How is the concrete volume calculated?

It multiplies length by width by depth (converting depth into the same unit) to give the volume in cubic metres or cubic feet, then divides by the yield per bag and rounds up.

What yield should I enter per bag?

A standard 25 kg bag yields roughly 0.011 m³ (about 0.4 ft³) of mixed concrete; a 20 kg bag yields slightly less. Check the bag for the exact figure. The default is 0.011 m³.

How many bags of concrete do I need for 1 cubic metre?

At the default yield of 0.011 m³ per 25 kg bag, one cubic metre needs about 91 bags (1 ÷ 0.011 = 90.9, rounded up). Always check your bag's stated yield, as it varies by mix and size.

Should I add extra for waste?

Yes. It is common to add 5-10% to the volume to allow for spillage, uneven sub-base and over-excavation. Increase your dimensions or bag count slightly rather than ordering exactly the calculated amount.

Is my data sent anywhere?

No. Every calculation runs entirely in your browser and nothing is uploaded to any server.

Concrete Calculator — Gera Tools

This calculator estimates the volume of concrete you need for a slab, path, footing or pad, and how many bags of dry mix to buy. Enter the length, width and depth, set the yield per bag printed on the packaging, and it returns both the cubic volume and a whole-bag count. It supports metric (m, mm depth, m³) and imperial (ft, in depth, ft³) units.

How it works

The tool converts the depth into the same unit as the length and width, then multiplies the three to get the volume:

volume = length × width × depth

In metric, depth in millimetres is divided by 1,000 to metres; in imperial, depth in inches is divided by 12 to feet. The bag count is the volume divided by the yield per bag, rounded up to a whole bag:

bags = ⌈ volume ÷ yield per bag ⌉

A standard 25 kg bag yields roughly 0.011 m³, which is the default.

Example

A slab 3 m × 3 m at 100 mm deep:

volume = 3 × 3 × (100 ÷ 1000) = 3 × 3 × 0.1 = 0.9 m³ bags = ⌈ 0.9 ÷ 0.011 ⌉ = ⌈ 81.8 ⌉ = 82 bags

So that slab needs 0.9 m³ of concrete, or about 82 standard 25 kg bags. All computed instantly in your browser.