Garage Floor Calculator

1. Enter the garage length and width in feet and the field thickness in inches.
2. For a thickened edge, enter its width and total depth; enter 0 for both to pour a flat slab.
3. Set the number of garages and a 5 to 10 percent waste allowance.
4. Read the result: total cubic yards, split into the field slab and the thickened edge, plus bags and weight.

Here is what each result line means:

A garage floor calculator estimates the concrete for a garage slab, including the thickened edge that most garage floors have around the perimeter. It answers how much concrete to order for a floor that has to carry vehicles, not just foot traffic.

A garage floor is a slab, usually 4 inches thick in the field for a residential garage and 5 to 6 inches for heavier use, often with a deepened edge or footing where the walls and door sit. The tool sizes the field slab and the edge separately and adds them. For a plain floor without the thickened edge, the general concrete calculator in the related tools below covers it. To work out the mix, use the concrete mix calculator or the cement calculator.

The field slab is length times width times thickness. The thickened edge is extra concrete: a strip around the perimeter deepened below the field slab, so it is the perimeter length times the strip width times the extra depth.

1. Find the field slab volume in cubic feet from length, width and field thickness.
2. Find the perimeter, 2 times length plus width, and multiply by the edge width and the extra depth below the field for the edge volume.
3. Add the field and edge, multiply by the number of garages, and add the waste allowance.
4. Divide by 27 for cubic yards, and by the bag yield for a bag count.

Suppose you are pouring a 2-car garage 24 by 24 feet at 4 inches in the field, with a 12 inch wide thickened edge 12 inches deep, and 10 percent waste.

Calculation: the field slab is 24 x 24 x 0.333 = 192 cu ft. The perimeter is 96 feet, and the edge adds 96 x 1 x (8 / 12) = 64 cu ft. Together that is 256 cubic feet, or 281.6 with 10 percent waste, which is 10.43 cu yd, about 470 eighty-pound bags and roughly 21 short tons.

The lesson: the thickened edge adds about a third again over the bare field slab here, so leaving it out would under-order the pour badly.

Field Thickness

Thickness multiplies the whole floor area, so it is the biggest lever. A 5 or 6 inch floor for heavy vehicles uses far more concrete than a 4 inch residential slab.

The Thickened Edge

The deepened perimeter carries the walls and door and stiffens the slab. On a large garage it can add a third again over the field slab, so it must be in the estimate.

Sub-base and Over-excavation

A soft or uneven sub-base soaks up concrete below grade. This is the main reason real pours beat the geometry, and why a waste allowance exists.

Reinforcement

Rebar or wire mesh does not change the concrete volume but does affect cost and the pour. Size it from your plans; this tool estimates the concrete only.

A garage floor can be a flat slab of uniform thickness, or a flat field with a deepened edge. The edge adds concrete but spreads the wall and door loads and resists cracking at the perimeter.

Enter 0 for the edge width and depth to price a flat slab, or the edge sizes to include the deepened perimeter. For the mix water, the water cement ratio calculator helps you get it right.

Ordering Ready-mix for a Garage

A garage slab is usually a few cubic yards, so size the order in cubic yards and add waste before you call the supplier.

Pricing the Pour

Use the cubic yards with a price per yard, or the bag count for a small detached garage you might hand-mix.

Checking the Thickened Edge

See how much the deepened perimeter adds, so it is in the order and not a surprise short-fall on the day.

1. Leaving Out the Thickened Edge

The deepened perimeter is real concrete, often a third again on a large garage. Forgetting it is the classic way to run short.

2. Using Patio Thickness

A garage carries vehicles, so 4 inches is the minimum and 5 to 6 inches is common. A thin slab cracks under load.

3. Mixing Up Feet and Inches

Entering thickness or edge sizes in the wrong unit throws the result off. Keep length and width in feet and thicknesses in inches, as labelled.

4. Forgetting a Waste Allowance

An uneven sub-base and spillage mean the exact geometry runs short. Keep 5 to 10 percent.

5. Treating the Estimate as a Structural Spec

Volume is not strength. Thickness, reinforcement and the edge detail for a load-bearing floor come from a code or an engineer, not this tool.

The arithmetic is exact; the usefulness depends on the dimensions, the edge sizes and the waste allowance you enter.

A 24 by 24 foot garage at 4 inches with a thickened edge needs about 10.4 cubic yards with waste; without the edge it is closer to 8. Enter your size, thickness and edge above for the exact figure.

Commonly 4 inches for a residential garage and 5 to 6 inches for heavier vehicles or workshops, often with a thickened edge around the perimeter. The required thickness comes from your plans or local code; this tool estimates volume.

A deepened strip of concrete around the slab perimeter, often about 12 inches wide and 8 to 12 inches deep, that carries the walls and garage door and stiffens the slab against cracking. It adds concrete beyond the flat field slab.

Most do, to support the walls and door and resist edge cracking, though a floor poured inside existing foundation walls may not. Enter 0 for the edge width and depth to price a flat slab if yours has no thickened edge.

A lot: a 2-car garage is roughly 10 cubic yards, which is hundreds of bags, so garage floors are almost always ready-mix. The tool shows the bag count mainly for a small detached garage you might hand-mix.

About 150 pounds per cubic foot, so a 24 by 24 foot, 4 inch floor with a thickened edge weighs around 21 short tons of concrete. The sub-base must be compacted to carry it without settling.

Yes, a 5 to 10 percent waste allowance for an uneven sub-base and spillage. Running short mid-pour on a slab this size risks a cold joint, so the margin is worth it.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• QUIKRETE Concrete Mix 1101 data sheet (QUIKRETE; bag yields 0.60, 0.45 and 0.30 cu ft).
• National Ready Mixed Concrete Association (NRMCA; ready-mixed concrete and ordering).
• American Cement Association (American Cement Association; what concrete and cement are).