Concrete & Masonry calculators
Slabs, footings, post holes and steps — anything you pour or set.
- Concrete Post Hole CalculatorBags per hole and for the whole fence
- Concrete Footing CalculatorConcrete, bags and rebar for a strip footing
- Brick CalculatorBricks, mortar bags and ties
- Concrete Slab CalculatorYards or bags, plus mesh and rebar
- Concrete Block CalculatorBlocks, mortar and core fill
- Concrete Steps CalculatorConcrete for a stoop or flight of steps
What is in this category
Four calculators for the things you mix, pour and cannot undo. A slab for a patio or shed base, a strip footing under a wall, a post hole for a fence or deck post, and a flight of steps.
Concrete is the one material where running short mid-pour genuinely ruins the job. Stop halfway and the joint between the first batch and the second is a permanent weakness called a cold joint. Everything below is aimed at making sure that does not happen.
Everything here is volume, not area
Concrete is sold by the cubic yard from a truck and by the cubic foot in a bag, and the two are far apart: a cubic yard is 27 cubic feet. That is forty-five 80 lb bags — over a ton of material to lift, open and mix before the first batch starts going off.
So the first number to work out is always the volume, and the second is which way to buy it:
| Bag | Yield | Per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 bags |
| 60 lb | 0.45 cu ft | 60 bags |
| 50 lb | 0.375 cu ft | 72 bags |
| 40 lb | 0.30 cu ft | 90 bags |
Around one cubic yard is where ready-mix usually wins on both price and effort.
What catches people out with concrete
The hole is never the size you dug it. Trench walls slump, the bottom is uneven, and concrete finds every low spot. Five to ten per cent over the calculated volume is realistic in firm ground, and fifteen in anything loose.
Frost depth is local and not negotiable. Anything structural has to bear below the frost line, which ranges from a few inches in the south to over four feet in the north. Your building department publishes the figure. It is not a number to estimate.
Reinforcement has to sit in the concrete, not under it. Rebar laid on the dirt and poured over does nothing — it needs concrete all around it, which means sitting it up on chairs before the pour.
Exterior concrete wants air entrainment. In any climate that freezes, the tiny air bubbles give water somewhere to expand into. Without them the surface flakes apart within a few winters. Bagged mixes generally are not air-entrained; ready-mix can be ordered that way.