Concrete Slab Calculator
Enter the length, width and thickness of your slab to see how many cubic yards of concrete or bags of mix to buy. Below the calculator, a size table gives the answer for 17 common slabs, from an 8 × 8 ft pad to a 30 × 40 ft shop floor, at 4, 5 and 6 in thick.
How to use the concrete slab calculator
- Keep Slab selected and enter the length and width in feet. For an L-shaped slab, split it into two rectangles and work them out one after the other; for several identical pads, set Number of identical slabs.
- Enter the thickness in inches: 4 in is usual for patios, walks and shed bases, 5–6 in where cars or heavy loads go.
- Leave the waste allowance at 10% unless your base is very flat and well compacted; 5% is enough for a small formed pad.
- Read the cubic yards and the bag counts, then open Cost estimate to compare bags with a ready-mix quote.
The slab volume formula
A slab is a rectangular box, so its volume is length × width × thickness. The thickness is measured in inches, so divide it by 12 first to turn it into feet:
Volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12
Divide the cubic feet by 27 to get cubic yards, the unit ready-mix plants sell in. Add your waste allowance, then divide by the yield of one bag to count bags: about 0.60 ft³ for an 80 lb bag, 0.45 ft³ for 60 lb and 0.30 ft³ for 40 lb, according to the QUIKRETE and Sakrete product data. Bags are always rounded up.
Worked example: a 16 × 12 ft patio slab, 4 in thick
This is the slab loaded in the calculator above.
- Volume: 16 × 12 × 4 ÷ 12 = 64 ft³, which is 2.37 yd³ before waste.
- Add 10%: 64 × 1.10 = 70.4 ft³, or 2.61 yd³.
- Bags: 70.4 ÷ 0.60 = 117.3, rounded up to 118 bags of 80 lb mix. With 60 lb bags you need 157; with 40 lb bags, 235.
- Weight: 64 ft³ × 150 lb ≈ 9,600 lb of finished concrete, close to 5 tons.
At over 2½ cubic yards this slab is a ready-mix job for most people. Mixing 118 bags by hand takes a crew most of a day, and the first batches start to set before the last ones are placed.
How thick should a slab be?
Thickness changes your order more than anything else: going from 4 to 5 in adds 25% more concrete, and 6 in adds 50%. As a rule of thumb, patios, walkways, shed bases and hot-tub pads are poured 4 in thick (check the tub maker's requirement), while driveways, garage floors and slabs carrying heavy loads are usually 5–6 in, often with reinforcement.
Every slab also needs a firm, well-compacted base, usually several inches of crushed stone. Work that out with the gravel calculator. If the slab needs a permit, your local building department sets the minimum thickness, base and reinforcement, so ask before you build the forms.
Concrete slab size table: cubic yards and 80 lb bags at 4, 5 and 6 in
| Slab size | Area | 4 in: yd³ | 4 in: 80 lb bags | 5 in: yd³ | 5 in: 80 lb bags | 6 in: yd³ | 6 in: 80 lb bags |
|---|---|---|---|---|---|---|---|
| 8 × 8 ft | 64 ft² | 0.87 | 40 | 1.09 | 49 | 1.3 | 59 |
| 10 × 10 ft | 100 ft² | 1.36 | 62 | 1.7 | 77 | 2.04 | 92 |
| 10 × 12 ft | 120 ft² | 1.63 | 74 | 2.04 | 92 | 2.44 | 110 |
| 10 × 20 ft | 200 ft² | 2.72 | 123 | 3.4 | 153 | 4.07 | 184 |
| 12 × 12 ft | 144 ft² | 1.96 | 88 | 2.44 | 110 | 2.93 | 132 |
| 12 × 16 ft | 192 ft² | 2.61 | 118 | 3.26 | 147 | 3.91 | 176 |
| 12 × 20 ft | 240 ft² | 3.26 | 147 | 4.07 | 184 | 4.89 | 220 |
| 12 × 24 ft | 288 ft² | 3.91 | 176 | 4.89 | 220 | 5.87 | 264 |
| 14 × 14 ft | 196 ft² | 2.66 | 120 | 3.33 | 150 | 3.99 | 180 |
| 16 × 16 ft | 256 ft² | 3.48 | 157 | 4.35 | 196 | 5.21 | 235 |
| 16 × 20 ft | 320 ft² | 4.35 | 196 | 5.43 | 245 | 6.52 | 294 |
| 20 × 20 ft | 400 ft² | 5.43 | 245 | 6.79 | 306 | 8.15 | 367 |
| 20 × 24 ft | 480 ft² | 6.52 | 294 | 8.15 | 367 | 9.78 | 440 |
| 24 × 24 ft | 576 ft² | 7.82 | 352 | 9.78 | 440 | 11.73 | 528 |
| 24 × 30 ft | 720 ft² | 9.78 | 440 | 12.22 | 550 | 14.67 | 660 |
| 30 × 30 ft | 900 ft² | 12.22 | 550 | 15.28 | 688 | 18.33 | 825 |
| 30 × 40 ft | 1,200 ft² | 16.3 | 734 | 20.37 | 917 | 24.44 | 1,100 |
Cubic yards and bags include 10% waste. 80 lb bags at 0.60 ft³ each (45 per yd³ before waste). Above about 1 yd³, compare a ready-mix quote.
Slab tips before you pour
- Measure the base, not just the forms. A slab that is 4 in deep at the edges but 5 in over a dip uses a quarter more concrete in that spot. Grade and compact the base, then check the depth in several places with a string line.
- Thickened edges add up. Many slabs have a deeper perimeter or a thickened strip under a wall. Work that part out as a footing with the footing calculator and add it to the slab total.
- Ask the plant about minimum orders. Ready-mix plants often charge extra for small loads and have their own ordering increments. Give them the cubic yards including waste.
- Pour in one go. Running out halfway leaves a cold joint, a weak line where fresh concrete meets concrete that has already started to set. That is why the waste allowance exists.
- Check the diagonals. Before you order, measure both diagonals of the forms: if they match, the slab is square and the calculator's area is right.
Frequently asked questions
How many yards of concrete do I need for a 20×20 slab?
At 4 in thick, a 20 × 20 ft slab is 133.3 ft³, or 4.94 yd³. With a 10% allowance, order about 5.43 yd³. At 6 in thick it is 7.41 yd³ before waste, or 8.15 yd³ with 10%.
How many bags of concrete do I need for a 10×10 slab?
With a 10% allowance, a 10 × 10 ft slab needs 62 bags of 80 lb mix at 4 in thick, 77 bags at 5 in and 92 bags at 6 in. That is over a cubic yard, so compare the cost with a ready-mix delivery.
How much concrete is in a 12×12 slab?
At 4 in thick, a 12 × 12 ft slab is 48 ft³, or 1.78 yd³. Add 10% and you need about 1.96 yd³, or 88 bags of 80 lb mix.
Can I pour a slab with bagged concrete?
Yes, but it is hard work above about 1 yd³ (45 bags of 80 lb mix). A small pad, a set of steps or a few post holes are easy bag jobs; a full patio or garage floor is usually easier and more even with ready-mix and enough helpers.
How do I calculate an L-shaped or odd-shaped slab?
Split it into rectangles, work out each one and add them together. A round section can be done with the Circle option on the concrete calculator. Keep the same thickness and waste for every piece.
More concrete calculators
- Concrete CalculatorSlabs, footings, sonotubes, walls, steps and post holes: cubic yards, 40/60/80 lb bags and cost.
- Concrete Slab Cost CalculatorPrice a slab with bags or ready-mix using your local prices.
- Concrete Footing CalculatorStrip and pad footings: cubic yards and bags per footing or per run.
- Sonotube CalculatorConcrete per tube for 6–24 in form tubes, and bags for every tube.
- Concrete Steps CalculatorConcrete for solid steps on grade, with an optional top landing.
- Fence Post Concrete CalculatorBags of concrete per post for 4×4 and 6×6 posts by hole size.
- Concrete Bags CalculatorHow many 40, 60 or 80 lb bags make a yard, and how many per pallet.
Related calculators
- Gravel CalculatorCubic yards and tons of gravel, pea gravel or crushed stone, with bags and cost.
- Paver CalculatorPavers for a patio or path, plus gravel base, bedding sand and edging length.
- Concrete Block Calculator8×8×16 blocks for a wall, minus doors and windows, plus mortar bags and optional core fill.
- Paver Base CalculatorGravel base and bedding sand for a patio, walk or driveway, by use.