How to Estimate Concrete

Concrete is the least forgiving material to estimate. You cannot go back for more halfway through a pour, offcuts are not a thing, and the units change depending on who you are buying from.

Three tools cover the pour itself. If you are laying a patio or building a wall on top of it, that is a different guide.

First: bags or a truck?

This is the decision everything else follows from, and it is a physical question rather than a financial one.

The Concrete Bag Calculator works out bags for a small pour, and it also tells you where bags stop being sensible — around 45 of them, which is roughly two tons of lifting and mixing against a setting clock. Past that point you want ready-mix delivered, and the arithmetic changes to cubic yards.

Work this out before anything else. A job you have mentally filed as "a few bags" and which is actually a truck is a different day entirely.

Slabs go in cubic yards

The Concrete Slab Calculator gives cubic yards from length, width and thickness, with a waste allowance built in so you are not short.

Take the waste allowance seriously here. Subgrade is never perfectly flat, and the low spots are exactly where the extra goes. Coming up short on a slab is not an inconvenience — it is a cold joint through the middle of your finished surface.

Posts: subtract the post

The Post Hole Concrete Calculator does one thing the obvious method does not: it subtracts the volume of the post itself from the hole.

Forget that and you over-order by about a seventh. On one post that is nothing. On a fence line it is real money and a lot of unnecessary lifting. This tool sits in both this group and the fencing group for that reason — it is the overlap between the two jobs.

The short version

1. Bags or truck first — around 45 bags is the line.
2. Slabs in cubic yards, and respect the waste allowance.
3. Subtract the post from the post hole.
4. Confirm the unit your supplier quotes in before you order anything.

Every one of these has a printable with its own assumptions on it — worth having in your pocket at the merchant's counter, where the units question comes up.