Concrete Calculator

Calculate Concrete

m
m
cm

Volume

0.90 m³

Cubic Yards

1.18

Pre-mix Bags

61

20 kg bags

How This Calculation Works

The concrete calculator finds the volume of your slab, footing, or pad by multiplying length × width × depth. Because depth is usually measured in centimetres or inches while length and width are in metres or feet, the calculator converts depth to the matching unit before multiplying.

The result is shown in cubic metres and cubic yards. It is also converted to the number of pre-mixed concrete bags you would need. A standard 20 kg bag yields roughly 0.015 m³ of mixed concrete, and an 80 lb bag yields about 0.6 ft³ (0.017 m³).

For projects over 1 m³ (1.3 yd³), ordering ready-mix concrete from a truck is usually cheaper and easier than mixing dozens of bags by hand.

Common Mistakes to Avoid

  • Underestimating depth. A patio slab should be at least 10 cm (4 in) thick. Driveways typically need 12–15 cm (5–6 in). Cutting depth to save money is the most common cause of cracked concrete.
  • Not accounting for sub-base. The concrete sits on a compacted gravel sub-base, usually 10–15 cm deep. Don't include the sub-base thickness in the concrete depth — they are separate layers.
  • Forgetting formwork losses. Some concrete is lost to imperfect formwork, uneven ground, and the mixing process. Add 5–10% to your calculated volume.
  • Wrong bag count. Pre-mix bags vary by brand and size (20 kg, 25 kg, 40 kg, 60 lb, 80 lb). Always check the yield printed on the bag rather than using a generic number.

Worked Example

Scenario: A garden path slab 3 m long, 1 m wide, and 10 cm deep.

Step 1 — Convert depth: 10 cm = 0.10 m.

Step 2 — Volume: 3 × 1 × 0.10 = 0.30 m³.

Step 3 — Cubic yards: 0.30 ÷ 0.7646 = 0.39 yd³.

Step 4 — Bags: 0.30 ÷ 0.015 = 20 bags of 20 kg pre-mix. Buy 20–22 bags (adding a couple extra for spillage).

Frequently Asked Questions

How thick should a concrete slab be?
For a patio or garden path, 10 cm (4 in) is the minimum. For a driveway, use 12–15 cm (5–6 in). For a garage floor or heavy-load area, 15 cm (6 in) or more is recommended, often with reinforcing mesh or rebar.
When should I use ready-mix instead of bags?
As a rule of thumb, if you need more than 1 m³ (about 65 bags of 20 kg), ordering ready-mix concrete from a truck is faster, cheaper, and results in a more consistent pour.
Do I need to add reinforcement?
For slabs over 10 cm thick and driveways, wire mesh or rebar is strongly recommended. It prevents cracking from soil movement, temperature changes, and heavy loads. For small garden pads, fibre-reinforced concrete may be sufficient.
How long does concrete take to cure?
Concrete reaches about 70% of its strength in 7 days and full strength in 28 days. You can walk on it after 24–48 hours, but avoid heavy loads for at least a week. Keep it moist during the first few days for best results.