Figure out exactly how much concrete you need. Pick your shape, enter the dimensions, and get the cubic yards, cubic feet, and the number of 40, 60, or 80 lb bags — with a waste factor built in so you don't come up short mid-pour.
Over ~2 yards — order ready-mix; bagging this many is impractical. Ready-mix plants have a minimum load (often 3–4 yd) and charge a short-load fee below it, so round your order up to the next whole yard.
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Concrete is ordered by the cubic yard. To get there, calculate the volume of your pour in cubic feet and divide by 27 — because a yard is three feet, and 3 × 3 × 3 = 27 cubic feet in a cubic yard. The one rule that trips everyone up: every dimension has to be in feet before you multiply. A 4-inch slab is 4 ÷ 12 = 0.333 ft, not 4 — get that wrong and you're off by twelve times. This calculator handles the conversion for you.
Then, for any shape: cubic yards = cubic feet ÷ 27.
If you're mixing bags instead of ordering ready-mix, here's the yield — a full cubic yard is 27 cubic feet:
An 80 lb bag does not make a full cubic foot — assuming it does under-buys you by 40%. Below roughly one cubic yard, bagging is usually cheapest because ready-mix carries a load minimum (often 3–4 yards) and a short-load fee. Over about two yards, order ready-mix. See the full cure-time chart once it's down, and check whether today's conditions are safe to pour before the truck leaves the yard.
Add 10% for slabs on grade (uneven subgrade and over-excavation eat volume) and about 5% for formed work. Coming up short mid-pour forces a cold joint — a permanent weak seam — so it's always safer to over-order slightly and round bag counts up. The calculator adds your waste factor before it gives you the number.
Multiply length × width × thickness with every dimension in feet to get cubic feet, then divide by 27 for cubic yards. A 4-inch slab is 0.333 ft, not 4 — the #1 mistake. Add ~10% waste for slabs on grade.
An 80 lb bag yields about 0.60 ft³, so it takes 45 bags per cubic yard (27 ÷ 0.60 = 45). A 60 lb bag = 60 per yard; a 40 lb bag = 90. (An 80 lb bag does not make a full cubic foot — a common under-buying error.)
A 10 × 10 ft slab at 4 inches is 33.3 ft³ ≈ 1.23 yd³. With 10% waste, order ~1.36 yd³ — round up to the next whole yard for a ready-mix order.
Add 10% for slabs on grade and about 5% for formed work (walls, columns, footings). Running short causes a cold joint, so over-order slightly and round bag counts up.
Under ~1 yard, bags usually win (ready-mix has a 3–4 yard minimum plus a short-load fee). 1–2 yards is a toss-up. Over ~2 yards, ready-mix wins and bagging is impractical.
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