Concrete Calculator: Volume, Cement Bags, Sand and Aggregate

Enter the length, width and thickness of the slab or member, choose a nominal mix and the wastage. The calculator gives the concrete volume and the cement, sand and aggregate to order.

FreeNo sign-upRuns in your browserUpdated 2 Nov 2026
A common slab is 100 to 200 mm.
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How to use this calculator

  1. Choose feet or metres and enter the length and width.
  2. Enter the thickness in millimetres.
  3. Pick the mix. M20 (1:1.5:3) is common for slabs and beams.
  4. Read the volume, cement bags, sand and aggregate.

The formula

Volume = Length × Width × Thickness × Number of pieces. Volume with wastage = Volume × (1 + Wastage%). Dry volume = Wet volume × 1.54. Each material = Dry volume × its part ÷ sum of parts. 1 bag of cement (50 kg) = 0.0347 m³.

Worked example

A 12 ft × 10 ft slab, 150 mm thick, with 5% wastage and an M20 mix (1:1.5:3): volume 1.672 m³, with wastage 1.756 m³ (62 cft). Dry volume 2.704 m³ gives about 14.2 bags of cement, 0.74 m³ of sand and 1.47 m³ of aggregate.

Understanding your result

The factor 1.54 allows for the extra material that is needed to make 1 m³ of compacted concrete, because the dry materials shrink in volume when mixed with water. The chart shows the cement, sand and aggregate share by volume.

When to use it

  • Estimate material for a slab, footing, plinth or column.
  • Compare the cost of different mixes.
  • Plan deliveries so that site storage is not overloaded.

Assumptions and limits

These are planning estimates. Real use depends on material quality, site conditions and workmanship. Confirm quantities with your engineer or contractor before ordering. Mixes are nominal volume-batch ratios. For structural members, follow the mix design from your structural engineer.

Frequently asked questions

How do I calculate the volume of concrete?

Multiply length, width and thickness, all in metres. A 12 ft × 10 ft slab 150 mm thick is about 1.67 m³.

How many bags of cement are needed for 1 m³ of M20 concrete?

About 8 bags of 50 kg, including a small wastage allowance.

Why multiply by 1.54?

Dry materials lose volume when mixed and compacted, so about 54% extra dry volume is needed to make 1 m³ of concrete.

What wastage should I allow?

5% is common. Use more for uneven ground, difficult access or pumped concrete.

What does M20 mean?

The characteristic compressive strength is 20 N/mm² after 28 days. The nominal mix 1:1.5:3 is cement : sand : aggregate.