Concrete Calculator
Figure out exactly how much concrete you need for a slab, footing, column, or set of steps. Enter your dimensions and get the volume in cubic yards plus the number of 40, 60, or 80 lb bags — with a cost estimate.
How to calculate concrete
Concrete volume is Length × Width × Thickness. Convert the result to cubic yards by dividing cubic feet by 27 (ready-mix is sold by the cubic yard). For bags, divide the cubic feet by each bag's yield.
Concrete bag yields
| Bag size | Yield (cu ft) | Bags per cu yd |
|---|---|---|
| 40 lb | 0.30 | ~90 |
| 60 lb | 0.45 | ~60 |
| 80 lb | 0.60 | ~45 |
Why order a little extra?
Concrete estimates rarely land exactly on the jobsite number. Uneven subgrade, small over-digs, form gaps, spillage, and cleanup waste can all use part of your calculated volume before the pour is finished.
For planning, this calculator includes a 10% waste allowance in the result summary. If you are ordering bags, round up to the next whole bag. If you are ordering ready-mix, confirm minimum order rules and short-load fees with your supplier.
Bags vs. ready-mix
Small patch jobs and short forms are often practical with bags mixed on site. Once the result climbs toward a cubic yard, the bag count gets heavy fast: one cubic yard is roughly 45 bags of 80 lb mix, 60 bags of 60 lb mix, or 90 bags of 40 lb mix.
Use the cubic-yard number and the bag counts together. The cubic-yard number helps when comparing ready-mix delivery, while the bag count helps when planning pickup, lifting, mixing time, and extra bags.
Related calculators
Concrete slab calculator · Concrete footing calculator · Gravel base calculator
Common slab sizes
| Slab | Thickness | Concrete |
|---|---|---|
| 10 × 10 ft | 4 in | ~1.23 cu yd |
| 12 × 12 ft | 4 in | ~1.78 cu yd |
| 20 × 20 ft | 4 in | ~4.94 cu yd |
| 24 × 24 ft (garage) | 4 in | ~7.11 cu yd |
FAQ
How many 80 lb bags of concrete are in a cubic yard?
About 45 bags, since each 80 lb bag yields roughly 0.60 cubic feet and a cubic yard is 27 cubic feet.
How much concrete for a 10×10 slab at 4 inches?
About 1.23 cubic yards (33.3 cubic feet). Add 5–10% for waste.
Ready-mix or bags?
Over roughly 1 cubic yard, ready-mix delivery is usually cheaper and far less work than mixing bags by hand.