Concrete Slab Calculator
Estimate how much concrete a rectangular slab needs. Enter the length, width, and thickness and the calculator returns the volume in cubic yards (cubic metres in metric), the number of 80-lb bags including a waste allowance, and — if you add a price — an estimated material cost. It is built for patios, sheds, walkways, and small footings.
Calculate
Default result: 1.36 yd³
Result
Concrete volume
- 80-lb bags
- 62
How to calculate
Enter the slab's length and width (feet or metres) and its thickness (inches or centimetres — thickness is usually a different unit than length). The waste percentage defaults to 10% to cover spillage and an uneven subgrade; change it if your site needs more or less. To see a cost estimate, type your supplier's price per cubic yard; leave it blank to skip cost. The result updates as you type.
cubic_yards = (length_ft × width_ft × thickness_ft) / 27, where thickness_ft = thickness_in / 12. A handy identity is yd³ = (L_ft × W_ft × thickness_in) / 324 (since 324 = 12 × 27). In metric, m³ = length_m × width_m × (thickness_cm / 100). The volume is then multiplied by (1 + waste% / 100), and bags = ceil(total_ft³ / 0.60) for 80-lb bags. Variables: L and W are the slab footprint, thickness is the pour depth, waste% is the extra-material allowance, and 0.60 ft³ is the yield of one 80-lb bag.
Example calculation
A 10 ft × 10 ft slab poured 4 inches thick is (10 × 10 × 4) / 324 = 1.235 cubic yards. Adding the default 10% waste allowance brings it to about 1.36 cubic yards (≈ 36.7 ft³), which fills ceil(36.7 / 0.60) = 62 eighty-pound bags.
- volume
- 1.36 yd³
- bags80
- 62 bags
Assumptions
- The waste allowance defaults to 10% (sources recommend 5–10% for spillage and an uneven subgrade) and is fully editable.
- An 80-lb bag of concrete mix yields about 0.60 ft³. This is a nominal manufacturer figure and varies slightly by brand, mix, and water content.
- The bag count includes the waste allowance and is always rounded up — you cannot buy a fraction of a bag.
- Thickness is entered in its own unit (inches or centimetres), independent of the length and width unit, so a 4-inch slab is not mistaken for 4 feet.
- Bagged mix is practical for small slabs; for a large pour, ordering ready-mix concrete by the cubic yard is usually cheaper and easier than mixing dozens of bags.
- When you open a shared link the inputs come from the URL first, then from your last-used values saved in this browser, then from the calculator's defaults.
Common mistakes
- Forgetting the waste allowance. Ordering the exact theoretical volume leaves no margin for spillage or an uneven subgrade — a short pour can mean a cold joint or a second trip.
- Entering thickness in feet instead of inches. A 4-inch slab entered as 4 feet overestimates concrete by 12×; keep thickness in inches (or centimetres) using its own unit control.
- Rounding the volume before counting bags. Round only the final bag count up; rounding the volume first can drop or add a whole bag.
Frequently asked questions
How many 80-lb bags of concrete are in a cubic yard?
About 45. One cubic yard is 27 ft³ and an 80-lb bag yields roughly 0.60 ft³, so 27 / 0.60 ≈ 45 bags. The calculator works this out for your exact volume and rounds up.
Should I use bags or order ready-mix concrete?
Bagged mix is fine for small slabs — a few square feet up to a small patio. Larger pours quickly need dozens of bags, so ordering ready-mix delivered by the cubic yard is usually cheaper, faster, and gives a more consistent slab.
Does the bag count include waste?
Yes. The bag count is based on the volume after the waste allowance is applied, then rounded up to whole bags. Set the waste percentage to 0 to see the no-waste count.
Why is the number of bags rounded up?
You cannot buy a fraction of a bag, and running short mid-pour risks a cold joint. The calculator always rounds the bag count up to the next whole bag.
Can I change the waste percentage?
Yes. The waste field defaults to 10% but you can set any value. Use a higher allowance for a rough or sloped subgrade and a lower one for a clean, well-prepared form.