How to Estimate Concrete for Any Project
To estimate concrete, multiply length by width by thickness in feet, divide by 27 to get cubic yards, add 4 to 10 percent for waste, then multiply by the delivered price per yard. That's the whole method for how to estimate concrete on a slab, patio or driveway, and it's the same arithmetic our calculators run.
The five-step estimate
Cubic yards = length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27
- Measure in feet. Length, width and thickness. Thickness is usually in inches, so divide by 12: a 4 inch slab is 0.3333 ft. Round length and width up, not down.
- Find cubic feet. Length × width × thickness. A 20 by 20 ft patio at 4 inches is 20 × 20 × 0.3333 = 133.3 cubic feet.
- Convert to cubic yards. A cubic yard is 27 cubic feet, so 133.3 ÷ 27 = 4.94 cubic yards.
- Add waste. NRMCA's CIP 31, Ordering Ready Mixed Concrete, says the "quantity of concrete ordered should be 4% to 10% more than an estimate calculated from the plan dimensions." 4.94 becomes 5.14 to 5.43 yards, so order 5.5.
- Price it. Multiply by the delivered price per cubic yard ($160 in our defaults as of 2025), then add finishing labor. Small loads usually carry a short-load or minimum-load charge, so ask the supplier.
Skip the math: the concrete cost estimator does all five steps
Worked example: a 20 by 24 slab at 4 inches
| Step | Working | Result |
|---|---|---|
| Area | 20 × 24 | 480 sq ft |
| Thickness in feet | 4 ÷ 12 | 0.3333 ft |
| Volume | 480 × 0.3333 | 160 cu ft |
| Cubic yards | 160 ÷ 27 | 5.93 yd³ |
| Plus 4 to 10% waste | 5.93 × 1.04 to 5.93 × 1.10 | 6.16 to 6.52 yd³ |
| Order | half yard size inside that range | 6.5 yards |
6.5 yards is about 10 percent over the plan volume, the top of NRMCA's range. On rough, hand-graded subgrade, round up to 6.75 instead.
A load this size is small for a ready-mix truck, and small loads usually carry a short-load or minimum-load charge on top of the $160 a yard for the concrete, so ask the supplier what theirs is. That charge is a common surprise on a small-job invoice. Ordering extra to get past it just buys concrete you'll throw away, so order to the next quarter or half yard and stop.
Mistakes that wreck an estimate
Inches as feet
4 inches is 0.3333 ft, not 0.4. On a 480 sq ft slab, using 0.4 adds 32 cubic feet, more than a cubic yard.
No waste allowance
Running short mid-pour means a cold joint or a second delivery with its own fees. Both cost more than a quarter yard extra.
Uneven subgrade
A dished base holds more concrete than the flat math says. If the grade is rough, use the top of the 4 to 10 percent range.
Forgetting the extras
Thickened edges, interior beams and an apron are extra volume outside length × width × thickness. Estimate them separately and add them.
Volume as the price
A finished job also pays for forming, base, steel, finishing, jointing and cleanup. A quote well above your material number can still be fair.
Ready-mix or bags
Bag yields come from the QUIKRETE Concrete Mix No. 1101 data sheet; bags per yard is 27 divided by the yield.
80 lb bag
45
bags per cubic yard
- Yield
- 0.60 cu ft
60 lb bag
60
bags per cubic yard
- Yield
- 0.45 cu ft
40 lb bag
90
bags per cubic yard
- Yield
- 0.30 cu ft
Forty-five 80 lb bags at our default bag price come to about $360 a yard, against $160 a yard delivered, before your own mixing time. Bags make sense for small pours: a 4 by 4 ft pad at 4 inches is 5.33 cubic feet, about 9 of the 80 lb bags. Past about a yard, a truck saves mixing dozens of bags against the clock. CIP 31 puts truck mixer capacity at 8 to 12 cubic yards.
Buying Quikrete specifically? The Quikrete calculator gives the bag count for each of its mixes.
Your estimate vs a contractor's quote
Your estimate covers the concrete. A contractor's quote covers the whole job: forming, base, reinforcement, the pour, finishing, jointing, cleanup, overhead and margin. Don't compare the two directly. Ask for the quote itemized so you can check the concrete line against your own number, and ask whether it's a fixed price or an estimate that can move.
The free concrete estimate template lays those lines out in order, and covers deposits, sales tax and how long a price should hold.
Where to go next
- How much does concrete cost? Current per-yard prices.
- Concrete cost per square foot when you know the area.
- Patio cost, foundation cost and driveway cost, including concrete vs asphalt.
- Driveway resurfacing cost when deciding whether to save a slab.
- Concrete estimating software if you bid jobs for a living.
Only need the yardage? Try how much concrete do I need or the slab calculator.
All concrete estimate guides
- How Much Does Concrete Cost?What concrete costs in 2026: price per cubic yard, per square foot, delivery and short-load fees, and what drives the number. Real ranges and a free estimator.
- Concrete Cost Per Square FootWhat concrete costs per square foot in 2026 by thickness and finish, how to convert square feet to cubic yards, and a fast way to price your slab.
- Concrete Patio CostConcrete patio cost in 2026 by size and finish, from plain to stamped. See per square foot prices, what drives them, and how to price your own patio.
- Concrete Estimate TemplateA free concrete estimate template you can print or edit. Line items for material, labor, delivery, and waste, plus how to fill it in and price a job.
- Concrete Estimating SoftwareA plain guide to concrete estimating software in 2026: takeoff tools, full estimating suites, what to look for, and when a free calculator is enough.
- Concrete Foundation Cost
- Driveway Resurfacing Cost
- Stamped Concrete CostStamped concrete costs $8 to $20 per square foot installed, roughly double a plain slab. See the tiers, project totals, stamped vs pavers, and sealing costs.
Frequently asked questions
How do you estimate concrete for a job?
Measure length, width and thickness in feet, multiply them for cubic feet, and divide by 27 for cubic yards. Add 4 to 10 percent for waste, the range in NRMCA's CIP 31, then multiply by the delivered price per yard for the material cost.
How much extra concrete should I order for waste?
NRMCA's CIP 31 says to order 4 to 10 percent more than the plan dimensions give. Use the low end on a well-formed slab over a flat base and the high end on rough subgrade, which holds more concrete than the flat math suggests.
How many yards of concrete do I need for a 20 by 24 slab?
At 4 inches thick, 20 × 24 × 0.3333 is 160 cubic feet, or 5.93 cubic yards. With 4 to 10 percent waste that's 6.16 to 6.52 yards, so order about 6.5. That's a small load for a truck, and small loads usually carry a short-load or minimum-load charge, so ask the supplier.
How many 80 lb bags make a cubic yard?
45. QUIKRETE lists an 80 lb bag of Concrete Mix at about 0.60 cubic feet, and a cubic yard is 27 cubic feet. A 60 lb bag takes 60 per yard and a 40 lb bag takes 90.
Is it cheaper to mix bags or order a truck?
Price 45 of the 80 lb bags at your local store against the delivered price of a yard plus any short-load fee. Bags suit small pours, where a truck's short-load fee weighs most. Past a yard or so you're mixing dozens of bags by hand against the clock, and a truck saves that work.