Concrete Calculator
Estimate concrete volume, the number of bags, cement/sand/aggregate quantities by mix strength, saw-cut spacing, and concrete needed on graded or uneven land.
Subtract the volume of a footing, pit, recess or void from the total — useful when a slab has a cut-out. Uses the same unit system as above.
Enter the depth from finished slab level down to natural ground at each corner. The slab top is assumed flat and level; volume uses the average corner depth × plan area.
Rule of thumb: cut depth ≈ ¼ of slab thickness; joint spacing ≈ 24–36× thickness. Cut early (within 6–18 hours). Higher-strength, low-shrinkage mixes tolerate the wider end of the spacing range.
How to work out how much concrete you need
This concrete calculator estimates how much concrete you need and helps you order and place it correctly. Choose metric or imperial and every field stays in that system. Volume is reported in cubic metres (metric) or cubic feet (imperial), with bag counts for your chosen bag weight. The advanced options add a cement, sand and aggregate mix estimate (cement bags plus bulk sand and aggregate by concrete strength and water-cement ratio), saw-cut spacing and depth guidance, an option to subtract a footing, pit or void, and a graded-land mode that works out concrete volume over uneven ground from four corner depths. Everything runs locally in your browser — nothing is installed or uploaded.
To calculate concrete volume by hand: multiply length × width × thickness for a slab, or π × radius² × height for a round column. Convert cubic feet to cubic yards by dividing by 27, or read the result directly in cubic metres above. Always add a 5–10% waste allowance.
Bags of concrete per cubic metre & cubic yard
Use this quick reference to estimate bag counts before you open the calculator:
| Bag size | Yield | Bags per m³ | Bags per yd³ |
|---|---|---|---|
| 20 kg | ≈0.010 m³ | ≈100 | ≈76 |
| 30 kg | ≈0.015 m³ | ≈67 | ≈51 |
| 40 kg | ≈0.020 m³ | ≈50 | ≈38 |
| 60 lb | ≈0.45 ft³ | ≈79 | ≈60 |
| 80 lb | ≈0.60 ft³ | ≈59 | ≈45 |
Yields are typical for pre-mixed bagged concrete; check your supplier's stated yield for exact figures.
Worked examples
Concrete for a 5 m × 3 m shed slab (100 mm thick)
Volume = 5 × 3 × 0.1 = 1.5 m³. With a 5% waste allowance that's about 1.58 m³, or roughly 158 × 20 kg bags — at this size, ready-mix delivery is usually cheaper than bags.
Concrete for a 10 ft × 10 ft slab (4 inch thick)
Volume = 10 × 10 × (4 ÷ 12) = 33.3 ft³ = 1.23 cubic yards, about 56 × 80 lb bags.
Concrete for round deck footings (300 mm diameter × 600 mm deep, ×6)
Each = π × 0.15² × 0.6 ≈ 0.042 m³; six footings ≈ 0.25 m³. Use the round/column mode above and set quantity to 6.
Frequently asked questions
How much concrete do I need for a slab?
Multiply length × width × thickness for the volume, then read it in cubic metres or cubic yards. A 5 m × 3 m slab at 100 mm needs 1.5 m³. Enter your dimensions above for an instant figure including waste.
How many bags of concrete are in a cubic metre?
About 100 × 20 kg bags, 67 × 30 kg bags, or 50 × 40 kg bags per cubic metre (see the table above).
How many 80 lb bags are in a cubic yard?
Roughly 45 × 80 lb bags, or about 60 × 60 lb bags, per cubic yard (27 ft³).
How do I calculate concrete volume on uneven ground?
Measure the depth from finished slab level to natural ground at each corner, average those depths, and multiply by the plan area. The graded-land option does this for you.
What cement, sand and aggregate ratio should I use?
≈1:2.5:3.5 for 20 MPa general concrete, ≈1:1.5:3 for 32 MPa structural, ≈1:1.2:2.4 for 40 MPa. Lower water-cement ratios mean higher strength.
How deep and how far apart should saw cuts be?
Cut to about ¼ of slab thickness and space joints roughly 24–36× the thickness (≈2.4–3.6 m for a 100 mm slab; ≈8–12 ft for a 4″ slab). Cut early, within 6–18 hours.
Should I order extra concrete?
Yes — add 5–10% for spillage and uneven sub-base. Use the waste-allowance setting so it's already included.