Concrete Floor Calculator
This concrete floor calculator works out how much concrete you need to pour a floor slab, in cubic yards, cubic feet, and 40, 60, or 80 lb bags. Enter the room length and width, or type in the square footage you already know, set the thickness, and you have an instant material estimate for a garage, basement, shed, or shop floor.
How to use this calculator
- Enter the floor length and width and pick their unit, or skip them and type the floor area straight into the area field if you already know your square footage. Not sure of the area? Work it out first with the concrete square footage calculator.
- Enter the thickness. Four inches is standard for a residential floor, and 5 to 6 inches suits a garage or shop floor that takes vehicles.
- Pouring more than one identical room or bay? Set the number of rooms/bays and the volume multiplies for you.
- Choose a waste allowance (10% is a safe default) to cover spillage and uneven subgrade.
- Read the cubic yards for ready-mix delivery, or the bag count if you are mixing by hand. Topping an existing floor instead of pouring a new one? Use the self-leveling concrete calculator.
The formula
A floor is a flat slab, so the concrete you need is simply the floor area times the thickness. Keep the area in square feet and the thickness in feet, multiply them for cubic feet, then divide by 27 to convert to cubic yards:
Cubic yards = (Floor area sq ft x Thickness ft) / 27
If you are starting from length and width instead of an area, multiply those first: area equals length times width. For a full-slab job with edges and footings, the concrete slab calculator covers the same math with a slab-first layout.
Worked example. A 20 ft by 20 ft garage floor at 4 inches thick is 400 sq ft x 0.333 ft = 133.3 cubic feet, which is 133.3 / 27 = about 4.94 cubic yards (round up to 5 for ordering).
Bump that same floor to 6 inches thick and it climbs to about 7.4 cubic yards, which is why floor thickness drives the concrete bill as much as the footprint does. See the concrete floor thickness guide for how thick to pour yours.
Frequently asked questions
How much concrete do I need for a floor?
Multiply the floor area by the thickness. Keep the area in square feet and the thickness in feet, multiply for cubic feet, then divide by 27 for cubic yards. A 400 sq ft floor at 4 inches thick is about 4.9 cubic yards before waste.
How many yards of concrete for a 20x20 floor?
A 20 ft by 20 ft floor is 400 sq ft. At 4 inches thick that is about 4.94 cubic yards, so you would order 5. At 6 inches thick the same floor needs about 7.4 cubic yards.
Can I enter the floor area instead of length and width?
Yes. If you already know the square footage, type it straight into the area field and set the thickness. The calculator uses your area directly, so you can skip the length and width entirely.
How thick should a concrete floor be?
Four inches is standard for a residential floor, basement, or shed. Step up to 5 or 6 inches for a garage or shop floor that carries vehicles. See the concrete floor thickness guide for details by use.
How many bags of concrete for a floor?
Find the volume in cubic feet, then divide by the bag yield: about 0.60 ft³ per 80 lb bag, 0.45 ft³ per 60 lb bag, or 0.30 ft³ per 40 lb bag. Floors past about half a cubic yard are usually cheaper as ready-mix.