How to measure a slab
Measure the length and width inside the forms, then decide the finished thickness of the concrete. Leave out the gravel or crushed stone base: it's bought separately and doesn't count toward the concrete volume.
- Rectangles: length × width. For an L-shaped patio, add two rectangles as separate parts.
- Round slabs: measure straight across through the center for the diameter.
- Odd shapes: if you already know the area (from a plan or a measuring app), choose "Slab by area" and enter it directly.
- Cut-outs: add a part for the hole (around a tree or drain) and set it to subtract.
How slab concrete is calculated
A flat slab is length × width × thickness. The calculator converts inches to feet (4 in is 0.3333 ft), multiplies, and divides cubic feet by 27 to get cubic yards. Here is the default 10 × 10 ft slab at 4 in, step by step:
Slab
- Length: 10 ft
- Width: 10 ft
- Thickness: 4 in ÷ 12 = 0.3333 ft
- Area: 10 ft × 10 ft = 100 ft²
- Volume: 100 ft² × 0.3333 ft = 33.3333 ft³
Total and order
- In cubic yards: 33.3333 ft³ ÷ 27 = 1.2346 yd³
- With 10% extra: 1.2346 yd³ × 1.1 = 1.358 yd³
- Order, rounded up to the next 0.25 yd³: 1.5 yd³
Thickness drives the total more than people expect. The same 10 × 10 ft slab at 6 in is 1.85 cubic yards instead of 1.23, 50% more concrete and 30 more 80-lb bags.
Slabs with a thickened edge
Many shed bases and garage slabs are deeper around the perimeter. The calculator adds the flat slab to the deeper strip below it, counting each corner once. A 20 × 20 ft slab at 4 in is 4.94 yd³ flat; with a 12 in wide edge that goes 12 in deep it becomes 6.81 yd³.
Round slabs
A circle's area is π × radius². A round slab 10 ft across and 4 in thick is 0.97 yd³, about 48 80-lb bags with 10% extra.
Slab sizes at 4, 5 and 6 inches
Exact volume, the order rounded up to the next ¼ yd³ with 10% extra, and bags including the extra.
| Slab | Thick | Exact yd³ | Order yd³ | 80-lb bags | 60-lb bags |
|---|---|---|---|---|---|
| 4 × 4 ft | 4 in | 0.2 | 0.25 | 10 | 14 |
| 4 × 4 ft | 5 in | 0.25 | 0.5 | 13 | 17 |
| 4 × 4 ft | 6 in | 0.3 | 0.5 | 15 | 20 |
| 8 × 8 ft | 4 in | 0.79 | 1 | 40 | 53 |
| 8 × 8 ft | 5 in | 0.99 | 1.25 | 49 | 66 |
| 8 × 8 ft | 6 in | 1.19 | 1.5 | 59 | 79 |
| 10 × 10 ft | 4 in | 1.23 | 1.5 | 62 | 82 |
| 10 × 10 ft | 5 in | 1.54 | 1.75 | 77 | 102 |
| 10 × 10 ft | 6 in | 1.85 | 2.25 | 92 | 123 |
| 10 × 12 ft | 4 in | 1.48 | 1.75 | 74 | 98 |
| 10 × 12 ft | 5 in | 1.85 | 2.25 | 92 | 123 |
| 10 × 12 ft | 6 in | 2.22 | 2.5 | 110 | 147 |
| 12 × 12 ft | 4 in | 1.78 | 2 | 88 | 118 |
| 12 × 12 ft | 5 in | 2.22 | 2.5 | 110 | 147 |
| 12 × 12 ft | 6 in | 2.67 | 3 | 132 | 176 |
| 12 × 16 ft | 4 in | 2.37 | 2.75 | 118 | 157 |
| 12 × 16 ft | 5 in | 2.96 | 3.5 | 147 | 196 |
| 12 × 16 ft | 6 in | 3.56 | 4 | 176 | 235 |
| 16 × 16 ft | 4 in | 3.16 | 3.5 | 157 | 209 |
| 16 × 16 ft | 5 in | 3.95 | 4.5 | 196 | 261 |
| 16 × 16 ft | 6 in | 4.74 | 5.25 | 235 | 313 |
| 20 × 20 ft | 4 in | 4.94 | 5.5 | 245 | 326 |
| 20 × 20 ft | 5 in | 6.17 | 7 | 306 | 408 |
| 20 × 20 ft | 6 in | 7.41 | 8.25 | 367 | 489 |
| 24 × 24 ft | 4 in | 7.11 | 8 | 352 | 470 |
| 24 × 24 ft | 5 in | 8.89 | 10 | 440 | 587 |
| 24 × 24 ft | 6 in | 10.67 | 11.75 | 528 | 704 |
Slab tips
- Check the minimum thickness. Building codes set 3½ in as the minimum thickness for concrete slab-on-ground floors. Bagged-mix slab instructions commonly call for at least 4 in. The calculator warns you below these.
- Uneven ground eats concrete. A low spot half an inch deep across a whole patio adds up quickly. Level and compact the base, and keep the waste allowance at 10% or higher if it's rough.
- Split big pours. A 64 ft³ patio (12 × 16 ft at 4 in) is 118 80-lb bags. Compare that with ready-mix using your local prices.
- Order for the whole job at once. Add steps, footings or post holes as extra parts so one ready-mix order covers everything.
Questions people ask
How many yards of concrete for a 12 × 12 slab?
A 12 × 12 ft slab at 4 in thick is 1.78 cubic yards. With 10% extra that's 1.96 cubic yards, so order 2 yd³, or buy 88 80-lb bags.
How many bags of concrete for a 10 × 10 patio?
At 4 in thick, a 10 × 10 ft patio takes 62 80-lb bags or 82 60-lb bags, including 10% extra. That's 1.23 cubic yards before the extra.
What is the minimum thickness for a concrete slab?
The International Residential Code requires slab-on-ground floors to be at least 3½ in thick. Bagged concrete mix is made for pours of 2 in or more, so anything thinner needs a different product. The calculator warns you if a slab is thinner than these figures.
How do I calculate a slab with a thickened edge?
Add the flat slab to the deeper strip around its edge. For a 20 × 20 ft slab at 4 in with a 12 in wide edge that goes 12 in deep, the flat slab is 4.94 cubic yards and the thickened edge adds 1.88, for 6.81 cubic yards in total. Choose "Slab with thickened edge" in the calculator to do this automatically.
How much concrete for a 24 × 24 garage slab?
At 4 in thick, a 24 × 24 ft slab is 7.11 cubic yards (order 8 yd³ with 10% extra). At 5 in it's 8.89 cubic yards (order 10 yd³). At this size a ready-mix truck is usually more practical than bags.
Do I include the gravel base in the thickness?
No. Measure only the concrete. If you're pouring 4 in of concrete over 4 in of gravel, enter 4 in. Gravel is bought separately, by the ton or cubic yard.
Related calculators
Sources
- QUIKRETE Concrete Mix No. 1101 product data sheet, retrieved October 2, 2026
- SAKRETE High-Strength Concrete Mix technical data sheet, retrieved October 2, 2026
- International Residential Code 2021, Section R506.1, retrieved October 2, 2026
- NRMCA, Concrete in Practice 31: Ordering Ready Mixed Concrete, retrieved October 2, 2026
- NRMCA, Concrete in Practice 36: Structural Lightweight Concrete, retrieved October 2, 2026
Last updated October 2, 2026. Read how we calculate, including formulas, waste defaults and rounding.