How Many Yards of Concrete for a 20 x 20 Slab?
A 20 by 20 foot slab (400 square feet) poured 4 inches thick needs 4.94 cubic yards of concrete. With 10% extra, order about 5.5 cubic yards. At 5 inches order about 7 yards and at 6 inches about 8.5 yards.
Concrete needed for a 20 x 20 slab
A 20 × 20 slab is a typical one-car-plus workshop floor or a large patio. The slab is 400 square feet. Ready-mix is sold by the cubic yard, so multiply the area by the thickness in feet and divide by 27.
At 4 inches thick: 400 × 0.3333 = 133.3 cubic feet, and 133.3 ÷ 27 = 4.94 cubic yards. Add 10% and the number to order is 5.43, which most suppliers will round to 5.5 yards.
20x20 slab concrete chart
| Thickness | Exact yards | With 10% extra | Round to order | Approx. weight |
|---|---|---|---|---|
| 4 in | 4.94 | 5.43 | 5.5 | 10.9 tons |
| 5 in | 6.17 | 6.79 | 7 | 13.6 tons |
| 6 in | 7.41 | 8.15 | 8.5 | 16.3 tons |
Fresh concrete weighs about 2 tons per cubic yard (roughly 4,000 lb), which matters for truck access and for any driveway, lawn or sewer line the truck must cross.
Thickness for this slab
A workshop or garage floor is commonly 4 to 6 inches thick with reinforcement and sits on a compacted base; vehicles push toward the thicker end. Each extra inch of thickness adds about 1.23 cubic yards to this slab, so a small change in the spec moves the order noticeably.
Ordering from a ready-mix plant
- Most plants sell in half-yard steps above a minimum load, and charge a short-load fee for small orders, so ask for the total delivered price.
- Tell the dispatcher the slab size, thickness and the exposure (freeze-thaw, de-icing salt) so they quote a suitable mix.
- Have forms, base and finishing tools ready before the truck arrives; discharge time is limited.
- If you are close to a round number, order the larger one. Running short mid-pour leaves a cold joint.
Check other sizes with the concrete calculator, see the cost with the concrete cost calculator, or measure an odd shape with the square footage calculator.
Common mistakes
- Ordering the exact volume and running short on a high spot.
- No clear path for the truck, or no plan for washing out the chute.
- Pouring in very hot, cold or rainy weather without a plan.
- Not asking about the mix design when the slab will see freezing and salt.
Other sizes
How Many Yards of Concrete for a 24x24 Slab · How Many Yards of Concrete for a 10x20 Slab
Calculators for this guide
Frequently asked questions
How many yards of concrete do I need for a 20x20 slab at 4 inches?
4.94 cubic yards exact, and about 5.5 yards to order with 10% extra.
How many yards for a 20x20 slab at 6 inches?
7.41 cubic yards exact, or about 8.5 to order.
How many bags of concrete are in a yard?
About 45 bags of 80 lb mix, 60 bags of 60 lb, or 90 bags of 40 lb make one cubic yard.
How much does a concrete truck hold?
A standard ready-mix truck carries about 8 to 10 cubic yards, so this slab fits in one load.
What is a typical slab thickness?
Four inches is common for patios and walks; garages and driveways are often 5 to 6 inches. Follow your local code.
Sources and further reading
- Concrete driveways and slabs — American Cement Association
- National Ready Mixed Concrete Association — NRMCA: ready-mix industry resources
- NIST Special Publication 811: conversion factors — National Institute of Standards and Technology
More guides
Estimates only. Always confirm quantities with your supplier or a qualified contractor before ordering, and add extra for waste and site conditions.