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Concrete slab calculator
Estimate a concrete slab, patio or floor from its length, width and thickness.
Your estimate
Enter dimensions and select Calculate concrete.
Plan the quantity. Check the order.
This is a geometric quantity estimate, not a structural design or a supplier order. Confirm field dimensions and ordering requirements before the pour. Results use full calculation precision and round only for display.
Measurements stay in this browser session. No account, payment, or upload is needed.
← All calculatorsHow to calculate slab concrete
- Choose US customary or metric units.
- Enter the dimensions and quantity of identical sections.
- Set a waste allowance suited to the job, then select Calculate concrete.
- Review the base volume and total with your supplier.
Formula
Volume = length × width × thickness. Convert inches to feet by dividing by 12; divide cubic feet by 27 for cubic yards. Convert millimetres to metres by dividing by 1,000 before multiplying.
Worked examples
A 20 ft × 12 ft slab at 4 inches thick uses 20 × 12 × (4 ÷ 12) = 80 ft³, or 2.963 yd³ before waste. With 5% extra: 3.111 yd³.
A 6 m × 4 m slab at 100 mm thick uses 6 × 4 × 0.1 = 2.4 m³; with 5% extra: 2.52 m³.
Before ordering
Measure thickness separately from length and width. Split thickened edges into additional non-overlapping sections. Confirm the finished subgrade and forms before ordering.
Common questions
Can I calculate in metric units?
Yes. Select Metric and choose metres, centimetres or millimetres beside each dimension. Results include cubic metres, cubic feet and cubic yards.
How much extra concrete should I order?
The tool starts at 5% and accepts 0–20%. Actual allowances depend on measurement accuracy, forms, excavation and placement. Confirm the quantity and delivery increments with the supplier.
Does this calculate reinforcement or structural capacity?
No. It estimates geometric concrete volume. It does not design reinforcement, foundations or concrete mixes.