compacted gravel.

Volume. 24 ft × 30 ft × 4 in. Working in feet, that is 24 × 30 × 0.3333 = 240 cubic feet, which is 240 ÷ 27 = 8.89 cubic yards.

Waste. The subgrade was screeded but not laser-graded, and the pad has a thickened edge that was not measured separately. At 10 percent, the order becomes 9.78 cubic yards.

What to order. Ready-mix is sold in quarter-yard increments, so this is a 10 cubic yard order. That is one full truck for most fleets, which is the outcome you want — a second truck for a quarter yard costs a short-load fee that can exceed the price of the concrete.

If you bagged it instead. At 0.60 cubic feet per 80-pound bag, 264 cubic feet is 440 bags. That is roughly 17½ tons of material, moved by hand, and it will not be a monolithic pour. Nobody bags a garage pad.

The last line is the point of the exercise. The calculator will happily tell you the bag count for a 9-yard pour, and the bag count is arithmetically correct, but the answer to “should I bag this?” is decided long before you get there.

Common mistakes

Ordering the exact number. The calculated volume assumes a perfectly flat subgrade and forms that do not move. Neither is true. Ten percent is the usual allowance for a slab on prepared ground; on rough or unprepared subgrade, fifteen is not excessive. Running short means a cold joint down the middle of a slab that was supposed to be monolithic, and that joint is permanent.

Rounding down to save a quarter yard. Concrete is the cheap part of the job. The expensive parts are the crew standing on site, the pump if you have one, and the second delivery. A quarter yard of surplus costs less than any of them.

Forgetting the thickened edge. Many slabs are 4 inches in the field and 8 to 12 inches at the perimeter. If yours has one and you enter 4 inches, you will be short by the volume of a trench around the whole slab. Calculate the field slab here, and add the edge separately as its own rectangle.

Using bag counts from a different product. The yields in this tool come from one manufacturer’s data sheet, and they are not universal. Sand and topping mixes yield around ten percent more per pound than standard concrete mix. Check the bag you are actually buying.

Ordering

Ready-mix plants sell in quarter-yard increments with a minimum, usually one cubic yard, and charge a short-load fee below three or four yards. The fee is per delivery and it does not scale with how far under you are, so a 2.5-yard order and a 3.5-yard order can cost the same.

Below about one and a half to two cubic yards, bags are usually the better answer — that is roughly 68 to 90 bags at 80 pounds, which is a long day for two people but does not involve a truck. Above that, order ready-mix.

Call the plant with the volume, the thickness and the day you want it. They will ask about access, whether they can reach the forms from the truck, and what mix design you need. If you are pouring in the cold, ask about accelerator; if it is over 90°F, ask about retarder. Those conversations change the number on the ticket more than a quarter yard either way.