Concrete Calculator

Order too little concrete and you're left with a half-finished base and a cold joint. Order too much and you're paying to get rid of it. Working out the right amount takes a few minutes with a tape measure and a little maths.

Our Concrete Calculator does the maths for you. Enter your measurements and it shows how much concrete you need in cubic metres, 25kg bags and tonnes, plus the cement, sand and gravel to mix your own.

How to work out concrete volume

Concrete is measured in cubic metres (m³). For a slab, base or footing:

Volume = length × width × depth

Measure the depth in metres, so 100mm is 0.1m. A 4m × 3m shed base 100mm deep needs 4 × 3 × 0.1 = 1.2m³.

For round post holes or columns:

Volume = π × radius² × depth

A 300mm hole is 0.15m in radius. At 600mm deep, that's 3.142 × 0.15 × 0.15 × 0.6 = about 0.042m³ per hole.

Always add 5% to 10% for waste, uneven ground and spillage.

How deep should concrete be?

• Shed bases, paths and patios: 75mm to 100mm.
• Driveways and garage floors: 100mm to 150mm, usually on a well-compacted hardcore base.
• Fence posts: holes about a third of the post's height above ground, often 600mm deep.
• Foundations for walls and extensions: depths are set by Building Regulations and your building control officer.

How to use the calculator (Concrete Calculator)

1. Choose what you're pouring: a slab or base, a strip footing or trench, or round post holes.
2. Enter the length, width and depth, or the diameter and depth for post holes.
3. Enter how many you need.
4. Set the extra for waste. 10% is a sensible default.

You'll see the total volume, its approximate weight, how much ready-mix to order, how many 25kg bags of pre-mixed concrete you'd need, and the cement, sand and gravel for a 1:2:4 mix.

A worked example

With the calculator's default settings, you're laying a 4m × 3m base, 100mm deep, with 10% extra:

• Volume: 4 × 3 × 0.1 = 1.2m³, or 1.32m³ with waste
• Weight: about 3.2 tonnes
• Ready-mix: order 1.5m³
• Or about 121 bags of 25kg pre-mixed concrete

That many bags would be a long day's mixing, which is why ready-mixed concrete usually makes sense for anything over about 1m³.

Bags, mix your own or ready-mix?

• 25kg bags of pre-mixed concrete are best for post holes and small repairs. Just add water. Each bag makes roughly 0.011m³.
• Mixing your own from cement, sharp sand and gravel is cheaper per m³ but hard work. A 1:2:4 mix by volume suits most footings, bases and slabs.
• Ready-mixed concrete arrives in a truck, mixed to the right strength. Mini-mix suppliers deliver smaller loads and can often barrow it to where you need it.

For a 1:2:4 mix, 1m³ of concrete needs roughly 320kg of cement (about 13 × 25kg bags), 0.7 tonnes of sharp sand and 1.4 tonnes of 20mm gravel.

Before you pour

• Check the rules for driveways. Since 2008, paving more than 5m² of a front garden with an impermeable surface like concrete needs planning permission unless rainwater drains to a lawn or border.
• Prepare the base. Dig out to the right depth, compact a hardcore sub-base and set up timber formwork.
• Plan for the weather. Avoid pouring in frost or heavy rain, and keep new concrete damp for a few days while it cures.

Thinking of a different finish? Compare the cost of a new driveway, a resin patio or a retaining wall with our other home calculators.