Slab concrete calculator
Anything flat, poured on grade.
Decimals fine. Bag counts round up.
At 20 × 20 ft and 4 inches you need
4.94 cubic yards
133.3 cubic feet · 18,670 lb of concrete · order 5.43 yd³ with the 10% waste allowance
Order ready-mix. At 4.94 yd³ you are 3.7 times past the point where bagging pays. That is one truck, one morning — against 223 bags to lift, mix and place before the first batch sets.
Two rules, each where it applies. Bag counts carry no waste allowance — a partial bag is not a thing you can buy, and an unopened one goes back — so the headline count is the volume and the figure beside it is the same count with waste. The ready-mix figure carries its allowance inside because a truck will not come back for a quarter yard.
The inch that costs $240
Thickness runs from the top of the compacted gravel to the finished surface. Measure from the dirt and you count the base as concrete: on this pour, one inch of that error is 1.23 extra cubic yards — about $240 of ready-mix, or 56 bags nobody needed. Set a stake to the finished height before you order.
The arithmetic, in the open
How this page gets its numbers
- Volume, cubic feet
- 20 × 20 × (4 ÷ 12) = 133.3 ft³
- Cubic yards
- 133.3 ft³ ÷ 27 = 4.94 yd³
- 80 lb bags
- ceiling(133.3 ÷ 0.60 ft³ a bag) = 223 bags, no waste allowance; with 10% waste, 245
- 60 lb bags
- ceiling(133.3 ÷ 0.45 ft³ a bag) = 297 bags, no waste allowance; with 10% waste, 326
- Weight
- 133.3 ft³ × 140 lb per ft³ = 18,670 lb
- Order allowance
- 4.94 yd³ plus 10% = 5.43 yd³ ordered
- Truck loads
- ceiling(5.43 yd³ ordered ÷ 10 yd³ a load) = 1 — a full truckload is typically about 10 cubic yards
Bag counts round up. A partial bag is not a thing you can buy, so a calculated 222.2 bags is 223 bags. where to put the tape.
Slab presets
The 8 sets people ask for most
| Slab | Area ft² | Cubic yards | 80 lb bags→ with 10% waste | Buy as |
|---|---|---|---|---|
| 10 × 10 ft | 100 | 1.23 | 56→62 | Bags |
| 20 × 20 ft | 400 | 4.94 | 223→245 | Ready-mix |
| 30 × 40 ft | 1,200 | 14.81 | 667→734 | Ready-mix |
| 20 × 30 ft | 600 | 7.41 | 334→367 | Ready-mix |
| 30 × 30 ft | 900 | 11.11 | 500→550 | Ready-mix |
| 12 × 12 ft | 144 | 1.78 | 80→88 | Ready-mix |
| 40 × 60 ft | 2,400 | 29.63 | 1,334→1,467 | Ready-mix |
| 30 × 50 ft | 1,500 | 18.52 | 834→917 | Ready-mix |
What a bag holds
Every bag size, and what it yields
| Bag size | Yield ft³ | Bags a yard |
|---|---|---|
| 40 lb bag | 0.300 | 90 |
| 50 lb bag | 0.375 | 72 |
| 60 lb bag | 0.450 | 60 |
| 80 lb bag | 0.600 | 45 |
| 90 lb bag | 0.675 | 40 |
A different shape
The other 7 calculators
Asked about this shape
Questions
Where exactly do I measure thickness from?
From the top of the compacted gravel base to the finished surface of the slab. The gravel is not concrete, and the mistake is expensive: one inch of it counted as concrete on a 20 × 20 ft slab is 1.23 cubic yards. Compact the base, set a stake to the finished height, and measure down from that.
How many 80 lb bags are in a cubic yard?
45, and you buy 45. An 80 lb bag of mix yields about 0.60 cubic feet once water is in it, and a cubic yard is 27 cubic feet. A 60 lb bag yields about 0.45 cubic feet, so 60 of those make a yard. Every bag count on this site is rounded up, because a store will not sell you nine tenths of a bag.
Bags or ready-mix — where is the line?
Around 60 bags of 80 lb, which is 36 cubic feet or 1.33 cubic yards. Under that you can mix it in a day with a rented mixer and one helper, and you pay only for what you open. Over it you are buying several pallets, handling two tons or more by hand, and racing the set on the first batch you placed. Some plants charge a short-load fee below about 3 yards, so ask when you book.
How thick does a slab need to be?
Four inches over a compacted base for a patio, shed floor or a car. Five to six inches where a loaded truck, an RV or a hoist will sit, with reinforcement. Thicker concrete is not a substitute for a base that was never compacted — that slab cracks at whatever thickness you pour it. Your local code decides, not this page.
How much extra should I order?
10 percent, and where it appears depends on what you are buying. Ready-mix carries it inside the ordered figure, because a truck will not come back for a quarter yard. Bag counts do not: the headline count is the volume, and the plus figure sits beside it, because bags are bought one at a time and an unopened one goes back. Either way the allowance covers spillage, a base that is not perfectly flat and the thickness you gain at the edges of the form.
What is a mixer-truck load?
A full truckload is typically about 10 cubic yards, and that is the figure this site plans with. Ask when you book: drums vary, and a plant will tell you what it is sending. The load count here is worked from the ordered volume — the one that already includes the 10% allowance — because that is what actually has to fit on the truck.
Does this include rebar, forms or labour?
No. The price band is concrete delivered: your own price per yard on the ordered volume, taken 15% either side of the middle to cover what a quote actually varies by. Forms, gravel, mesh or bar, a pump if the truck cannot reach, and finishing are all separate. Get the plant to quote your postcode before you budget.
Bag yields: QUIKRETE Concrete Mix data sheet No. 1101. Price basis: Concrete Financial Insights weighted average selling price for 2025, escalated with U.S. Bureau of Labor Statistics PPI series PCU327320327320 (2026-07). Prices vary by region — enter your own quote above.
Updated 2026-08-25