Common slab sizes at 4" thick
| Slab | Exact yd³ | With 10% extra | 80 lb bags |
|---|---|---|---|
| 4 × 4 (AC pad) | 0.20 | 0.22 | 10 |
| 8 × 8 | 0.79 | 0.87 | 40 |
| 10 × 10 | 1.23 | 1.36 | 62 |
| 10 × 12 (shed) | 1.48 | 1.63 | 74 |
| 12 × 12 | 1.78 | 1.96 | 88 |
| 12 × 16 | 2.37 | 2.61 | 118 |
| 20 × 20 | 4.94 | 5.43 | 245 |
| 24 × 24 (garage, 4") | 7.11 | 7.82 | 352 |
Over about 2.5 yards, bags stop being realistic. Above, a 20 × 20 slab works out to 245 bags, roughly 9.8 tons of lifting and mixing.
Thickened edges
Many shed and garage slabs have a thickened edge, a perimeter that's 8–12" deep and about 12" wide. To include it, figure the slab at its normal thickness here. Then switch to the Footing tab on the main calculator. Enter the perimeter length, a 12" width, and the extra depth only (for example 4" if the edge is 8" and the slab is 4"). Add the two results together.
Before you order
- Measure the actual forms, not the plan. Forms often end up ½–1" wider.
- Check the base with a string line. Every ½" low across a 10 × 10 slab costs another 0.15 yd³.
- Ask the plant about its short-load fee and how long it allows to unload (often 5–7 minutes per yard).
Common questions
How much concrete do I need for a 10×10 slab?
At 4" thick, a 10×10 slab is 1.23 cubic yards exactly, or about 1.36 yd³ with 10% extra. That's 62 × 80 lb bags, or a 1.5-yard ready-mix order.
How much concrete for a 12×12 slab?
At 4" thick: 1.78 yd³ exactly, about 1.96 yd³ with 10% extra. Order 2 yards of ready-mix, or 88 × 80 lb bags.
How much concrete for a 20×20 slab?
At 4" thick: 4.94 yd³ exactly, about 5.43 yd³ with 10% extra. Order 5.5 yards. At 6" (for a garage), it's 7.4 yd³ exactly, so order 8.25 yards with extra.
Do I need gravel under a concrete slab?
Usually, yes. 4" of compacted gravel (crusher run or ¾" crushed stone) gives drainage and an even base. Use the gravel calculator with the same length and width.
Should a slab have rebar or wire mesh?
Driveways, garage floors and slabs on expansive soil normally get rebar (often #3 or #4 on a 16–24" grid) or fiber-reinforced mix. Small patios often get by with wire mesh or fiber. Check your local code. This calculator doesn't size reinforcement.
Prices last checked October 2026. Quantities only. This is not structural advice.