How much concrete for a 20×20 slab?
A 20 × 20 ft slab 4 inches thick needs 4.94 cubic yards of concrete, or 5.43 cubic yards with 10% extra, so order about 5½ yards. In bags that would be 245 bags of 80 lb mix, which is a truck job.
How to work it out
- 20 ft × 20 ft = 400 sq ft
- 400 sq ft × 4 in ÷ 12 = 133 cu ft ÷ 27 = 4.94 cu yd
- + 10% extra = 5.43 cu yd = 147 cu ft
- 147 cu ft ÷ 0.6 cu ft per 80 lb bag = 245 bags
Four hundred square feet is a two-car parking pad, a large patio or a small shop floor. At this size bags stop being a real option for most people: the question is how many yards to order and how to place them before they set.
Adjust for your project
More options: Extra · Price
- Or bags of mix (incl. extra)
- 245 bags · 80 lb bag
- Concrete before the extra
- 4.94 cu yd (133 cu ft)
- Slab area
- 400 sq ft
- Weight of the concrete (≈ 145 lb/cu ft)
- 19,333 lb (10 tons)
- 245 bags is a lot of mixing. Compare a ready-mix delivery (ask about minimum loads and short-load fees) with the bags vs ready-mix calculator.
- Total to buy counts this as ready-mix. Switch “How will you buy it?” to bags if you are mixing it yourself.
- Plan control joints about 1 in deep every 6–8 ft (Quikrete’s guidance) so shrinkage cracks follow your lines.
20×20 slab by thickness (10% extra included)
| Thickness | Concrete to order | Concrete before the extra | Weight of the concrete (≈ 145 lb/cu ft) |
|---|---|---|---|
| 4 in | 5.43 cu yd | 4.94 cu yd | 19,333 lb |
| 5 in | 6.79 cu yd | 6.17 cu yd | 24,167 lb |
| 6 in | 8.15 cu yd | 7.41 cu yd | 29,000 lb |
| 8 in | 10.9 cu yd | 9.88 cu yd | 38,667 lb |
Ordering five and a half yards
With the extra, the slab needs 5.43 yards at 4 inches. Order 5.5 and you will finish with a few wheelbarrows to spare for a step pad or post holes — have a spot ready to use or dump it. A standard mixer truck carries well over that, so it is one delivery. If the slab is for vehicles, the thickness comes from your plans and local code; at 6 in the same footprint jumps to 7.41 yards before extra and 8.15 with it.
Bagged, 245 bags of 80 lb mix weighs close to ten tons. Nobody should plan to mix that by hand for one continuous slab, and stopping halfway leaves a cold joint across the middle.
Placing a slab this size
A 20 ft span is too wide to reach with a hand float, so plan on a bull float, a screed long enough to ride both side forms (or a center screed rail) and at least three people: one on the chute or wheelbarrow, one raking and one screeding. Divide the slab into panels with control joints about every 6–8 ft, which here means joints at roughly 6⅔ ft — a 3 × 3 grid of panels. A slab this size usually sits on 4 in or more of compacted stone; see how much gravel under a concrete slab for the base math. For a typical detached garage footprint, compare how many yards of concrete for a 24×24 slab.
Different size? Use the full Concrete Calculator.
What you’ll need
- Bull float with handleMid-range price
Smooths and levels a slab you can’t reach across, right after screeding. Rent or buy one for anything wider than about 6 ft.
See options on Amazon (opens in a new tab) - Aluminum screedMid-range price
A straight, light screed strikes off the concrete level with the forms. A straight 2×4 works too if it really is straight.
See options on Amazon (opens in a new tab) - Concrete finishing broomMid-range price
Drags a non-slip texture into a walkway or patio once the surface has stiffened.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How much concrete for a 20×20 slab 6 inches thick?
7.41 cubic yards before extra and 8.15 cubic yards with 10%. Order about 8¼ yards.
How many cubic yards of concrete for a 20×20 slab 5 inches thick?
6.17 cubic yards, or 6.79 cubic yards with 10% extra.
How many 80 lb bags of concrete for a 20×20 slab?
About 245 bags at 4 inches with 10% extra. At that count, order ready-mix.
Other common sizes
- How much concrete for a 10×10 slab — 1.36 cu yd with 10% extra (62 bags of 80 lb)
- How much concrete for a 12×12 slab — 1.96 cu yd with 10% extra (order 2 yd)
- How many yards of concrete for a 24×24 slab — 7.82 cu yd with 10% extra at 4 in
- How much concrete for a 30×40 slab — 16.3 cu yd with 10% extra at 4 in
- How many bags of concrete for a 4×4 slab — 10 bags of 80 lb (14 of 60 lb), 4 in thick
- How many bags of concrete for an 8×8 slab — 40 bags of 80 lb, 4 in thick
- How much concrete for a 10×12 shed — 1.63 cu yd with 10% extra; about 2 cu yd with a thickened edge
Sources
- Quikrete Concrete Mix (No. 1101) data sheet (opens in a new tab) — Yields: 40 lb ≈ 0.30, 50 lb ≈ 0.375, 60 lb ≈ 0.45, 80 lb ≈ 0.60, 90 lb ≈ 0.675 cu ft; unit weight ≈ 140 lb/cu ft; for pours 2 in thick or more; 6–9 pints of water per 80 lb bag; control joints every 6–8 ft; cure 5 days at 70 °F or above, 7 days at 50–70 °F; protect from freezing for 48 hours.
- NRMCA Concrete in Practice 31 — Ordering Ready Mixed Concrete (opens in a new tab) — Truck mixers carry 8 to 12 cubic yards (about 6 to 9 m³); order 4–10% more than the plan estimate; a loaded truck weighs about 80,000 lb; fresh concrete can cause severe chemical burns.
- NRMCA Concrete in Practice 2 — Scaling Concrete Surfaces (opens in a new tab) — Order air-entrained concrete (about 6% air) for exterior slabs exposed to freezing and de-icers; avoid de-icing chemicals in the first winter.
- Iowa DOT Instructional Memorandum 340 — Weight per cubic foot, yield and air content of concrete (opens in a new tab) — Normal-weight concrete is typically 140–150 lb per cu ft at 5 to 8 percent air.
- OSHA 3351 — Preventing Skin Problems from Working with Portland Cement (opens in a new tab) — Wet portland cement can cause caustic burns that may not hurt at first and in severe cases reach the bone.
- NIST Handbook 44, Appendix C — General Tables of Units of Measurement (opens in a new tab) — 1 cubic yard = 27 cubic feet; 1 cubic foot = 1,728 cubic inches.