Unkillable Tools / Calculator / Concrete slab calculator
Volume, bag count and delivered cost for a slab, with the waste allowance that stops you ordering short.
Change the thickness — driveways need more than patios.
Change the rates, the labels and the maths to match your business, then export it as a single HTML file you paste into your site. Free, no account, and it keeps working whatever happens to us.
Open the builderVolume in cubic feet is length × width × thickness in feet, so a 4-inch slab uses 4/12. Divide by 27 for cubic yards, which is how ready-mix is sold.
Always add a waste allowance. Ten percent is the common default and covers spillage, uneven subgrade and the fact that forms are never perfect. Running short mid-pour is far more expensive than over-ordering slightly, because a cold joint in the middle of a slab is a permanent defect.
Ready-mix is sold in whole yards, so the cost line rounds up. A calculator that quotes 3.26 yards and prices 3.26 yards is quietly wrong about what you'll be invoiced.
An 80 lb bag yields about 0.6 cubic feet, so a cubic yard takes roughly 45 bags. The bag count is included mainly so people can see the crossover point for themselves.
Past about half a cubic yard, bags stop making sense on labour alone — that's around 22 bags to mix by hand. Showing both numbers side by side makes the decision obvious without you having to argue it.
It's a rectangular slab calculator. For footings, run it once per section and add the results, or build a custom version with the fields you need.
Build a version with metres and cubic metres — change the labels and drop the /27 conversion. The engine is unit-agnostic.
Yes, that's what the builder is for. Free, and the export is a single HTML file.
Hosted calculator builders charge $5–$115 a month and stop rendering when you exceed your monthly impressions. This one exports a file that runs on your own site and cannot be switched off.
Build one free