How much concrete do you need for an 18×18 slab?

A 18×18 ft slab poured 4 inches thick needs 4 cubic yards (108 cu ft) of concrete. With 10% extra for waste, buy 198 bags of 80 lb premix or order 4.4 yards of ready-mix.

Change the numbers

The calculator below is pre-filled for this project. Adjust any value — including feet and inches — and the results update instantly.

pcs
ftin
ftin
in
%
Add prices to estimate cost (optional)
$
$

Enter today’s prices from your store or supplier; we never guess prices for you.

Tip: enter feet and inches in separate boxes, or use decimals (10 ft 6 in = 10.5 ft).

Your results

Concrete to buy (incl. 10% extra)4.4 cu yd
80 lb bags198
60 lb bags264
40 lb bags396
Exact volume108 cu ft (4 cu yd)
How much concrete do you need for an 18×18 slab — diagram

Concrete for an 18×18 slab at different thicknesses

The slab covers 324 square feet. Every extra inch of thickness adds about 1 cubic yards. Quantities below include 10% for waste.

ThicknessCubic yards80 lb bags60 lb bags40 lb bags
3.5 in3.85174231347
4 in4.4198264396
5 in5.5248330495
6 in6.6297396594
8 in8.8396528792

Bags or ready-mix for this size?

At 4.4 cubic yards, bags mean mixing 198 bags weighing roughly 7.9 tons. Ready-mix delivery is usually the practical choice at this size, and it gives a more consistent slab.

How heavy is an 18×18 slab?

Normal-weight concrete weighs about 150 lb per cubic foot, so a 4-inch 18×18 slab weighs roughly 16,200 lb (8.1 tons) once cured. Make sure your base is compacted gravel on firm, undisturbed soil.

The math

18 ft × 18 ft × (4 in ÷ 12) = 108 cu ft 108 cu ft ÷ 27 = 4 cu yd +10% waste = 118.8 cu ft 118.8 ÷ 0.6 cu ft per 80 lb bag = 198 bags (rounded up)

Common questions

How many 80 lb bags of concrete for an 18x18 slab?

198 bags for a 4-inch slab including 10% extra (180 with no extra). At 6 inches thick you need 297 bags.

How many yards of concrete for an 18x18 slab 6 inches thick?

6.6 cubic yards including 10% extra, or 6 yards exactly.

How many 60 lb bags for an 18x18 slab?

264 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.

Similar sizes

12×24 slab14×14 slab16×16 slab16×20 slab20×20 slab20×24 slab20×30 slab24×24 slab

Related calculators

Paver CalculatorPavers, base gravel and bedding sand for patios and walkways.Gravel CalculatorTons and cubic yards of gravel, crushed stone or sand for driveways and paths.Fence CalculatorPosts, rails, pickets and concrete bags for a wood privacy or picket fence.