Concrete slab calculator
Find out how much concrete you need for a slab, patio or path, how many bags that is, and an optional cost estimate using your own prices.
Your estimate
Enter the slab’s length, width and thickness to see how much concrete you need.
How the concrete calculation works
The calculator multiplies length, width and thickness to get the slab’s volume, then adds your waste allowance. For bags, it divides by the yield of one bag and rounds up.
- Thickness in feet = thickness in inches ÷ 12. In metric, centimeters ÷ 100 gives meters.
- Volume = length × width × thickness, in cubic feet (or cubic meters).
- Cubic yards = cubic feet ÷ 27.
- Volume with waste = volume × (1 + waste %).
- Bags = volume with waste ÷ yield per bag, rounded up to a whole bag.
- Cost (optional) = volume with waste × your price, or bags × your price per bag.
Worked example: a 10 ft by 10 ft slab, 4 inches thick
Volume = 10 ft x 10 ft x (4 in / 12) = 33.33 cubic feet. Dividing by 27 gives 1.23 cubic yards.
Add 10% for waste (uneven ground, spillage, a slab that ends up slightly thicker): 1.2346 x 1.1 = 1.36 cubic yards (rounded). This is the amount to order if you are buying ready-mixed concrete.
If you are mixing from bags, take the volume with waste (36.67 cubic feet) and divide it by the yield printed on your bag. With an assumed yield of 0.60 cubic feet per bag, that is 61.1, so you would buy 62 bags. Bag yields vary by brand and size, so always check the label of the bag you plan to buy.
Metric version: 3 m x 3 m x 0.10 m = 0.90 cubic meters, or 0.99 cubic meters with 10% waste.
Assumptions and limits
- Waste allowance. The default is 10% to cover uneven ground, spillage and forms that flex. Change it to suit your site.
- Rounding. Volumes are shown to 2 decimal places and bags are rounded up to a whole bag. Costs use the unrounded volume.
- Bag yield is an assumption. The yield filled in for each bag size is a starting point, not a fact about your product. Check the yield printed on your bag and change it if it differs.
- Thickness. Confirm it against your local requirements, and with a professional for structural work.
- Not included: gravel subbase, rebar or mesh, formwork, delivery minimums or short-load charges, pumping, and local permits or rules.
- Costs are estimates, not quotes. There are no default prices; costs use only the prices you enter.
Practical tips
- Measure thickness in several places. Ground is rarely perfectly flat, and thickness has a big effect: on a 4 in slab, one extra inch means 25% more concrete.
- Check your bag label. Yields vary between brands and mixes, so use the figure printed on the bag you’re buying.
- Compare bags with ready-mix. A large slab means a lot of bags to mix by hand. Ask local suppliers for a ready-mix price and whether they have a minimum order, then enter both prices to compare.
- Don’t cut it fine. Running out partway through a pour can leave a weak joint between the old and new concrete. It’s better to have a little left over.
Frequently asked questions
How many bags of concrete do I need for a 10 × 10 slab?
At 4 in thick with 10% waste, a 10 ft × 10 ft slab needs about 36.67 cu ft of concrete. That is 62 bags at 0.60 cu ft per bag, 82 bags at 0.45 cu ft, or 123 bags at 0.30 cu ft. Those are the yields this calculator fills in for 80, 60 and 40 lb bags, but check the label on yours.
How do I work out cubic yards of concrete?
Multiply length in feet by width in feet by thickness in inches divided by 12 to get cubic feet, then divide by 27. For example, a 20 ft × 12 ft slab 6 in thick is 120 cu ft, or 4.44 cu yd. With 10% waste, that is 4.89 cu yd.
How thick should a concrete slab be?
It depends on what the slab will carry, the ground underneath it and your local building requirements, so this calculator doesn’t recommend one. Check your local rules, and ask a professional for anything structural.
How much will my concrete slab cost?
Enter your supplier’s price per cubic yard (or cubic meter) for ready-mix, or the price of one bag, and the calculator multiplies it by the amount you need. There are no built-in prices because they vary by place and over time. Delivery, labor, forms and reinforcement are not included, so treat it as an estimate.
How much concrete does an 80 lb bag make?
The yield is printed on the bag and varies by product. This calculator fills in 0.60 cu ft for an 80 lb bag as a starting assumption. If your bag states a different yield, enter that instead and the bag count updates.
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