Concrete & Masonry
Concrete Slab Calculator
Enter your slab dimensions in feet and inches to get the cubic yards of ready-mix to order, with a wastage allowance and an optional cost estimate.
Formula tested · Local units · No sign-up
Project inputs
Enter measurements
Use your preferred units. Results update automatically.
Show the calculation methodFormula, conversions, rounding, and assumptions+
Volume = length × width × thickness. Because thickness is usually measured in inches while length and width are in feet, the calculator converts everything to a common unit before multiplying — a 12 ft × 10 ft slab at 4 inches is 12 × 10 × 0.333 = 40 cubic feet, not 480.
Ready-mix in the US is sold by the cubic yard (27 cubic feet), so the result is divided by 27 and then rounded up to the next quarter yard, which is how most plants take orders.
Real-world example
Worked example: 12 ft × 10 ft patio slab, 4 in thick
- Convert thickness: 4 in ÷ 12 = 0.333 ft.
- Volume: 12 × 10 × 0.333 = 40 cubic feet.
- Convert to yards: 40 ÷ 27 = 1.48 cubic yards.
- Add 10% wastage: 1.48 × 1.10 = 1.63 cubic yards.
- Round up to the ordering increment: 1.75 cubic yards.
Order 1.75 cubic yards. At an example price of $150 per yard with 8% sales tax, that's $262.50 + $21.00 = $283.50.
Before you start
How to measure
- Measure the formed length and width of the slab at the top of the forms, in feet.
- Measure thickness in inches at several points if the sub-base is uneven, and use the largest value — a slab that averages thicker than planned is the most common reason pours come up short.
- For L-shaped slabs, split the shape into rectangles, calculate each one, and use the 'number of slabs' field or add the volumes.
Local guidance
Notes for United States
- US ready-mix is ordered in cubic yards; most plants have a minimum load (often 1 yard) and charge a short-load fee for small orders — ask when you call.
- A nominal 4-inch slab is common for patios and walkways; driveways are often 5–6 inches. Local code and your soil conditions govern — check with your building department.
- Sales tax varies by state and locality and is entered manually; supplier quotes usually exclude it.
Quick reference
Slab thickness quick reference (typical nominal values)
| Application | Common nominal thickness |
|---|---|
| Walkway / patio | 4 in |
| Garage floor | 4–6 in |
| Residential driveway | 5–6 in |
| Shed base | 4 in |
Nominal values for planning only — soil, loads and local code determine the real requirement.
Good to know
Common mistakes to avoid
- Multiplying feet by inches without converting — this overstates volume by 12×.
- Ordering the exact calculated volume with no wastage: uneven sub-base and spillage almost always consume 5–10% extra.
- Forgetting that thickened edges and footings add volume beyond the flat slab.
- Comparing supplier quotes without checking whether delivery and short-load fees are included.
Need help?
Frequently asked questions
How many cubic yards do I need for a 10×10 slab, 4 inches thick?
10 × 10 × (4/12) = 33.3 cubic feet = 1.23 cubic yards exactly. With 10% wastage that's 1.36, so most people order 1.5 cubic yards.
Should I order concrete by the bag or ready-mix truck?
As a rule of thumb, above roughly one cubic yard the mixing labour of bagged concrete becomes significant and ready-mix is usually more practical. An 80 lb bag yields about 0.6 cubic feet, so one cubic yard is roughly 45 bags.
Does this calculator include rebar or mesh?
No. It estimates concrete volume only. Reinforcement depends on the slab's use and local code — see the rebar calculator (coming soon) or ask your supplier.
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About this calculator
- Written by:
- BuildMeasure Editorial Team
- Technically reviewed by:
- Pending independent technical reviewer (formula unit-tested; see methodology)
- Last reviewed:
- 2026-07-16
- Formula version:
- 1.0.0
- Region reviewed for:
- United States
- Spotted an error?
- Report a correction
Methodology
- Volume is computed as length × width × thickness × number of slabs, converted internally to SI units (metres) before any arithmetic to avoid unit drift.
- The wastage allowance is applied to the exact volume, then the total is rounded UP to the next 0.25 of the regional ordering unit, because ready-mix suppliers typically sell in quarter-unit increments.
- The cost estimate simply multiplies the suggested order quantity by the price you enter, then applies the tax rate you enter. No prices are built in.
- The formula is covered by automated unit tests, including hand-calculated worked examples, and is versioned (see formula version on this page).
Sources & standards
- Unit definitions: 1 ft = 0.3048 m and 1 yd³ = 0.764554857984 m³ (exact international definitions).
- Ordering increments: Quarter-yard ordering is standard US ready-mix practice; confirm with your plant.
This tool provides a material estimate for planning purposes only. It is not a quotation, and it does not size reinforcement, check ground conditions, or replace professional structural advice. Confirm quantities and mix specification with your supplier before ordering.