How Many Bags of Concrete for a 20x20 Slab? (2026 Guide)

The slab math: square feet first

Every estimate starts with the surface area: 20 × 20 = 400 sq ft. The thickness only decides how many cubic feet sit under those 400 sq ft. The rule of thumb across our guides is cu ft = area (sq ft) × thickness (in) ÷ 12, so a 4 in pour is 400 × 4 ÷ 12 = 133.3 cu ft.

From there, count bags or yards. An 80-lb bag makes about 0.6 cu ft and a 60-lb bag about 0.45 cu ft, so divide the cubic feet by the bag's yield; for ready-mix, divide by 27 (the cu ft in a cu yd). The 4 in pour needs 133.3 ÷ 0.6 ≈ 222 bags of 80-lb mix (or about 296 at 60 lb), or 133.3 ÷ 27 ≈ 4.9, so order about 5 cu yd of ready-mix. The concrete calculator does these conversions for any size you type in.

Bags of concrete for a 20×20 slab, by thickness

Counts below are whole 80-lb bags (0.6 cu ft each) for the 400 sq ft pad:

Read the ready-mix column: from the standard 4 in pour up to a heavy 6 in slab you're looking at roughly 5–7.5 cu yd. Even the thinnest row means ~195 bags — more than 15,000 lb of mix — so bagged concrete is impractical at any realistic thickness for a 20×20 footprint, and the ready-mix yardage is the number to budget around. Building something smaller? The bags of concrete per square foot guide gives the 4 in rule of thumb you can scale by hand.

Worked example: the standard 4-inch pour

Take the most common patio and pad spec, 20×20 at 4 in: area 400 sq ft, so 400 × 4 ÷ 12 = 133.3 cu ft of concrete. In bags, 133.3 ÷ 0.6 ≈ 222 bags of 80-lb mix — 222 × 80 lb is nearly 17,760 lb (about 9 tons) of dry mix to haul, plus a mixer, water and hours of batching. In ready-mix, 133.3 ÷ 27 ≈ 4.9, so order about 5 cu yd.

That contrast is the whole story: ready-mix costs more per cubic yard than bagged mix, but one truck places the slab in well under an hour with two helpers, while 222 bags means 200-plus individual batches. For a 20×20 pad we strongly recommend ready-mix at about 5 cu yd (4 in), 6.2 cu yd (5 in) or 7.4 cu yd (6 in). Most suppliers charge short-load fees below a few cubic yards, so this footprint comfortably clears the minimum.

Pouring tips that keep a 20×20 slab sound

• Compare ready-mix with bags on total cost. A bag is cheap; 222 of them, plus mixer rental and a full day of labor for two people, usually exceeds the delivered price of ~5 cu yd of ready-mix — and the truck does the mixing. Reserve bags for pads you can pour from a wheelbarrow, like the 10×10 slab.
• Build the base before you pour. Concrete on soft topsoil settles and cracks. Excavate to depth, then level and compact about 4 in of gravel or well-drained fill so the slab has uniform support, and grade the base so water runs off rather than pooling under the pad.
• Pour in one continuous pass. A 4 in pour is ~133 cu ft that starts setting as soon as it's mixed. Have your forms, reinforcement and finishing tools ready and enough helpers to screed and finish each truckload — stopping mid-pour creates a weak cold joint that shows up as a crack later.

Picking a different footprint? The how much concrete for a 10×10 slab guide walks a smaller pour end to end, and the bags of concrete per square foot guide is the shortcut for quick estimates. For odd shapes or mixed thicknesses, let the concrete calculator handle the conversions.

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