Concrete Calculator
It works out the volume of concrete a slab, footing or pad needs, in cubic feet, yards and meters, and converts that into a count of pre-mixed bags.
Concrete needed
1.36 yd³
Cubic yards
1.36
Cubic feet
36.7
Cubic meters
1.04
40 lb bags
123
50 lb bags
98
60 lb bags
82
80 lb bags
62
At over a cubic yard, ready-mix delivery is normally cheaper and far less work than mixing 62 bags by hand.
What it calculates
It works out the volume of concrete a slab, footing or pad needs, in cubic feet, yards and meters, and converts that into a count of pre-mixed bags.
Why it matters
Concrete is ordered by the cubic yard and cannot be topped up once a pour begins. Under-ordering ruins a slab; over-ordering wastes money.
Who it's for
DIY builders pouring a patio or shed base, contractors ordering ready-mix, and anyone estimating a footing.
Formula
- V
- Volume = length × width × thickness
- yd³
- Cubic yards — 27 cubic feet
- w
- Waste allowance for spillage and uneven subgrade
Worked example
A 10 ft × 10 ft slab, 4 inches thick
- 1Convert thickness: 4 in ÷ 12 = 0.333 ft
- 2Volume = 10 × 10 × 0.333 = 33.33 cu ft
- 3Cubic yards = 33.33 ÷ 27
1.23 cubic yards, or 56 bags of 80 lb mix
How the concrete calculator works
Volume is length times width times thickness, with the thickness converted from inches to feet first — that division by twelve is where most errors happen. Dividing cubic feet by 27 gives cubic yards, the unit ready-mix is sold in. Bag counts come from the yield printed on the bag: an 80 lb bag makes about 0.6 cubic feet, a 60 lb bag about 0.45.
Volume is length times width times thickness, with thickness converted from inches to feet first. That division by twelve is where most estimating errors happen — forget it and you will over-order by a factor of twelve.
Bag counts come from the yield printed on the bag: an 80 lb bag makes about 0.6 cubic feet and a 60 lb bag about 0.45. Above roughly one cubic yard, ready-mix delivery usually beats bags on both cost and effort.
Common mistakes
- Forgetting to convert thickness from inches to feet, which overstates volume twelvefold.
- Ordering no waste allowance — subgrade is never perfectly level, and 5–10% extra is standard.
- Ignoring that a bagged pour of more than about a cubic yard is far cheaper as ready-mix.
Tips and best practice
- Above roughly one cubic yard, ready-mix delivery usually beats bags on both cost and effort.
- Add 10% on an uneven or unformed subgrade; the extra concrete finds every low spot.
Frequently asked questions
How much concrete do I need for a 10x10 slab?
At 4 inches thick, 33.3 cubic feet or 1.23 cubic yards — about 56 bags of 80 lb mix, before any waste allowance.
How many cubic feet are in a cubic yard?
Twenty-seven, because a cubic yard is 3 ft × 3 ft × 3 ft. That factor of 27 is why a slab measured in feet has to be divided down before ordering ready-mix.
How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb mix, 60 of 60 lb, or 90 of 40 lb, based on their standard yields.
How much waste should I allow?
5% on a level, well-formed subgrade and up to 10% on rough ground. Running short mid-pour creates a cold joint, which is far worse than a little waste.
What thickness does a slab need?
4 inches suits patios and shed bases; driveways and anything carrying vehicles usually need 5–6 inches. Check local building codes.
Related calculators
Methodology & trust
- Formula source
- Standard volume calculation with published bag yields.
- Last updated
- 2026-07-28
- Privacy
- Every calculation runs in your browser. No inputs are sent to a server or stored.
- Accessibility
- Keyboard navigable, labeled inputs and WCAG AA color contrast.