Use this free concrete calculator to work out how much concrete you need for a slab, footing, or driveway in seconds. Enter the length, width, and thickness in feet and inches or in meters and centimeters, add waste if you wish, and the concrete calculator shows the volume in cubic meters, cubic feet, and cubic yards, plus the number of bags to buy — no sign-up, no account, and no personal information needed. Your numbers are calculated in your browser and are not sent to us.
Contents: Formula · How to use · Examples · Tips · FAQ · Related tools
No sign-up required. This concrete calculator works in your browser, so the dimensions you enter are not stored or sent to us. Bag yields are typical values for premixed concrete; check the yield printed on your bag before ordering.
Concrete Calculator Formula
A rectangular slab is a box, so its volume is length times width times thickness. This concrete calculator first converts all measurements to one system, then applies the volume formula, then converts the result to metric and imperial units.
Volume = Length × Width × Thickness
Cubic yards = Cubic feet ÷ 27
Bags needed = Round up ( Volume in cubic feet × (1 + Waste ÷ 100) ÷ Yield of one bag )
For example, a slab 10 ft × 10 ft × 4 inches is 10 × 10 × (4 ÷ 12) = 33.33 cubic feet, or 1.23 cubic yards. With 10% waste, that is 36.67 cubic feet. At 0.60 cubic feet per 80 lb bag, 36.67 ÷ 0.60 = 61.1, rounded up to 62 bags. The concrete calculator shows the same volume in cubic meters, so the result works for ready-mix orders in the United States, the United Kingdom, and Canada.
How to Use the Concrete Calculator
Choose feet and inches or meters and centimeters, then type the length, width, and thickness. In the imperial system, enter the thickness in inches — a common slab is 4 inches. In the metric system, enter the thickness in centimeters — 10 cm is about 4 inches. Choose your bag size, set the waste percentage (10% is a good default for a simple slab), and tap Calculate Concrete.
Measure the longest and widest points, and allow for uneven ground, because a sub-base that is not level uses more concrete than the exact volume. For shapes that are not rectangles, split the area into rectangles, work out each part, and add the volumes together before using the concrete calculator.
Concrete Calculator Examples
Example 1 — 10 ft × 10 ft slab, 4 inches thick, 80 lb bags, 10% waste. Volume 33.33 cu ft (0.94 cu m, 1.23 cu yd). Bags: 33.33 × 1.10 ÷ 0.60 = 61.1, rounded up to 62 bags.
Example 2 — 20 ft × 12 ft driveway, 4 inches thick, no waste allowance in the example. Volume 20 × 12 × (4 ÷ 12) = 80 cu ft, or 2.96 cu yd. With 80 lb bags and 10% waste, that is 80 × 1.10 ÷ 0.60 = 146.7, rounded up to 147 bags.
Example 3 — 3 m × 4 m slab, 10 cm thick. Volume 3 × 4 × 0.10 = 1.20 cu m, or 42.38 cu ft. With 10% waste and 80 lb bags, that is 42.38 × 1.10 ÷ 0.60 = 77.7, rounded up to 78 bags.
These concrete calculator examples show why rounding up matters: buying the exact calculated number of bags would leave you short once waste and uneven ground are counted.
Concrete Calculator Tips
Order or buy a little extra for footings, steps, and uneven sub-bases, and check the strength grade your project needs — a driveway usually needs a stronger mix than a garden path. For background on concrete mixes and curing, see Wikipedia's article on concrete. Do not pour in heavy rain or freezing weather without advice, and allow the slab to cure before loading it.
Concrete Calculator FAQ
What does a concrete calculator do? A concrete calculator finds the volume of concrete for a slab from its length, width, and thickness, and converts that volume into bags or cubic yards.
How many 80 lb bags make a cubic yard? About 45 bags, because a cubic yard is 27 cubic feet and an 80 lb bag yields about 0.60 cubic feet: 27 ÷ 0.60 = 45.
How thick should a concrete slab be? A garden path is often 4 inches (10 cm) thick, while a driveway is often 5 to 6 inches thick. Follow local building guidance for your project.
Can I use meters and centimeters? Yes. Choose meters and centimeters, enter the length and width in meters and the thickness in centimeters, and the concrete calculator shows cubic meters as well as cubic feet and cubic yards.
Does the concrete calculator include waste? Yes. Set the waste percentage — 10% is a good default — and the bag count includes it.
What if my slab is not a rectangle? Split it into rectangles, calculate each part with the concrete calculator, and add the volumes together before ordering or buying bags.
Related Tools
Try the Tile Calculator for floors, the Paint Calculator for walls, the Meters to Feet converter, and the Ratio Calculator for mix ratios. Return to the PDGPT homepage to browse all free calculators by category.