Concrete Calculator

How to use this calculator

1. Find the pour that matches your job — each of the five above is its own calculator, so an answer stays on screen while you work out the next.
2. Enter each measurement and choose the unit beside it. Every measurement carries its own unit, so a slab measured in feet with a thickness in inches needs no converting by hand.
3. Set the quantity if you are pouring several identical footings or slabs.
4. Read the volume in cubic feet, cubic yards and cubic metres, then the weight and the number of 60-lb and 80-lb bags beneath it.

Mixing units is fine and often quicker: a 16 ft by 10 ft pad at 4 inches thick can be entered exactly that way. Each measurement is converted to feet before the volume is worked out.

How concrete estimating works

Concrete is sold by volume, so every estimate comes down to turning a shape's dimensions into cubic yards. Work out the volume in cubic feet, then divide by 27 — there are 27 cubic feet in a cubic yard. The weight follows from the density of the mix, and the bag counts follow from the weight.

The five pours

• Slabs, square footings or walls — length × width × thickness. The everyday rectangular pour.
• Hole, column or round footings — a cylinder: π × (diameter ÷ 2)² × depth.
• Circular slab or tube — a ring: the outer cylinder less its bore, π ÷ 4 × (d₁² − d₂²) × height.
• Curb and gutter barrier — the flat gutter flag plus the curb standing on it, multiplied along the run.
• Stairs — the platform slab plus a stack of treads. Each step up sits on the one below, so the concrete grows faster than the number of risers.

The formula

For dimensions measured in feet:

cubic yards = (length × width × depth) ÷ 27

• length × width × depth — the volume in cubic feet, with depth in feet (a 4-inch slab is 0.333 ft).
• ÷ 27 — converts cubic feet to cubic yards, the unit ready-mix is ordered in.
• Weight — the volume times the density of the mix, 133 lbs per cubic foot (2,130 kg/m³) for standard pre-mix.
• Bags — that weight divided by the weight of a bag, so a 60-lb bag goes further in count than an 80-lb one.

Choose metres instead and the tool converts the dimensions to feet first, so the method and the 27-cubic-feet-per-yard constant stay the same underneath.

A worked example

Say you are pouring a 12 ft × 12 ft patio, 4 inches thick. Four inches is one-third of a foot, so the volume is 12 × 12 × 0.333 = 48 cubic feet. Divide by 27 and that is 1.78 cubic yards, or about 1.36 cubic metres.

At the standard pre-mix density that is 48 × 133 = 6,384 lbs, or about 80 × 80-lb bags and 107 × 60-lb bags — a lot of hauling and mixing, which is exactly why a pour this size is usually ordered as ready-mix. Order 10% more than the estimate and it rises to roughly 1.96 cubic yards.

Bags or ready-mix?

Bagged pre-mix suits small jobs — footings, fence posts, a single small pad. Each bag covers only a fraction of a cubic foot, so the count climbs fast:

Bag size	Yield per bag	Bags per cubic yard
60 lb	~0.45 cu ft	~60
80 lb	~0.60 cu ft	~45

For anything much over a cubic yard, ordering ready-mix by the yard is usually cheaper and far less work than mixing dozens of bags by hand.

Order a little more than the estimate

Some concrete is always lost to spillage, an uneven subgrade and slight over-excavation, and running short mid-pour is far more costly than a little leftover. A margin of about 5–10% covers it on a typical slab; add more for a rough or sloping site. The calculator reports exactly what the shape holds, so add that margin to the figure it gives you rather than expecting it to be built in.

How long concrete takes to set and cure

Fresh concrete is firm enough to walk on within 24–48 hours and strong enough to drive on after about a week, but it keeps gaining strength for weeks, reaching most of its rated value at 28 days. Keeping the surface damp through the first days — curing — measurably increases the finished strength.

These are estimates; confirm quantities with your supplier and allow for your own site conditions. For a room's floor area rather than a pour, see the square footage calculator, or the volume calculator for cylinders, columns and other shapes.