Concrete Calculator: Slabs

How much concrete, and how many bags.

Pick a slab, a footing, a column or a flight of stairs, type the dimensions, and read the answer in cubic yards, cubic metres and cubic feet at once — with the bag count for 40, 60 and 80 lb bags beside it.

EVERY BOX SAYS ITS OWN UNIT
A slab is twelve feet long and four inches deep at the same moment, and no app should make you convert one into the other. Each box is labelled with the unit it wants. Type 12' 6", 12 6, 12.5 or 4 1/2 — all of them mean the same thing here, and a comma decimal works as well as a point.

WASTE, ADDED IN THE RIGHT ORDER
Spillage, an uneven subgrade and over-excavation are real, so the allowance goes on the volume before the bags are counted, never after. Bags always round up, because merchants sell whole ones.

STAIRS DONE PROPERLY
A flight poured solid is a staircase prism, not a box and not half of one: each step sits on the concrete of every step below it. Getting that wrong orders too little concrete, which is the expensive direction.

A COLUMN IS MEASURED ACROSS
An 8" tube is eight inches across, not eight inches of radius. Reading it the other way orders four times what the job needs.

WHEN A BAG COUNT STOPS BEING THE ANSWER
Past about a cubic yard the app says so. That is forty-odd bags and a ton and a half to carry; ready-mix is cheaper and very much faster, and an app that answers the arithmetic without saying that has not answered the question.

JOBS — THE PAID UNLOCK
A real job is never one pour. Save a job, add the slab, the forty feet of footing, the six piers and the landing, and read one total with each pour's own waste in it. Cost it at your delivered price per cubic yard. Get the rebar grid — counting the closing bar, which is the one everyone drops — and the mesh sheets, allowing for the overlap that makes a 5 × 7 sheet cover 4.5 × 6.5. And if you are mixing it yourself, the cement, sand, stone and water for a 1:2:3, 1:2:4 or 1:3:5 mix, batched to the dry volume rather than the wet one.

OFFLINE, AND IT STAYS THAT WAY
No account, no sign-in, no adverts, and not one network request. Saved jobs live on your phone and nowhere else. Nothing about you is collected, which is why the privacy label says so.

One payment for the unlock, yours forever. The calculator itself is free and always will be.