Rectangular Concrete Slab Calculator

A rectangular slab is the most common concrete pour for driveways, garage floors, sheds, patios and footpaths. The volume is straightforward — length times width times depth — but the catch is the units. Concrete is ordered in cubic yards (US) or cubic metres (everywhere else), but you measure dimensions in feet, inches, metres or centimetres. This calculator handles the conversion and adds the standard 10% waste buffer.

How to measure

For length and width, measure the longest two perpendicular sides of the slab footprint. Use a tape measure on the formwork or chalk line, not the rough excavation — formwork is what actually contains the concrete.

For depth, measure from the top of the formwork to the prepared subgrade at several points. If the subgrade is uneven, average the depths or use the deepest point and accept slight over-pour. Typical residential slab depths:

• Footpaths and patios: 100 mm (4 in)
• Driveways: 100–150 mm (4–6 in)
• Garage floors: 100–150 mm (4–6 in)
• Shed slabs: 100 mm (4 in) for light-duty, 125 mm (5 in) for heavier loads

How the calculation works

Volume is length × width × depth. The calculator converts every input into millimetres internally to avoid rounding errors, computes the cubic-millimetre volume, then converts to your chosen output unit:

• Metric: cubic metres (m³)
• Imperial: cubic feet (ft³) and cubic yards (yd³)

A 10% buffer is added to the raw volume — the ACI 318 industry standard for waste, spillage, uneven subgrade and the dribble lost in chutes. Order to the buffered figure.

Ordering ready-mix vs bagged concrete

Ready-mix is sold by the cubic yard or cubic metre and delivered in a truck. Most plants have a minimum order (often 1 yd³ or 1 m³), so for small slabs (< 1 m³) bagged concrete is usually cheaper. As a rough yield:

• An 80 lb bag yields about 0.6 ft³ of concrete (≈ 45 bags per yd³)
• A 20 kg bag yields about 0.01 m³ of concrete (≈ 100 bags per m³)

For anything over a couple of cubic metres, ready-mix is faster and almost always cheaper per unit.

Common gotchas

• Concrete is heavy. A cubic metre weighs about 2.4 tonnes. Make sure the access route can take a fully-laden truck.
• Pour day weather matters. Don’t pour into standing water, and avoid pouring within a few hours of heavy rain or a hard frost.
• Order the buffer. A short pour usually means a cold joint or a second truck call-out; both are more expensive than the 10% extra.

Use it on the jobsite

The Concrete Calculator app for iOS has this same calculator plus saved projects, history, four more calculator types (circular slabs, irregular slab areas, columns and post holes) and full offline support. Get it on the App Store.