How much do you need?
Free, accurate material calculators for your project — exact bag and yard counts, an estimated cost, and where to buy or get a quote. No sign-up.
Calculators
Every tool is a static page: type dimensions, get a materials list in the units you actually buy. Concrete and paint also have pre-filled common sizes so you can skip the form entirely.
Answers to the most common sizes
No need to open a calculator for these — the standard answers, at 4″ thick with a 10% waste factor for concrete and two coats on smooth walls for paint:
| Concrete slab | 80 lb bags | Ready-mix | Buy as |
|---|---|---|---|
| 10×10 slab | 62 | 1.5 yd³ | Ready-mix |
| 12×12 slab | 88 | 2 yd³ | Ready-mix |
| 16×20 slab | 196 | 4.5 yd³ | Ready-mix |
| 20×20 slab | 245 | 5.5 yd³ | Ready-mix |
| Room to paint | Wall area | Gallons (2 coats) |
|---|---|---|
| 10×12 room | 352 sq ft | 3 |
| 12×12 room | 384 sq ft | 3 |
| 12×14 room | 416 sq ft | 3 |
| 12×16 room | 448 sq ft | 3 |
For mulch and gravel the shortcut worth memorising is that one cubic yard covers 324 square feet one inch deep — so a bed of any size at any depth is just area × depth ÷ 324. A 200 ft² bed at 3″ works out at 1.85 cubic yards, or 25 bags of 2 cu ft.
Why the unit matters more than the volume
Most calculators stop at a volume. They tell you a slab needs 1.23 cubic yards and leave you in the aisle working out how many bags that is, on a phone, with a cart in the way. That conversion is where people get it wrong — and being one bag short mid-pour means a cold joint, a visible line, and a weak spot that stays there for the life of the slab.
Everything here converts to what you actually buy: whole bags, gallons, cubic yards in the quarter-yard steps ready-mix dispatches in, and tons in the half-ton steps quarries sell in. It applies a waste factor before rounding up rather than after, shows you what that factor is, and tells you when a different buying route is cheaper — bulk instead of bags, a 5-gallon pail instead of four singles, a truck instead of an afternoon of mixing.
What it costs
Ballpark numbers so you can decide whether to DIY or call a contractor. Always verify locally.
Guides
How to order, how much waste to add, and how to read a bag. Each guide links back to the calculator it belongs with.
How we work
Every constant on this site comes from a manufacturer data sheet or an extension-service publication, is independently re-derived before it ships, and is locked into an automated test suite so a published answer can’t quietly change. The formulas, the rounding rules, the waste factors and their sources are all written up in how we calculate — and you can check any answer by hand from what’s printed on the page. More about who runs this and how it’s funded and the editorial policy.