Concrete Calculator
Calculate how much concrete you need for a slab, patio or driveway in cubic yards, plus the number of 60 lb and 80 lb premix bags.
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How to Calculate
Multiply length × width (in feet) × thickness (in inches ÷ 12) to get cubic feet, then divide by 27 to convert to cubic yards.
An 80 lb bag of premix yields about 0.60 ft³ and a 60 lb bag about 0.45 ft³. Ready-mix trucks are usually worth it above roughly 1 cubic yard.
Add 5–10% for spillage, uneven subgrade and over-excavation.
See our methodology for how every calculator on this site is built.
Worked Example: A 12 × 16 ft Patio
Say you're pouring a back patio 12 ft by 16 ft at the standard 4 inches thick.
- Area: 12 × 16 = 192 ft²
- Thickness in feet: 4 ÷ 12 = 0.333 ft
- Volume: 192 × 0.333 = 64 ft³
- Cubic yards: 64 ÷ 27 = 2.37 yd³
- With 10% extra: 2.37 × 1.10 = 2.61 yd³
Ready-mix plants usually sell in quarter-yard steps, so you would order 2.75 yd³. If you tried to do this with bags, 64 ft³ × 1.10 = 70.4 ft³, and 70.4 ÷ 0.6 = 118 bags of 80 lb mix. That is over 9,400 lb of concrete to lift, mix and place before the first batch starts to set. For a pour this size, call the plant.
How Far a Cubic Yard Goes
One cubic yard is 27 ft³. Spread at different thicknesses it covers:
| Slab thickness | Area per cubic yard | Typical use |
|---|---|---|
| 3 in | 108 ft² | Overlay, light walkway (not recommended over soft soil) |
| 4 in | 81 ft² | Patios, sidewalks, shed floors |
| 5 in | 64.8 ft² | Driveways for cars, garage floors |
| 6 in | 54 ft² | Driveways with pickups or RVs |
| 8 in | 40.5 ft² | Heavy equipment pads |
Where Concrete Estimates Go Wrong
The subgrade is never flat
A form set at 4 inches over a base that dips half an inch in spots turns into a 4.5 inch slab in those spots. Across a 200 ft² patio, an average extra quarter inch adds about 0.15 yd³. That's why the waste allowance exists. Rake and compact the base to string lines and you can stay near 5%. Rough base, plan on 10%.
Thickened edges
Many slabs get a thickened edge or turned-down footing around the perimeter, often 8 to 12 inches deep and about a foot wide. Calculate that strip separately and add it. People forget it constantly and come up short at the last corner.
Short loads
Most ready-mix suppliers charge a short-load fee for orders under a minimum, often 3 to 5 yards. On a 2 yard job that fee can be a large part of the bill. Ask about it when you call, and ask whether a smaller local supplier or a trailer-mix rental is cheaper.
Running out mid-pour
Being 0.25 yd³ short on a truck pour is worse than being 0.25 yd³ over. A second truck means a cold joint and another delivery charge. Have a small form ready (a post footing, a step pad) to dump any leftover into.
Practical Tips
- Set forms, compact the base and wet it lightly before the truck arrives. Dry ground pulls water from the bottom of the slab.
- Cut control joints at a spacing of roughly 2 to 3 times the slab thickness in feet: about every 8 to 10 ft on a 4 inch slab, and at least a quarter of the slab depth.
- Don't add water at the truck to make it easier to spread. It weakens the mix and leads to dusting and cracking.
- Keep the slab damp or covered for the first several days. Slow curing is stronger curing.
Rough Cost
Ready-mix typically runs somewhere around $130 to $200 per cubic yard delivered in much of the US, plus short-load and fuel fees. An 80 lb bag of premix is usually $5 to $8. Bags look cheaper per bag but come out to roughly $250 to $350 per cubic yard once you count 45 of them, so they only make sense for small jobs or places a truck can't reach.
Sources
Common questions
- How much concrete do I need for a 10x10 slab?
- How many 80 lb bags of concrete for a 10x10 slab 4 inches thick?
- How much concrete do I need for a 12x12 patio?
- How much concrete for a 20x20 garage slab 6 inches thick?
- How many yards of concrete for a 24x24 garage?
Frequently Asked Questions
How thick should a concrete slab be?
4 inches is standard for patios, walkways and shed floors. Driveways that carry cars are usually 5–6 inches.
How many 80 lb bags make a yard of concrete?
About 45 bags of 80 lb premix make one cubic yard (27 ÷ 0.6).
Should I order ready-mix or use bags?
Bags are practical for small jobs under about 1 yd³. Above that, ready-mix delivery is faster and often cheaper.
How long before I can walk or drive on new concrete?
Foot traffic after about 24–48 hours in mild weather. Wait 7 days before parking a car and closer to 28 days for heavy trucks.
Do I need rebar or wire mesh in a 4 inch slab?
For patios and walks, fiber-reinforced mix or welded wire mesh is common. Driveways and slabs on poor soil benefit from #3 or #4 rebar on 18–24 inch centers.