How to Calculate Concrete for a Slab, Step by Step
Length × width × thickness, ÷ 27, plus waste. Four steps, two unit conversions, and one worked example you can follow with a pencil.
Working out concrete is genuinely simple arithmetic — the errors all come from units. Length and width arrive in feet, thickness in inches, and the order goes in in cubic yards, so there are two conversions hiding inside one small formula. Miss either and the answer is out by a factor of 12 or 27, which is usually discovered when the truck arrives.
This guide does the calculation by hand so you can see every step. When you want it done instantly — and checked — the concrete calculator runs the same arithmetic with the formula printed beside the result.
The formula
For any rectangular slab: volume in cubic feet equals length (ft) × width (ft) × thickness (in) ÷ 12. Divide by 27 to get cubic yards — a cubic yard is a 3 × 3 × 3 ft cube, 27 cubic feet. Then add a waste allowance, because geometry assumes a perfectly flat subgrade and real ground is not.
That is the entire method. Two conversions to respect: inches to feet (÷ 12) for the thickness, and cubic feet to cubic yards (÷ 27) at the end.
The four steps
| Step | Operation |
|---|---|
| 1. Thickness to feet | inches ÷ 12 |
| 2. Volume | length × width × thickness, all in feet |
| 3. To cubic yards | cubic feet ÷ 27 |
| 4. Order quantity | cubic yards × (1 + waste %) |
A full worked example: 24 × 24 ft at 4 inches
The most common residential pour — a two-car garage slab.
Step 1: thickness in feet — 4 ÷ 12 = 0.333 ft. Step 2: volume — 24 × 24 × 0.333 = 192 cubic feet. Step 3: cubic yards — 192 ÷ 27 = 7.11 yd³. Step 4: with 10% waste — 7.11 × 1.10 = 7.82 yd³.
Plants sell in whole and half yards, so the order is 8 cubic yards. The extra 0.18 yd³ is cheap insurance: running short mid-pour means a cold joint — a permanent weakness — or an emergency second delivery at full price plus a short-load fee.
Slabs that are not rectangles
Split the shape into rectangles, calculate each, and add the volumes — exactly as you would for square footage. An L-shaped patio is two rectangles poured as one. A slab with a thickened perimeter edge is a flat slab plus a footing: work the edge out separately as a strip (length × width × depth, all converted to feet) and add it, rather than hoping the waste allowance absorbs it. A 4 inch slab with a 12 inch thickened edge holds noticeably more concrete than the flat arithmetic suggests.
For circular pads and piers, the cubic yard calculator handles the general volume case, and the concrete calculator has dedicated footing and column modes with the radius arithmetic built in.
Bags or ready-mix
Once you have the volume, the decision is how to buy it. The crossover is around one cubic yard.
An 80 lb bag yields about 0.6 cubic feet, so a cubic yard takes 45 of them — roughly 3,600 lb of material to move and mix. At $5–$8 per bag that is $225–$360 per yard, against $140–$180 delivered for ready-mix. Bags still win below a yard because they avoid the delivery and short-load fees entirely, and you can pour at your own pace.
Bags per cubic yard
| Bag size | Yield per bag | Bags per yd³ |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 50 lb | 0.375 ft³ | 72 |
How much waste to allow
Waste covers spillage, minor over-excavation and the subgrade never being perfectly flat — a dip of half an inch across a 24 ft slab is nearly a quarter of a yard on its own. It is not a substitute for measuring properly: check the dimensions first, then add the allowance.
Waste allowance by condition
| Condition | Allowance |
|---|---|
| Flat, well-prepared, screeded subgrade | 5% |
| Typical residential slab | 10% |
| Uneven subgrade or irregular shape | 12–15% |
| Hand-dug footing trenches | 15% |
The ordering checklist
When you call the plant, have five things ready: the volume in cubic yards (rounded up to how they sell — whole or half yards), the strength (3,000–4,000 psi is standard residential), the pour date and access details, whether you need any admixtures (accelerator in cold weather, retarder in heat), and a question about the short-load fee if you are under 3–4 yards.
Confirm how long the truck stays before standby charges start — typically there is a set unload window. Have the forms, reinforcement and enough hands ready before it arrives; concrete does not wait, and everything about a pour gets worse when the material sets faster than the crew can place it.
One thing not to do: pour a single slab in stages across separate days. Concrete bonds poorly to cured concrete without joint preparation, and casual staged pours produce cold joints exactly where the slab will crack. Size the order so the slab goes down in one continuous pour.
Run the numbers for your project
Frequently asked questions
How much concrete do I need for a 10 × 10 slab at 4 inches?
10 × 10 × 0.333 = 33.3 cubic feet, which is 1.23 cubic yards; with 10% waste, 1.36 yd³ — order 1.5. In 80 lb bags that is about 61 bags, right at the boundary where ready-mix starts to win despite the short-load fee.
How many bags of concrete make a cubic yard?
45 × 80 lb bags, 60 × 60 lb bags, or 72 × 50 lb bags — all about 3,600 lb of material. The counts come from manufacturer yields: 0.6, 0.45 and 0.375 cubic feet per bag respectively.
What strength of concrete does a slab need?
3,000–4,000 psi covers standard residential slabs — patios, garage floors, driveways. Exposure to freeze-thaw and de-icers argues for the 4,000 end with air entrainment. Anything structural should be specified by an engineer, not a rule of thumb.
Can I pour a slab in two stages?
Not casually. Fresh concrete does not bond properly to cured concrete, so a staged pour creates a cold joint — a crack you scheduled. If a slab must be poured in sections, it needs planned construction joints with dowels or keyways, which is a design decision, not an ordering convenience.
Where these figures come from
Costs in this guide are 2026 US planning ranges, consistent with the figures published on the concrete calculators and their named sources. The constants behind every calculation are listed on the methodology page and asserted by automated tests on every build; corrections are logged on the changelog.
Prices vary with region, access and season. Use this guide to plan and to sanity-check quotes — not as a substitute for one.