Concrete Slab Calculator
Enter finished slab dimensions. Ready-mix quantity uses volume; bag quantity uses the cured-volume yield printed on the bag, not bag weight alone.
How to use this calculator
Enter length, width, and finished thickness for each slab.
Choose ready-mix volume or a bag size and enter its published yield.
Review the default allowance and rounded-up bag total.
Formula
Order volume = Σ(length × width × thickness) × (1 + allowance). Bags = ceil(order volume ÷ bag yield).
Worked example
A 10 ft × 10 ft slab at 4 in thick is 33.333 ft³, or 1.235 yd³. With 10% extra it is about 1.358 yd³.
Assumptions and limits
- Default concrete density is 155 lb/ft³ and is editable.
- Default extra material is 10%.
- Bag yield is editable because product yields differ.
Sources
Related calculators
Frequently asked questions
Should concrete bags be estimated by weight?
No. Use the manufacturer’s stated mixed or cured volume yield; two products with the same bag weight can have different yields.
Can I use metric and US customary units in the Concrete Slab Calculator?
Yes. Unit changes convert the same canonical measurement; they do not reinterpret or round your original value.
When does the calculator round?
The calculation keeps full precision internally and rounds only the displayed result. Whole packages are rounded up at the final step.
Last updated · formula-v1 · content-2026-08-14 · Trusty Calculator Editorial Team