The concrete calculator turns the length, width and depth of a slab, pad or footing into cubic feet, cubic yards and a count of 80-lb bags, with a waste allowance applied after the conversion.
It is for rectangular pours: a patio slab, a shed pad, a walkway section, a strip footing. For fence-post holes use the fence calculator, which sizes the holes for you.
How to use it
- 01
Enter the footprint in feet
Length and width of the pour. For an L-shaped patio, split it into two rectangles and run the calculator twice. For a footing, use the footing's own width and length, not the wall it carries.
- 02
Enter the depth in inches
The residential code's least thickness for a concrete floor slab on the ground is 3½ inches (IRC R506.1). For a patio, walkway or driveway, ask your building department what it wants and whether your soil calls for more; thickened edges add volume the flat number does not show.
- 03
Set a waste allowance
The default is ten percent. It covers an uneven subgrade, forms that bow and the concrete left in the mixer. Raise it for a rough excavation; lower it only if the base is compacted and screeded flat.
What the concrete calculator measures
- Length, in feet. For an L-shaped pour, split it into two rectangles and run the calculator for each.
- Width, in feet. For a footing, the footing’s own width, not the wall it carries.
- Thickness, in inches. The residential code’s least thickness for a concrete slab-on-ground floor is 3½ inches (R506.1); ask the building department what a patio, walk or driveway needs.
- The base under the slab is a separate order: size the compacted gravel with the gravel calculator.
Not to scale. Cubic yards = length × width × (thickness ÷ 12) ÷ 27.
How the math works
Volume
Cubic feet = length × width × (depth ÷ 12). Cubic yards = cubic feet ÷ 27. A 12-by-10 slab at 4 inches is 40 cubic feet, or 1.48 cubic yards.
Bags
Bags = cubic yards × 27 × (1 + waste) ÷ 0.6, rounded up, where 0.6 cubic feet is the yield of one 80-lb bag of premixed concrete. The same slab with ten percent waste needs 74 bags. A 60-lb bag yields about 0.45 cubic feet, so multiply the count by 1.33 if that is what the store stocks.
Bags or a truck
Forty-five 80-lb bags make one cubic yard. Somewhere around that volume, ordering ready-mix becomes easier than mixing bags, and above it the truck is usually cheaper per yard — though plants charge a short-load fee under their minimum. Enter the store's bag price and the plant's price per yard, and the calculator totals both ways; add the short-load fee to the truck.
Worked examples
| Pour | Size and thickness | Cubic feet | Cubic yards | 80-lb bags | 60-lb bags |
|---|---|---|---|---|---|
| Shed pad | 10 × 8 ft, 4 in | 26.7 | 0.99 | 49 | 66 |
| Patio | 10 × 10 ft, 4 in | 33.3 | 1.23 | 62 | 82 |
| Slab at the code's least thickness | 12 × 10 ft, 3½ in | 35.0 | 1.30 | 65 | 86 |
| Shed pad | 12 × 12 ft, 4 in | 48.0 | 1.78 | 88 | 118 |
| Walkway | 30 × 3 ft, 4 in | 30.0 | 1.11 | 55 | 74 |
| Slab | 20 × 10 ft, 4 in | 66.7 | 2.47 | 123 | 163 |
Cubic feet and yards are the slab alone; the bag counts add 10 percent and round up, at Quikrete's yields of 0.6 cubic feet an 80-lb bag and 0.45 a 60-lb bag.
Where the number goes wrong
- Entering the depth in feet. The slider is in inches; a 4-foot slab is not a thing you want to pay for.
- Forgetting the base. Compacted gravel under the slab, wire mesh or rebar, and forms are separate purchases — use the gravel calculator for the base.
- Ignoring thickened edges and footings that drop below the slab; they can add a quarter of the volume on a small pad.
- Pouring in the public right-of-way without asking. A driveway apron is a permit from the road authority even where the slab on your lot is exempt.
- Mixing bags alone for a large pour. Concrete needs to be placed and finished before the first batch sets; a slab over a yard is a crew job.
Questions people ask
How many 80-lb bags of concrete are in a cubic yard?
Forty-five, at 0.6 cubic feet per bag. With ten percent waste the calculator rounds that to 50.
How much concrete do I need for a 10×10 slab?
At 4 inches thick, 33.3 cubic feet, or 1.23 cubic yards. With ten percent waste that is 62 bags of 80 lb or 82 of 60 lb.
How many square feet does a cubic yard of concrete cover?
A cubic yard is 27 cubic feet, which covers 324 square feet an inch deep, so divide 324 by the thickness in inches: 81 square feet at 4 inches, about 93 at 3½ inches and 54 at 6 inches, before waste.
How thick should a concrete slab be?
The International Residential Code sets 3½ inches as the least thickness for a concrete slab-on-ground floor (R506.1), and sends expansive soils to its foundation rules. For a patio, walkway or driveway, ask your building department what thickness it wants.
How thick should a garage or shed slab be?
The same 3½ inches under the code: R506.1 sets it for a concrete slab-on-ground floor, and the same section lists garages, utility buildings and other unheated accessory structures among the slabs that need no vapor retarder (R506.3.3). R506.1 sends expansive soils to the foundation rules; ask your building department what your lot needs.
Does a concrete patio need a permit?
At-grade flatwork on your own lot is often treated as exempt from a building permit, but impervious-cover limits, setbacks and drainage rules still apply. The patio permit brief covers the pattern and where it breaks.