Quick answer
For a rectangular slab, multiply length by width by 4 inches converted to feet. Divide the cubic-foot result by 27 to get cubic yards, then add a project-specific allowance for waste and uneven subgrade. The calculation estimates material quantity; slab thickness, reinforcement and subgrade design are separate decisions.
Step by step
- Measure the slab Use the actual finished length and width in feet and split irregular shapes into simple rectangles.
- Convert 4 inches Four inches equals 4 ÷ 12 = 0.3333 ft.
- Calculate volume Multiply length × width × thickness to get cubic feet, then divide by 27 for cubic yards.
- Add allowance Apply a visible allowance for waste, grade variation and ordering practice rather than hiding it in the formula.
Formula or rule of thumb
Example
A 20 ft × 30 ft slab at 4 inches is about 20 × 30 × 0.3333 = 200 ft³, or 7.41 yd³ before an ordering allowance.
Common mistakes
Do not assume a nominal 4-inch slab will be exactly 4 inches everywhere. Do not use the concrete quantity calculation to choose structural thickness or reinforcement. Confirm the supplier's minimum order and truck-delivery rules before purchasing.
Planning note: product yields, installation methods, site conditions and local requirements can change the final quantity. Use the exact manufacturer or supplier information before purchasing.
Decision guide
| Decision point | What the estimate tells you | What to verify |
|---|---|---|
| Measurements | Start with accurate project dimensions and the correct units. | Replace generic assumptions with site measurements. |
| Product data | Use the actual package yield, coverage or density. | Check the current manufacturer or supplier label. |
| Final decision | Treat the result as an early planning estimate. | Confirm final quantities and requirements before purchase. |
Authority and verification
RenoMetric provides transparent planning arithmetic, not a structural design or contractor quote. Verify final decisions with the current product label or supplier data, the applicable local requirements, and a qualified professional when the project is safety-critical.
- RenoMetric methodology — formulas, units and planning limitations.
FAQ
Is 4 inches enough for a slab?
Thickness depends on use, loads, soil, reinforcement and applicable design requirements; the calculator only converts dimensions to volume.
How much extra concrete should I order?
A modest allowance is common for estimating, but the right amount depends on grade variation, form accuracy, access and supplier minimums.
Should I order by cubic feet or yards?
Ready-mix suppliers commonly quote concrete by cubic yard, so convert the calculated cubic feet to cubic yards.