How Much Concrete for a 20×20 Slab?

4-inch slab estimate

Order quantity

5.432

yd³ of concrete

Includes 10% overage.

Base volume: 4.94 yd³

With 10% overage: 5.432 yd³

Confirm your supplier’s ordering increment. Bag counts below include the same overage.

Base volume (ft³)

133.33

cubic feet

60 lb bags

326

bags (inc. 10% waste)

80 lb bags

245

bags (inc. 10% waste)

Need a different size or thickness? Use the full calculator to get an estimate.

Calculate another size

Compare delivery for 5.432 yd³ with transporting and mixing 245 × 80 lb bags. Ask about minimum orders, access and delivery fees.

20 × 20 ft slab: thickness comparison

Same 400 ft² footprint, with 10% overage. Depths illustrate quantities, not structural recommendations. Bag yield: 0.60 ft³ per 80 lb bag.

Thickness	Base yd³	With overage yd³	80 lb bags
4 in	4.938	5.432	245
5 in	6.173	6.790	306
6 in	7.407	8.148	367
8 in	9.877	10.864	489

The 4-inch example needs 5.432 yd³ with overage, equivalent to 245 eighty-pound bags: 19,600 lb of dry mix. Plan truck access, placement equipment and labor.

Calculation method

The formula is: Volume = Length × Width × Thickness. Convert thickness from inches to feet first, then divide by 27 to get cubic yards.

20 ft × 20 ft slab, 4 in thick; step by step

Step 1; Volume 20 × 20 × (4 ÷ 12) ft = 133.33 cu ft
Step 2; Cubic Yards 133.33 ÷ 27 = 4.94 cu yd
Step 3; Add 10% Waste 133.33 × 1.10 = 146.67 cu ft
Step 4; 60 lb Bags 146.67 ÷ 0.45 = ~326 bags
Step 5; 80 lb Bags 146.67 ÷ 0.60 = ~245 bags

Practical Tips for a 20×20 Slab

Use the specified thickness. The 4-inch figure here is a quantity example, not a structural recommendation.
Allow for overage. 10% is a starting allowance, not a universal specification. Allow for uneven ground and spillage.
Compact your sub-base first. 4–6 inches of gravel under the slab prevents settling and cracking.
Don't thin the mix with extra water. It makes mixing easier but weakens the concrete significantly.
Cure for at least 7 days. Keep the slab moist after pouring. This dramatically increases final strength.
Round up, not down. You cannot pour half a slab and come back; you'll get a visible cold joint.

Assumptions

QUIKRETE Concrete Mix technical data provides the 0.45 ft³ and 0.60 ft³ bag yields used here. Other mixes can have different yields.

◦ Slab dimensions: 20 ft × 20 ft at 4 inches thick. Changing thickness changes all totals.
◦ Concrete yield: 60 lb bags = 0.45 cu ft; 80 lb bags = 0.60 cu ft. Standard manufacturer figures.
◦ Waste factor: 10% applied. Increase to 15% for rough terrain or irregular shapes.
◦ Results are estimates only: Confirm quantities with your concrete supplier before ordering.

Questions

A 20×20 ft slab at 4 inches thick requires 133.33 cubic feet (4.94 cubic yards) of concrete. With a 10% waste factor, that's about 326 bags of 60 lb mix or 245 bags of 80 lb mix.

Compare the complete delivery quote with transporting and mixing 326 sixty-pound bags or 245 eighty-pound bags. Confirm supplier minimums, truck access and available mixing equipment.

Use the thickness specified for your project. The table above shows how quantity changes at different depths; it does not determine structural adequacy.

One cubic yard equals 27 cubic feet. At standard yields, you need about 60 bags of 60 lb mix or 45 bags of 80 lb mix per cubic yard; before adding a waste factor.

Setting, drying and curing are different processes. Follow the selected mix instructions and the project specification for curing, walking and loading times; weather and temperature affect them.