Concrete slab estimator
Concrete looks solid, but ordering it is basically a volume-and-mistakes calculation. A slab that’s “just 4 inches thick” over a big area turns into multiple cubic yards fast — and “almost enough” concrete is the kind of mistake that hardens in real time.
For developers: API access
Same result via GET request (use current inputs above):
curl -s "https://howdeedo.com/api/calc/concrete-slab-estimator?lengthFt=20&widthFt=10&thicknessInches=4&wastePercent=10"fetch("https://howdeedo.com/api/calc/concrete-slab-estimator?lengthFt=20&widthFt=10&thicknessInches=4&wastePercent=10").then(r => r.json())Get an API key for higher limits and stable access.
Good to know
Thickness is everything. A 10' × 20' slab at 4 inches is 2.47 cubic yards. At 6 inches it’s 3.70. Same footprint, 50% more concrete. If you’re changing thickness, you’re changing the whole project.
Overage is cheaper than panic. Shorting concrete by even a small amount can ruin finish quality and force ugly patches. A small waste factor is usually cheaper than a second truck (or a bad day).
Forms lie. If your forms aren’t square, your “rectangle” isn’t a rectangle. Measure both diagonals. If they match, it’s square; if they don’t, your volume estimate is already off.
Methodology, disclaimers & sources
How it works
- Volume (cu ft) = length (ft) × width (ft) × thickness (ft)
- Convert to cubic yards: cu yd = cu ft / 27
- Add waste/overage if enabled, then round up for ordering
Details & assumptions
Assumes a flat, uniform slab thickness. Real slabs vary due to grade, forms, and thickened edges. This is a planning estimate — concrete suppliers may order in increments and may recommend minimums.