Calculate the net shape first. Add an explicit allowance only after openings, thickened zones and actual dimensions are understood.
1. Break the pour into simple shapes
Use the issued dimensions and divide slabs, strips, pads, beams and thickened edges into separate prisms. Subtract openings only when they genuinely displace concrete and can be formed accurately.
Convert every dimension to one unit before multiplying. Millimetre thickness must become metres for a result in cubic metres. Keep a sketch showing which dimensions belong to each quantity.
2. Verify formwork and subgrade
Geometric volume assumes the built shape matches the drawing. Measure accessible forms, check subgrade level and identify irregular edges before ordering. Do not use extra concrete as a substitute for correcting unsuitable ground or formwork.
Confirm reinforcement cover, penetrations and construction joints with the design. This calculator estimates quantity; it does not determine strength, thickness, reinforcement or foundation adequacy.
3. State the allowance separately
Allowance covers plausible dimensional variation, handling and small losses. Record the percentage and reason instead of burying it in rounded dimensions. Large uncertainty is a reason to investigate the site, not automatically increase the order.
Ask the supplier about minimum loads, delivery increments, access, discharge time and how unused material is handled. Round only after applying the supplier’s actual ordering rules.
4. Match quantity planning to the pour plan
Check truck access, placing rate, crew, vibration, finishing, weather and curing arrangements. A correct volume does not prevent a cold joint or unsafe vehicle movement.
Create a final order sheet with mix specification, net volume, allowance, rounded order, delivery sequence and an authorised contact for changes.
Concrete quantity worksheet
| Item | Calculation | Check |
|---|---|---|
| Main slab | Length × width × thickness | Uniform thickness |
| Thickened edges | Separate cross-section × length | Avoid double counting |
| Openings | Subtract formed volume | Confirm they displace concrete |
| Allowance | Net volume × stated % | Document reason |
| Order rounding | Supplier increment | Apply last |
Worked example
An 8 m by 4 m slab
The specified thickness is 120 mm and the planning allowance is 7%.
Net = 8 × 4 × 0.120 = 3.84 m³ With allowance = 3.84 × 1.07 = 4.1088 m³
Apply the supplier’s delivery increment after this step. Thickened edges or beams must be calculated separately.
What to have ready
- Issued dimensions and pour sketch
- Verified forms and subgrade
- Mix specification and supplier increments
- Access, placing and curing plan
Common mistakes
- Multiplying metres by millimetres.
- Hiding allowance inside dimensions.
- Forgetting thickened zones or counting them twice.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.