Concrete Calculator

How to estimate concrete

Concrete is sold by volume: ready-mix trucks by the cubic yard in the United States and by the cubic meter almost everywhere else, and small jobs in premixed bags that you add water to. To estimate:

1. Choose the shape. A slab covers patios, floors, driveways and footpaths; a round column covers columns poured in cardboard tube forms, round footings and post holes; steps covers a solid flight poured on the ground.
2. Measure inside the formwork. In US units, enter long sizes in feet and thickness, diameter, rise and run in inches; in metric, meters and centimeters. Switching units converts what you entered.
3. Set How many for repeated identical pours, such as eight post holes, and a waste allowance for spillage, uneven ground and forms that bow. An extra 5 to 10 percent is a common rule of thumb; the right figure depends on your site.
4. Read the volume to order and the number of bags. Bag counts use the yields printed by the manufacturer; open Bag yields to match the bag you’re buying.

Concrete volume formulas

L, W, T

length, width and thickness of the slab

D, H

diameter and height (or depth) of the column or hole

W, R, D, n

width of the steps, rise per step, run (tread depth) per step and number of steps

The steps formula treats each step as a solid block from the ground up: the bottom one is one rise tall, the next two rises, and so on, so their heights add up to 1 + 2 + … + n = n(n + 1) ÷ 2 rises. It fits steps poured solid against the ground, not a sloping stair slab with open space underneath.

Then add the waste allowance and convert the units:

The conversion is exact, from the international foot of 0.3048 m. Bags are the volume to order divided by the yield of one bag, always rounded up to a whole bag.

Worked example

A 10 ft × 10 ft patio slab, 4 inches thick, with a 10% waste allowance:

1. Thickness in feet: 4 ÷ 12 = 0.3333 ft
2. Volume: 10 × 10 × 0.3333 = 33.33 ft³, or 33.33 ÷ 27 = 1.23 yd³
3. With 10% waste: 33.33 × 1.1 = 36.67 ft³
4. Cubic yards to order: 36.67 ÷ 27 = 1.36 yd³ (1.04 m³)
5. Bags: 36.67 ÷ 0.60 = 61.1, so 62 bags of 80 lb; or 82 bags of 60 lb (0.45 ft³ each), or 123 bags of 40 lb (0.30 ft³ each).

In metric, a 3 m × 3 m slab 10 cm thick is 3 × 3 × 0.10 = 0.9 m³; with 10% waste, 0.99 m³ (990 L), which is 990 ÷ 12.2 = 81.1, so 82 bags of 25 kg.

Bags or ready-mix?

A bag yields only a small volume: QUIKRETE’s technical data lists about 0.30 ft³ for a 40 lb bag, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb, so a full cubic yard (27 ft³) takes 45 bags of 80 lb, around 3,600 lb of dry mix to lift and mix. For 25 kg bags this calculator starts at 12.2 liters per bag, from the 0.43 ft³ that QUIKRETE’s Canadian licensee publishes; metric premixes differ between brands and countries, so check yours.

For larger pours, compare the bag count with a ready-mix delivery. Suppliers often have a minimum load or charge extra for small ones, so ask before you plan around it.

What the estimate leaves out

• Uneven ground. The volume assumes a flat base at exactly the thickness entered. A base that dips by half an inch adds 12.5% to a 4-inch slab; measure in several places and use the average.
• Rebar, mesh and pipes take up a little space, which is usually ignored and covered by the waste allowance.
• Shapes. For L-shaped or irregular slabs, split them into rectangles and add the volumes, or find the area with the square footage calculator and multiply by the thickness. For other solids, the volume converter changes units.
• Structural design. The calculator estimates quantity only. Slab thickness, footing size and reinforcement for anything that carries a load should follow your local building code or an engineer.