Concrete Calculator for Slabs, Patios & Pads
Enter the slab size and thickness. You'll get cubic yards for a ready-mix order and the exact number of 40, 60 or 80 lb bags, with a waste allowance included.
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- Feet or metric
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2 What to buy
Updates as you typeThat's over 1 cubic yard (about 62 bags of 80 lb mix). Mixing that by hand is a long, hard day. Get a quote for ready-mix delivery or rent a mixer.
| Slab area | 100 sq ft |
|---|---|
| Volume before waste | 33.33 cu ft |
| Volume with waste | 36.67 cu ft (1.36 yd³) |
| 40 lb bag (US) | 123 bags |
| 60 lb bag (US) | 82 bags |
| 80 lb bag (US) | 62 bags |
| 20 kg bag | 111 bags |
| 25 kg bag | 89 bags |
| 30 kg bag | 74 bags |
Shopping list
Quick answer
A 10 × 10 ft slab, 4 inches thick, needs 1.23 cubic yards (33.3 cu ft) of concrete. With 10% extra that's 62 bags of 80 lb mix, 82 bags of 60 lb or 123 bags of 40 lb. One cubic yard is 45 bags of 80 lb mix.
Concrete for common slab sizes
| Slab | 4" thick | 5" thick | 6" thick |
|---|---|---|---|
| 10 × 10 ft | 1.36 yd³ · 62 × 80 lb | 1.70 yd³ · 77 × 80 lb | 2.04 yd³ · 92 × 80 lb |
| 10 × 12 ft | 1.63 yd³ · 74 × 80 lb | 2.04 yd³ · 92 × 80 lb | 2.44 yd³ · 110 × 80 lb |
| 12 × 12 ft | 1.96 yd³ · 88 × 80 lb | 2.44 yd³ · 110 × 80 lb | 2.93 yd³ · 132 × 80 lb |
| 10 × 20 ft | 2.72 yd³ · 123 × 80 lb | 3.40 yd³ · 153 × 80 lb | 4.07 yd³ · 184 × 80 lb |
| 12 × 20 ft | 3.26 yd³ · 147 × 80 lb | 4.07 yd³ · 184 × 80 lb | 4.89 yd³ · 220 × 80 lb |
| 16 × 16 ft | 3.48 yd³ · 157 × 80 lb | 4.35 yd³ · 196 × 80 lb | 5.21 yd³ · 235 × 80 lb |
| 20 × 20 ft | 5.43 yd³ · 245 × 80 lb | 6.79 yd³ · 306 × 80 lb | 8.15 yd³ · 367 × 80 lb |
| 24 × 24 ft | 7.82 yd³ · 352 × 80 lb | 9.78 yd³ · 440 × 80 lb | 11.73 yd³ · 528 × 80 lb |
Cubic yards and 80 lb bags, each including 10% extra. Use the calculator above for your exact size.
How the slab calculation works
Concrete is sold by volume, so the math is length × width × thickness, with all three in feet. Convert the thickness from inches by dividing by 12, multiply, and you have cubic feet. Divide by 27 to get cubic yards, the unit ready-mix suppliers use.
Bagged mix is labelled by weight, but what matters is its yield once mixed with water. Standard concrete mix yields about 0.30 cu ft per 40 lb bag, 0.45 cu ft per 60 lb bag and 0.60 cu ft per 80 lb bag. The calculator divides your volume by those yields and rounds up. You can't buy part of a bag, and running short halfway through a pour is the worst outcome.
Worked example: a 10 × 10 ft patio, 4 inches thick
- Volume: 10 × 10 × (4 ÷ 12) = 33.3 cu ft
- Add 10% waste: 36.7 cu ft = 1.36 cubic yards
- 80 lb bags: 36.7 ÷ 0.6 = 62 bags (or 82 × 60 lb, or 123 × 40 lb)
- Ready-mix order: 1.5 yd³. Suppliers sell in quarter-yard steps.
62 bags is about 2.5 tons of material to lift, mix and place. That's why we flag anything over a yard as a ready-mix job.
Bags or ready-mix?
| Compare | Bagged mix | Ready-mix truck |
|---|---|---|
| Best for | Under ~1 yd³: small pads, steps, post footings | Over 1 yd³: patios, driveways, garage floors |
| Labor | Heavy. You mix every bag | Light on mixing, but you must place and finish fast |
| Cost watch-outs | Mixer rental, many trips to the store | Short-load fees on small orders, waiting-time charges |
Choosing the right thickness
- Walkways and patios: 4 inches on 4 inches of compacted gravel.
- Shed or hot tub pad: 4–6 inches. Use 6" under a hot tub, since a filled tub can weigh 3–5 tons.
- Driveways: 5–6 inches with rebar or wire mesh. Go thicker for heavy trucks.
- Garage floors: 4–6 inches. Check local code.
Tips that save a pour
- Measure the actual form depth in several spots. A 4" form on uneven ground is often 4½" in places, and that extra half inch adds about 12% to the concrete you need.
- Keep 1–2 spare bags. Most stores take unopened bags back.
- Space control joints 2–3 times the slab thickness in inches, read as feet: 8–12 ft apart for a 4" slab. They make the slab crack along straight lines instead of randomly.
Frequently asked questions
How many 80 lb bags of concrete are in a yard?
About 45. One cubic yard is 27 cubic feet, and an 80 lb bag yields about 0.6 cubic feet, so 27 ÷ 0.6 = 45 bags. With 60 lb bags it is 60, and with 40 lb bags it is 90.
How much concrete do I need for a 10×10 slab?
At 4 inches thick, a 10×10 slab needs 33.3 cubic feet (1.23 cubic yards). With 10% waste that is 62 bags of 80 lb mix or about 1.5 yards of ready-mix.
Should I add extra concrete for waste?
Yes. 5–10% covers uneven subgrade, spills and form bowing. Use the higher end if the ground isn't perfectly level or compacted.
Is it cheaper to buy bags or order ready-mix?
For small jobs under about one cubic yard, bags are usually cheaper because ready-mix suppliers charge short-load fees. Above that, ready-mix normally wins on cost and almost always on labor.
How many 80 lb bags of concrete are in a yard?
About 45. One cubic yard is 27 cu ft, and an 80 lb bag yields about 0.6 cu ft, so it takes 45 bags of 80 lb, 60 of 60 lb or 90 of 40 lb mix, before waste.
Last reviewed 2026-10-21. See how we calculate for our sources and assumptions.