The three measurements that control volume
A rectangular slab uses length x width x thickness. Enter length and width in feet, and record slab thickness in inches. Convert thickness to feet before multiplying: 4 inches equals 4/12 ft, or about 0.333 ft. Never skip that unit conversion.
Divide the cubic-foot total by 27 to get cubic yards because one cubic yard holds 27 cubic feet. This geometric volume describes a perfectly flat rectangular box, so it excludes uneven subgrades, thickened edges, footings, spillage, and mixer residue.
How do you calculate concrete cubic yards and bags?
Ready-mix concrete is ordered in cubic yards (1 yd³ = 27 ft³), while slab thickness is almost always measured in inches. Convert slab thickness to feet by dividing by 12, multiply by length and width to get cubic feet, and divide by 27:
where:
- L, W – Slab length and width in feet (find surface area first with the square footage calculator);
- V_cu yd – Exact geometric concrete volume in cubic yards (check haul weight with the concrete weight calculator); and
- Bags_80 – Estimated 80 lb premixed bags (⌈ V_cu ft ÷ 0.60 ⌉). To compare 40, 50, 60, and 80 lb bag sizes, open the concrete bag calculator.
Check the site before calculating
- 01
Measure inside the forms
Measure the exact interior length and width that the wet concrete will fill rather than trusting rough framing plans.
- 02
Check depth in several locations
A low or rutted subbase increases average slab depth. Pull a string line and measure depth at the center and edges.
- 03
Separate thickened sections
Calculate turn-down edges, grade beams, and footings as separate rectangular volumes instead of guessing one average depth.
- 04
Confirm how concrete will be supplied
Ready-mix suppliers batch by the cubic yard, while preblended bags rely on the printed yield for that specific bag weight.
Worked example: 12 ft by 20 ft by 4 in
Keep the exact fraction 4/12 ft during multiplication. Multiplying 12 x 20 x (4/12) gives 80 cubic feet. Divide 80 by 27 to get 2.96 cubic yards before any waste allowance.
An 80 lb concrete mix bag yields roughly 0.60 cubic feet, so 80 / 0.60 equals 133.3 bags. Round up to 134 full bags before adding waste. Check the printed bag label before ordering because lightweight or high-early mixes yield different volumes.
Price per bag never tells the whole story on a 134-bag pour. Lifting, mixing speed, truck access, crew size, and set time matter far more than simple arithmetic when you cross two cubic yards.
| Quantity check | Formula / basis | Result |
|---|---|---|
| Geometric volume | 12 × 20 × (4/12) ft | 80.0 cu ft |
| Ready-mix volume | 80 ÷ 27 cu ft/yd | 2.96 cu yd |
| 80 lb mix bags | 80 ÷ 0.60 cu ft/bag (133.3) | 134 bags (rounded up) |
| Project allowance | Documented separately | Added to exact total |
Where estimates usually go wrong
- Entering four inches as 0.4 ft instead of dividing inches by 12.
- Calculating from the nominal slab outline while ignoring thickened edges or separate footings.
- Assuming the excavated base is flat because the form tops look level.
- Using a generic bag yield that does not match the product you buy.
- Adding a percentage without first documenting the exact geometric volume.
- Rounding cubic yards down to fit an ordering increment.
Turn the result into an order conversation
Write down three separate numbers: exact geometric volume, your waste allowance, and the final order quantity. That keeps the takeoff easy to audit. When calling a ready-mix plant, ask about minimum load fees, truck washout space, chute reach, and short-load charges.
For bagged concrete, verify the exact bag yield, water ratio, and mixer batch size. Never add extra water to stretch a batch. If the slab supports structural loads or requires a building permit, have a licensed engineer or contractor review the design before you pour.