Concrete Calculator

How to Use the Concrete Calculator

Concrete is sold by volume and poured into a shape, so the only real question is what that shape holds. Get the volume right and everything else — bags, trucks, weight, cost — falls out of it. Get it wrong and you find out at the worst possible moment, with a truck idling and a slab half full.

1. Pick the shape you are pouring. Slabs and footings are both length by width by thickness; round columns and tube forms need a diameter and a height instead, and the fields change to match.
2. Enter the dimensions. Thickness is in inches while length and width are in feet, because that is how everyone actually measures a slab. The calculator handles the conversion.
3. Set a waste allowance. Ten per cent is the standard figure and it exists for good reasons: subgrade that is not perfectly level, spillage, and forms that bulge slightly under load.
4. Read the bag count and the yardage together. Below about thirty bags, mixing on site is reasonable. Above it, order ready-mix — the arithmetic on your own time is brutal.

Everything is computed in your browser. No dimensions are transmitted, nothing is stored, and the page works with the connection off.

Concrete Volume Formulas

Two shapes, two formulas, and one conversion that catches almost everyone: there are twenty-seven cubic feet in a cubic yard, not nine.

The twenty-seven is worth dwelling on. A cubic yard is three feet by three feet by three feet, and 3 × 3 × 3 is 27. People who divide by nine — the square-foot-to-square-yard conversion — order three times too little and discover it on delivery day.

Bag yield is the other figure worth checking rather than assuming. It is printed on every bag, and it varies by manufacturer and by mix. High-strength and fast-setting mixes often yield slightly less per pound than standard mix, which changes the bag count on a large job by enough to matter.

Example

Worked example one: a 20 ft by 12 ft patio slab, 4 inches thick. Four inches is 4 ÷ 12 = 0.3333 feet, so the volume is 20 × 12 × 0.3333 = 80 cubic feet. In cubic yards that is 80 ÷ 27 = 2.96 yd³, and in cubic metres 80 × 0.028317 = 2.27 m³.

Adding the standard 10% allowance gives 88 cubic feet, or 3.26 cubic yards. At 0.60 cubic feet per 80 lb bag that is 88 ÷ 0.60 = 146.67, so 147 bags — nearly six tons of dry mix, carried by hand. This is exactly the job where ready-mix wins. At $125 a cubic yard delivered, the concrete costs 3.26 × 125 = $407.41.

The cured slab weighs 88 × 150 = 13,200 lb, or 6.6 tons. That is worth knowing before you pour one over a suspended structure rather than over ground.

Worked example two: four tube forms, 10 inches across, 4 feet deep. The radius is 5 inches, which is 5 ÷ 12 = 0.41667 feet. The cross-section is π × 0.41667² = 0.5454 square feet. Each tube holds 0.5454 × 4 = 2.1817 cubic feet, and four of them hold 8.73 cubic feet — just 0.32 cubic yards.

With the 10% allowance that is 9.60 cubic feet, or 9.60 ÷ 0.60 = 16 bags of 80 lb mix. Here the answer flips: sixteen bags is a morning's work with a mixer, and no supplier will send a truck for a third of a yard without charging for a full load.

Bags or a Truck: Reading the Result

The single decision this calculator exists to inform is bags against a truck, and the crossover sits somewhere around thirty 80 lb bags — roughly half a cubic yard. Below that, mixing on site is cheaper and far more flexible. Above it, the labour becomes punishing and the risk of a cold joint between batches becomes real.

That cold-joint risk is the part people underestimate. Concrete starts setting within about an hour, so a slab poured in twenty separate batches over an afternoon is not one slab — it is twenty small ones stuck together, and it will crack along those lines. A single truck pour has no such seams.

Read the weight tile before pouring anything above ground. Concrete at 150 lb a cubic foot is heavier than most people picture: a 4-inch slab weighs 50 lb per square foot, so a modest 10 ft by 10 ft landing puts five thousand pounds onto whatever is underneath it.

The waste allowance is not padding. Subgrade that is an inch low across half a slab adds real volume, forms bow outwards under the weight of wet concrete, and some of every load stays in the barrow. Ten per cent covers ordinary conditions; on rough ground, fifteen is more honest.

If the pour is part of a wider project, the square footage calculator handles awkward room shapes and the gravel calculator covers the sub-base underneath the slab, which is usually four inches of compacted stone.

Five Things That Change the Number

Five things that change the number after you have measured correctly.

Slab thickness is rarely uniform. A slab specified at four inches over ground that dips by an inch is thicker than four inches wherever it dips, and concrete fills that dip before it fills anything else. Screeding the subgrade properly is cheaper than buying the extra yard.

Reinforcement displaces almost nothing. Rebar and mesh occupy well under one per cent of a typical slab volume, so there is no need to deduct for them. Chairs and spacers likewise. The waste allowance swamps the difference.

Mix design changes the yield. Fibre-reinforced, high-early-strength and fast-setting bagged mixes do not all yield the same volume per pound. The bag prints its own figure; use it rather than the 0.60 assumed here if it differs.

Trucks are sold in fixed increments. Ready-mix is ordered to the quarter yard and delivered in a mixer holding eight to ten yards. Ordering 3.26 yards means paying for 3.25 or 3.50, and the short-load fee below three yards can be worth more than the concrete itself.

Footings are governed by code, not by preference. Depth below frost line, width relative to the wall above, and reinforcement are all specified locally and often inspected. This calculator tells you what a footing of a given size holds; it cannot tell you what size that footing has to be.