Outdoor Build
Concrete Calculator
Calculate concrete for slabs, footings, and post holes in cubic yards and 60 or 80 lb bag counts.
Your answer
Over a cubic yard — get a ready-mix quote. Mixing this by hand is a very long day.
The concrete calculation, in plain terms
Concrete is bought by volume: length times width times thickness, with every dimension converted to the same unit. The trap is thickness, which is usually quoted in inches or centimetres while the other two are in feet or metres. A four-inch slab is a third of a foot, not four.
Post holes are a subtraction problem instead. The concrete fills the hole minus the volume the post itself occupies, so a wide post in a narrow hole needs surprisingly little mix.
Bags or ready-mix?
| Volume | Best option | Why |
|---|---|---|
| Under 0.25 cu yd | Bagged mix, hand mixed | A few bags in a barrow |
| 0.25-1 cu yd | Bagged mix with a hired mixer | Ready-mix minimums make delivery poor value |
| 1-3 cu yd | Ready-mix, borderline | Compare the short-load surcharge against 45 bags per yard |
| Over 3 cu yd | Ready-mix delivery | Hand mixing at this scale risks cold joints |
Thickness and base preparation
- Patio or shed base: 100 mm (4 in) over 100 mm of compacted hardcore.
- Driveway with car loading: 150 mm (6 in) over 150 mm of compacted sub-base, with mesh.
- Strip footing: depth to suit the ground, always below the frost line.
- Post holes: one third of the above-ground height, three times the post width across.
Curing decides the strength you actually get
Concrete hardens by chemical hydration, not by drying. Water lost to sun and wind in the first days stops that reaction at the surface — exactly where wear happens. Cover the pour with plastic sheeting or wet hessian and keep it damp for seven days. It is free, and it can be the difference between a slab that lasts and one that dusts.
Common mistakes
- Measuring thickness in inches and multiplying as if it were feet.
- Ordering exactly the calculated volume with no allowance for an uneven subgrade.
- Pouring in direct summer sun without shading or curing cover.
- Adding extra water to make the mix easier to place, which permanently weakens it.
- Setting posts without a gravel base, so water sits at the post bottom and rots it.
How many 80 lb bags make a cubic yard?
About 45 bags of 80 lb mix, or 60 bags of 60 lb mix. Above roughly one cubic yard, ready-mix delivery is usually cheaper and far less work.
How much extra concrete should I allow?
Five to ten per cent. Subgrade is never perfectly level and formwork bulges slightly, so a pour measured exactly always comes up short.
How deep should a post hole be?
One third of the above-ground post height, or below the frost line if that is deeper — whichever is greater. Diameter should be about three times the post width.
When can I walk on a new slab?
Usually after 24 to 48 hours. Full design strength takes 28 days, and keeping it damp for the first seven days matters more than anything else you do.
Last reviewed February 2026