Concrete Calculator
Estimate the concrete you need for a slab or rectangular pour — in bags or ready mix.
How to measure thickness
Measure from the compacted base to the finished surface. Standard patio and walkway slabs are 4 inches; driveways are typically 5–6 inches.
How we calculated it
We compute your slab’s volume — length × width × thickness in feet — and add a waste allowance, because running short mid-pour is the one concrete mistake you can’t fix. Buying bags? We divide by your bag’s yield. Ready mix? We convert to cubic yards, the unit trucks are ordered in.
Things that can change your result
- Uneven ground eats concrete — a slab that averages 4.5″ instead of 4″ needs 12% more.
- Bag yields differ: an 80 lb bag makes about 0.60 cubic feet, a 60 lb bag about 0.45.
- Ready-mix trucks have minimum orders — small pours often pay a short-load fee.
Buying tips
- Above roughly a cubic yard (about 45 eighty-pound bags), ready mix is usually less work and less risk.
- Never plan a pour to come out exactly even — the 10% allowance is your insurance.
- Have help and tools staged before the truck arrives; concrete doesn’t wait.
Common mistakes
- Rounding down on bags to save a few dollars, then running short with half the slab poured.
- Ignoring forms and sub-base in the schedule — the pour is the easy part.
- Ordering ready mix without confirming truck access to the site.
Frequently asked questions
How many bags of concrete do I need for a 10×10 slab?
At 4 inches thick with a 10% allowance, a 10×10 slab is about 36.7 cubic feet — around 62 eighty-pound bags. At that size, price out ready mix too.
How much does a yard of concrete cover?
One cubic yard covers 81 square feet at 4 inches thick — a 9×9 slab.
Bags or ready mix?
Under half a yard, bags. Over a full yard, ready mix. In between, weigh your mixer access, helpers, and the short-load fee.