How much concrete do you need for a 20×20 slab?

A 20×20 ft slab poured 4 inches thick needs 4.94 cubic yards (133.3 cu ft) of concrete. With 10% extra for waste, buy 245 bags of 80 lb premix or order 5.43 yards of ready-mix.

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Your results

Concrete to buy (incl. 10% extra)5.43 cu yd
80 lb bags245
60 lb bags326
40 lb bags489
Exact volume133.33 cu ft (4.94 cu yd)
How much concrete do you need for a 20×20 slab — diagram

Concrete for a 20×20 slab at different thicknesses

The slab covers 400 square feet. Every extra inch of thickness adds about 1.23 cubic yards. Quantities below include 10% for waste.

ThicknessCubic yards80 lb bags60 lb bags40 lb bags
3.5 in4.75214286428
4 in5.43245326489
5 in6.79306408612
6 in8.15367489734
8 in10.86489652978

Bags or ready-mix for this size?

At 5.43 cubic yards, bags mean mixing 245 bags weighing roughly 9.8 tons. Ready-mix delivery is usually the practical choice at this size, and it gives a more consistent slab.

How heavy is a 20×20 slab?

Normal-weight concrete weighs about 150 lb per cubic foot, so a 4-inch 20×20 slab weighs roughly 20,000 lb (10 tons) once cured. Make sure your base is compacted gravel on firm, undisturbed soil.

The math

20 ft × 20 ft × (4 in ÷ 12) = 133.33 cu ft 133.33 cu ft ÷ 27 = 4.938 cu yd +10% waste = 146.67 cu ft 146.67 ÷ 0.6 cu ft per 80 lb bag = 245 bags (rounded up)

Common questions

How many 80 lb bags of concrete for a 20x20 slab?

245 bags for a 4-inch slab including 10% extra (223 with no extra). At 6 inches thick you need 367 bags.

How many yards of concrete for a 20x20 slab 6 inches thick?

8.15 cubic yards including 10% extra, or 7.41 yards exactly.

How many 60 lb bags for a 20x20 slab?

326 bags of 60 lb concrete for a 4-inch slab with 10% extra.

Want the full method, tips and other sizes? See the concrete calculator guide.

Last reviewed November 1, 2026. Results are estimates — confirm with your supplier.

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