How Much Concrete for a 12x20 Patio? (2026 Guide)
How much concrete for a 12x20 patio? A 4-in-thick slab is 80 cu ft ≈ 3 cubic yards — about 134 bags of 80-lb mix. Confirm amounts with your supplier first.
Volume math for a slab
Two numbers drive the estimate: the slab's square footage and its thickness. The formula is:
Concrete (cu ft) = length (ft) × width (ft) × depth (in) ÷ 12
One cubic yard is 27 cu ft, so divide by 27 to convert. For bagged mixes, an 80-lb bag yields about 0.60 cu ft and a 60-lb bag about 0.45 cu ft — actual yields vary by mix, so check the bag label. Punch your dimensions into the concrete calculator to skip the arithmetic.
Worked example: a 4-in-thick 12×20 patio
Walk the 12×20 (240 sq ft) slab through the formula:
1. Area. 12 × 20 = 240 sq ft.
2. Thickness in feet. 4 in ÷ 12 = 0.333 ft.
3. Volume. 240 × 0.333 = 80 cu ft.
4. Ready-mix. 80 ÷ 27 ≈ 2.96 → order 3 cu yd. Ready-mix is billed in whole cubic yards, so round up — never down — and confirm the pour with your supplier.
5. Bags. 80 ÷ 0.60 ≈ 134 bags of 80-lb mix; at 60-lb, 80 ÷ 0.45 ≈ 178 bags.
Mixing about 134 bags by hand — over 10,000 lb of dry mix — isn't realistic for one pour. For a slab this size, rent a powered mixer with helpers or, better, order a ready-mix truck; both give more consistent strength than bag-by-bag hand mixing.
12×20 patio concrete reference
Volumes use length × width × depth in ÷ 12, and ready-mix amounts round up to whole yards:
| Slab size (ft) | Area (sq ft) | Thickness (in) | Volume (cu ft) | Ready-mix to order |
|---|---|---|---|---|
| 12 × 20 | 240 | 3.5 | 70 | 3 cu yd |
| 12 × 20 | 240 | 4 | 80 | 3 cu yd |
| 12 × 20 | 240 | 5 | 100 | 4 cu yd |
| 10 × 20 | 200 | 4 | 66.7 | 3 cu yd |
| 12 × 16 | 192 | 4 | 64 | 3 cu yd |
Read the 4-in row: 80 cu ft ≈ 2.96 cu yd, but a 3 cu yd order is the minimum you'd book. All figures here are estimates — your site's grade, subgrade and mix all shift the total, so verify against your actual measurements and confirm with your ready-mix supplier or the bag label.
Patio pour tips
- Compact a gravel base first — spread and tamp about 4 in of gravel so the slab sits on a firm, level, well-drained bed; soft or uneven ground is the top cause of cracked patios.
- Plan the joints — for a 12×20 rectangle, keep the slab about 4 in thick and cut control joints every 10 ft or so in each direction so random shrinkage cracks follow the scored lines instead.
- Ready-mix beats bags at this size — bagged mix suits small pads and repairs; at roughly 3 cu yd and 134 bags, a ready-mix delivery usually costs less per yard and pours in a single afternoon.
Pouring a different footprint? The same math covers a 10×10 slab, and fence-post holes follow the same length × width × depth volume approach per hole.
FAQ
How many cubic yards of concrete do I need for a 12x20 patio?
About 3 cubic yards at 4 in thick. The 240 sq ft slab holds 240 × 4 ÷ 12 = 80 cu ft, and 80 ÷ 27 ≈ 2.96 — so round up and order 3 cu yd, since ready-mix is delivered in whole yards. Confirm the exact pour with your supplier.
How many 80-lb bags of concrete does a 12x20 patio need?
About 134 bags at 4 in thick: 80 cu ft ÷ 0.60 cu ft per 80-lb bag ≈ 134. The same slab takes about 178 bags of 60-lb mix (0.45 cu ft each). Both counts assume the actual bag yield, so check your mix's label.
Should my patio be 4 or 5 inches thick?
4 in is enough for a residential patio poured over firm, well-drained ground. Step up to 5–6 in — with rebar or welded wire — anywhere vehicles will drive or park, and keep the slab on a compacted gravel base.
Need a different size?
Our free calculators give you the exact answer in seconds.