Concrete & Masonry
Concrete Slab Calculator
Enter your slab dimensions in metres and millimetres to get the cubic metres of ready-mixed concrete to order, with wastage and an optional VAT-aware cost estimate.
Formula tested · Local units · No sign-up
Project inputs
Enter measurements
Use your preferred units. Results update automatically.
Show the calculation methodFormula, conversions, rounding, and assumptions+
Volume = length × width × thickness. UK practice measures slab plan dimensions in metres and thickness in millimetres, so the calculator converts millimetres to metres before multiplying — a 4 m × 3 m slab at 100 mm is 4 × 3 × 0.1 = 1.2 m³.
Ready-mixed concrete is sold by the cubic metre; the result is rounded up to the next quarter cubic metre to match typical ordering increments.
Real-world example
Worked example: 4 m × 3 m garden room base, 100 mm thick
- Convert thickness: 100 mm = 0.1 m.
- Volume: 4 × 3 × 0.1 = 1.2 m³.
- Add 10% wastage: 1.2 × 1.10 = 1.32 m³.
- Round up to the ordering increment: 1.5 m³.
Order 1.5 m³. At an example price of £110 per m³ plus 20% VAT, that's £165.00 + £33.00 = £198.00.
Before you start
How to measure
- Measure the internal length and width of your shuttering in metres.
- Measure the depth from the top of the compacted sub-base to the finished level in millimetres, at several points — use the deepest reading.
- For irregular shapes, break the base into rectangles and add the volumes.
Local guidance
Notes for United Kingdom
- UK suppliers offer both 'ready-mixed' (batched at a plant) and volumetric (mixed on site) delivery; volumetric lorries charge for what you use and suit uncertain quantities.
- A 100 mm slab over 100 mm of compacted hardcore is a common build-up for sheds and garden rooms; driveways typically need 150 mm or more. Check whether your project needs building regulations approval.
- VAT at the standard 20% rate applies to most concrete supply; trade quotes are often ex-VAT, so check before comparing.
Quick reference
Slab thickness quick reference (typical nominal values)
| Application | Common nominal thickness |
|---|---|
| Shed or garden room base | 100 mm |
| Path or patio base | 75–100 mm |
| Domestic driveway | 150 mm+ |
| Garage floor | 100–150 mm |
Nominal values for planning only — ground conditions and loading determine the real requirement.
Good to know
Common mistakes to avoid
- Entering thickness in centimetres when the field is set to millimetres — a '10 cm' slab typed as 10 mm gives a tenth of the concrete you need.
- Ignoring wastage: uneven hardcore easily absorbs 5–10% extra volume.
- Not asking whether the quoted price includes VAT and delivery.
- Ordering before the shuttering is checked level — depth errors multiply across the whole slab.
Need help?
Frequently asked questions
How much concrete do I need for a 3 m × 3 m shed base, 100 mm thick?
3 × 3 × 0.1 = 0.9 m³ exactly. With 10% wastage that's 0.99 m³, so order 1 m³.
What is the difference between ready-mixed and volumetric concrete?
Ready-mixed is batched at a plant and delivered as a fixed quantity; volumetric trucks mix on site and you pay for the amount actually poured, which reduces the risk of over- or under-ordering.
Can I mix this quantity by hand instead?
Below roughly 0.5 m³, mixing bagged concrete with a small mixer is feasible. Above that, delivery is usually more practical — 1 m³ needs around 55–60 25 kg bags of dry mix.
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About this calculator
- Written by:
- BuildMeasure Editorial Team
- Technically reviewed by:
- Pending independent technical reviewer (formula unit-tested; see methodology)
- Last reviewed:
- 2026-07-16
- Formula version:
- 1.0.0
- Region reviewed for:
- United Kingdom
- Spotted an error?
- Report a correction
Methodology
- Volume is computed as length × width × thickness × number of slabs, converted internally to SI units (metres) before any arithmetic to avoid unit drift.
- The wastage allowance is applied to the exact volume, then the total is rounded UP to the next 0.25 of the regional ordering unit, because ready-mix suppliers typically sell in quarter-unit increments.
- The cost estimate simply multiplies the suggested order quantity by the price you enter, then applies the tax rate you enter. No prices are built in.
- The formula is covered by automated unit tests, including hand-calculated worked examples, and is versioned (see formula version on this page).
Sources & standards
- Unit definitions: Metric units used throughout; 1 m³ = 1,000 litres.
- Ordering increments: Quarter-cubic-metre increments are typical for UK ready-mix; confirm with your supplier.
This tool provides a material estimate for planning purposes only. It is not a quotation, and it does not size reinforcement, check ground conditions, or replace professional structural advice. Confirm quantities and mix specification with your supplier before ordering.