Free tool
Concrete Calculator
Enter your slab dimensions and get exactly how much concrete to order — cubic yards, bag counts, and ready-mix cost, with a waste factor built in. Free, no signup for the estimate.
Concrete needed
0 yd³
- Volume0 ft³
- With waste0 ft³
- Cubic yards0 yd³
- 80 lb bags0
- 60 lb bags0
- Ready-mix cost—
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Or download it now (PDF)How to calculate concrete for a pour
Ordering concrete is one of the few jobsite numbers you can’t fix after the fact. Come up short mid-pour and you get a cold joint; over-order by a lot and you’ve paid for concrete that goes in the spoil pile. Getting it right is just careful measuring and one unit trick.
1. Measure length, width, and thickness
Length and width are in feet; thickness is almost always in inches. A standard slab is 4″, a driveway or garage floor is often 5–6″, and footings are deeper. Keep thickness in inches for now — the next step converts it.
2. Convert to cubic yards
Concrete is sold by the cubic yard. Divide your thickness by 12 to get feet, multiply length × width × thickness-in-feet for cubic feet, then divide by 27 (there are 27 cubic feet in a cubic yard). For an odd-shaped pour, break it into rectangles, calculate each, and add them together.
3. Add a waste factor
Real subgrade is never perfectly flat, forms flex, and some concrete is always lost to spillage and set-up. Add 5–10% so a slightly deep spot doesn’t leave you scrambling for a second batch. On hand-dug footings where depth varies, lean toward the higher end.
4. Bags or ready-mix?
Under about a cubic yard, bagged concrete is usually cheaper and practical. Above that, the bag count climbs fast — roughly 45 eighty-pound bags per yard — and a ready-mix truck saves real money and a lot of mixing. The breakdown above shows bags and yards side by side so you can make the call.
Measuring the job is step one. Turning it into a priced, professional quote is faster inside CrewRivet. See the paint calculator, the job pricing calculator, or concrete estimating software.
Concrete calculator FAQ
- How much concrete do I need for a slab?
- Multiply length × width × thickness to get the volume, then convert to cubic yards. The trick is units: thickness is in inches, so divide it by 12 to get feet first. Length (ft) × width (ft) × (thickness ÷ 12) gives cubic feet; divide by 27 for cubic yards. Add 5–10% for waste. This calculator does all of it for you.
- How many 80 lb bags of concrete are in a cubic yard?
- An 80 lb bag yields about 0.60 cubic feet, and a cubic yard is 27 cubic feet — so roughly 45 bags of 80 lb per cubic yard. A 60 lb bag yields about 0.45 cubic feet, or about 60 bags per yard. Bags add up fast, which is why most crews switch to ready-mix on bigger pours.
- Should I order ready-mix or use bags?
- As a rule of thumb, once you pass about one cubic yard, ready-mix delivered by truck is usually cheaper and far less labor than mixing dozens of bags by hand. Below a yard — footings, post holes, a small pad — bagged concrete is typically the call. The calculator shows both so you can compare.
- Why add a waste factor?
- Real pours never match the math exactly: subgrade is uneven, forms bow, some concrete spills or sets before it is placed, and running short mid-pour means a cold joint. A 5–10% waste allowance covers it. Running a little over is cheap insurance; running out is not.
- Is the calculator free?
- Yes — the live estimate is free with no email required. Enter your email only if you want the free Job Pricing Kit (pricing worksheet, markup cheat-sheet, and invoice + quote templates) sent to your inbox.