How Many Bags of Concrete Do You Actually Need?

The math behind buying exactly the right amount, explained simply.

Start with the volume, not the area

Concrete is measured in cubic feet or cubic yards, so you need length x width x depth, not just square footage — depth is the number people forget most often.

Know your bag coverage

A standard 60 lb bag covers about 0.45 cubic feet when mixed; an 80 lb bag covers about 0.6 cubic feet — always check the specific bag you're buying since formulas vary slightly by brand.

Always round up, not down

Running short mid-pour is a bigger problem than having a partial bag left over — most experienced DIYers buy 10% extra to cover measurement error and spillage.

Depth matters more than people expect

A stepping stone might only need 2 inches of depth, while a load-bearing slab needs 4 inches or more — using our calculator's project type selector accounts for this automatically.

When the number feels too big for hand-mixing

If your calculation comes out to more than roughly 15-20 bags, renting a small mixer is worth the cost in saved time and effort.