Ordering concrete is one of the few parts of a project where a small arithmetic slip costs real money. Order too little and you are mixing bags in a panic while the first part of the pour starts to set. Order too much and you pay for concrete you have to get rid of. The math itself is simple; the mistakes come from units and from measuring the wrong thing. This guide walks through it step by step.
Step 1: Measure the space the concrete will fill
You need three measurements: length, width and depth (for a slab, the depth is its thickness). Measure theinside of your forms, because that is the space concrete actually fills.
- For uneven ground, take depth readings in several spots and use the average, or the deepest reading if you want a safe estimate.
- For an L-shape or other combined shape, split it into rectangles and calculate each one separately.
- Write down the unit of every measurement. Mixing feet and inches is the single most common source of error.
Step 2: Convert everything to feet
Concrete in the US is sold by the cubic yard (ready-mix) or by the bag, and bag yields are printed in cubic feet. Working in feet keeps things simple. To convert inches to feet, divide by 12.
| Thickness | In feet |
|---|---|
| 3 in | 0.25 ft |
| 4 in | 0.333 ft |
| 5 in | 0.417 ft |
| 6 in | 0.5 ft |
| 8 in | 0.667 ft |
Step 3: Calculate the volume
Multiply the three dimensions to get cubic feet, then divide by 27 to get cubic yards.
cubic feet = length (ft) × width (ft) × depth (ft) cubic yards = cubic feet ÷ 27
Why 27? A cubic yard is a cube that measures one yard (3 ft) on every side, and 3 × 3 × 3 = 27 cubic feet. In metric, multiply meters by meters by meters and you have cubic meters directly. One cubic yard is about 0.765 m³.
Worked example: a shed base
A base for a shed measures 12 ft by 8 ft and will be 4 in thick.
- Thickness: 4 ÷ 12 = 0.333 ft
- Volume: 12 × 8 × 0.333 = 32 cu ft
- Cubic yards: 32 ÷ 27 = 1.19 cu yd
Step 4: Add a margin for waste
Real pours use more concrete than the geometry says. Subgrade has dips, forms bulge a little under the weight, and some material is lost in the mixer, wheelbarrow or chute. A margin of 5–10% is common practice; the ready-mix industry group NRMCA suggests 4–10% (CIP 8). Lean toward the higher end when the ground is uneven, you are mixing by hand, or running short would be a serious problem.
For the shed base, 10% extra gives 32 × 1.10 = 35.2 cu ft, or 1.30 cu yd.
Step 5: Decide between bags and ready-mix
Bagged concrete
Premixed bags list their approximate yield on the label. For standard concrete mix, the yields commonly printed on US bags are about 0.30 cu ft for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb and 0.60 for 80 lb. These figures match the manufacturer'sdata sheet for QUIKRETE® Concrete Mix (No. 1101). Different products yield different amounts, so always check the bag you are buying.
bags = total cubic feet ÷ yield per bag, rounded up
For the shed base: 35.2 ÷ 0.60 = 58.7, so 59 bags of 80 lb. That is more than two tons of material to lift, mix and place, which is worth keeping in mind before you commit to bags.
Ready-mix delivery
Ready-mix comes in a truck, already mixed, and is priced per cubic yard. It is usually the better choice for larger pours because the whole slab can be placed at once. Ask whether your supplier has a minimum order or charges extra for small loads when you request a quote. When you call, give the supplier your volume including the waste margin and ask how they round orders.
Rule of thumb, not a rule: there is no single cutoff where ready-mix becomes cheaper than bags. It depends on local prices, delivery fees, access for the truck and how much labor you have. Price both.
Common mistakes to avoid
- Using inches as feet. 12 × 8 × 4 is 384, which would be a 12-yard order instead of a 1.2-yard one.
- Measuring outside the forms. Form lumber can add a few inches to each side.
- Ignoring extra depth at thickened edges, footings or post holes. Calculate them separately and add them in.
- Going by bag weight instead of yield. Two products with the same weight can yield different volumes.
What this guide does not cover
Volume is only part of planning a pour. How thick a slab must be, what strength of concrete to use, whether you need reinforcement, and what kind of base goes underneath depend on the job, your soil, your climate and local code. For anything structural, check with your local building department or a qualified professional.
Fresh concrete is also caustic and can burn skin, so wear gloves, long sleeves and eye protection, and follow the safety information on the product.
Skip the arithmetic
The Concrete Calculator does all of the steps above, including bag counts for each common size, and the Concrete Slab Calculator adds ready-mix cost. To budget a slab, continue withHow to Estimate Concrete Slab Costs.