How to use the concrete patio calculator calculator
- 1
Measure the patio outline
Measure to the inside of the forms, since that is the space concrete will fill. For a round patio, measure the diameter through the center. If the patio is already marked out, measure the marked area rather than the drawing.
- 2
Split irregular patios into sections
An L-shaped patio, a patio with a separate landing at the door, or one with a rounded corner is easier and more accurate as two or three sections than as one averaged shape.
- 3
Enter the thickness
Use the thickness from your plan. This calculator does not suggest a patio thickness, because that depends on what the patio will carry, the subgrade, and local requirements.
- 4
Add an allowance
Patios are often poured over a subgrade that has been graded by hand, which is rarely perfectly flat. An allowance percentage of your own choosing covers that and any spillage.
- 5
Compare bags and ready-mix
Enter the bag yield and price per bag, and your quoted price per cubic yard, to see both totals. A patio is often right at the point where the two routes are worth comparing directly.
Formula
Patio cubic yards = (square feet x thickness in inches) / 324
Equivalent to area times thickness in feet, divided by 27. For a circular patio, the area is pi times the radius squared.
Worked example
A rectangular patio 16 feet by 20 feet, poured 4 inches thick
- 1Find the area: 16 ft x 20 ft = 320 square feet.
- 2Apply the shortcut: 320 x 4 / 324 = 3.95 cubic yards.
- 3In cubic feet: 320 x 0.3333 = 106.67 cubic feet.
- 4At a 0.6 cubic foot bag yield: 106.67 / 0.6 = 177.8, rounding up to 178 bags.
The patio needs about 3.95 cubic yards, or 106.67 cubic feet, which is 178 bags at that example yield.
Worked examples
Circular patio
A round patio 12 feet in diameter at 4 inches thick
- 1Radius: 12 / 2 = 6 ft.
- 2Area: pi x 6^2 = 113.10 square feet.
- 3Convert: 113.10 x 4 / 324 = 1.40 cubic yards.
- 4In cubic feet: 113.10 x 0.3333 = 37.70 cubic feet.
The circular patio needs about 1.40 cubic yards. Note that it uses roughly a third of the concrete of the 16 by 20 rectangular patio despite not looking much smaller.
Patio plus a connecting walkway
The 16 ft x 20 ft patio with a 15 ft x 4 ft walkway, both 4 inches thick
- 1Patio area: 16 x 20 = 320 square feet.
- 2Walkway area: 15 x 4 = 60 square feet.
- 3Total: 320 + 60 = 380 square feet.
- 4Convert once: 380 x 4 / 324 = 4.69 cubic yards.
Together they need about 4.69 cubic yards. The walkway adds 0.74 cubic yards, which is easy to forget if you only measure the patio.
How to measure a patio
Measure to the inside face of the forms, since the forms define the volume that will be filled. If the patio is staked out but not yet formed, measure the marked outline and remember that the final formed dimensions may differ slightly.
Take each dimension in more than one place. Patios are often laid out by eye against a house wall, and a patio that is 16 feet at one end and 16 feet 4 inches at the other is common. Using the larger figure is the safer choice for ordering.
For a circular patio, measure the diameter through the center rather than estimating it from the radius at one point. A small error in the diameter has an outsized effect, because the area depends on the square of the radius.
Rectangular and circular patios compared
The two shapes behave differently in a way that surprises people. A circular patio 12 feet across looks comparable in size to a 12 by 12 rectangular one, but the circle is 113 square feet against the rectangle's 144, about 21 percent less.
That is simply what pi does: a circle inscribed in a square covers about 78.5 percent of it. If you are choosing between shapes on a fixed budget, a round patio buys you noticeably less usable area for the same footprint, or noticeably less concrete for the same nominal dimension.
It also means the diameter is worth measuring carefully. Going from 12 feet to 13 feet across raises the area from 113 to 133 square feet, an 18 percent jump from an extra foot.
Patios with several sections
Patios frequently have more than one part: a main area, a walkway to the door, a landing at a step, or an extension around a corner of the house. Each is straightforward on its own and awkward to average into a single rectangle.
Measure each part, enter it as its own section, and let the calculator total the areas before converting. Rounded corners can be handled as a rectangle plus a quarter circle, and an angled edge as a rectangle plus a triangle.
The walkway is the part most often left out, because it feels like a detail. In the example above it added 0.74 cubic yards to a 3.95 cubic yard patio, close to a fifth again. That is not a rounding error.
Steps are the other frequent omission, and they are harder to measure than they look. A step is not a flat area but a stack of solid blocks, so each tread has to be calculated as its own small slab at its own depth, and a set of three steps can quietly hold as much concrete as several square yards of patio. If your patio has a step down to the lawn or up to a door, measure each riser and tread separately and add them as sections rather than treating the whole step as part of the patio area.
Bags or ready-mix for a patio
Patios sit in the range where the choice is genuinely open. A small patio is a comfortable bag job. A larger one starts to involve a lot of mixing: the 16 by 20 example above works out to 178 bags at a 0.6 cubic foot yield, which is a serious amount of work to mix and place before the first batches begin to set.
Ready-mix removes that problem but brings a delivery charge and requires truck access to the site, which is not always available in a back garden. The calculator prices both routes from figures you enter so you can see the material difference, but the labor and access questions are yours to weigh.
One point worth thinking through: a patio poured in batches over several hours may show joints between them. Whether that matters for your patio, and whether the design allows for it, is a question for whoever is placing it rather than an arithmetic one.
Common mistakes
The usual problems on patio estimates:
- Leaving out the connecting walkway or the landing at the door.
- Measuring the outside of the forms rather than the inside face.
- Estimating a circular patio from a radius measured at one point rather than the diameter through the center.
- Assuming a round patio needs about the same concrete as a square one of the same nominal size.
- Choosing bags for a large patio without working out how many batches that means.
- Entering the thickness in inches without converting, or using a thickness borrowed from another project.
- Ordering the exact calculated figure over a hand-graded subgrade with no allowance.
Patio area to concrete cubic yards by thickness
Cubic yards for common patio sizes at a range of thicknesses. Thickness columns are reference values, not recommendations.
| Area | 3 in | 4 in | 5 in | 6 in |
|---|---|---|---|---|
| 64 sq ft | 0.593 | 0.79 | 0.988 | 1.19 |
| 100 sq ft | 0.926 | 1.23 | 1.54 | 1.85 |
| 144 sq ft | 1.33 | 1.78 | 2.22 | 2.67 |
| 200 sq ft | 1.85 | 2.47 | 3.09 | 3.7 |
| 256 sq ft | 2.37 | 3.16 | 3.95 | 4.74 |
| 320 sq ft | 2.96 | 3.95 | 4.94 | 5.93 |
| 400 sq ft | 3.7 | 4.94 | 6.17 | 7.41 |
| 500 sq ft | 4.63 | 6.17 | 7.72 | 9.26 |
Bag-yield math: bags per cubic yard
Mathematical bag-yield examples showing 27 cubic feet divided by each yield. Use the yield printed on the actual concrete bag or supplier documentation.
| Bag volume | Bags per cubic yard | Whole bags for 1 cubic yard |
|---|---|---|
| 0.33 cu ft | 81.82 | 82 |
| 0.45 cu ft | 60 | 60 |
| 0.6 cu ft | 45 | 45 |
| 0.75 cu ft | 36 | 36 |
| 1 cu ft | 27 | 27 |
Frequently asked questions
- How much concrete do I need for a 12 x 12 patio?
- A 12 by 12 foot patio is 144 square feet. At 4 inches that is 1.78 cubic yards, or 48 cubic feet. At a 0.6 cubic foot bag yield that is 80 bags. Use the thickness your plan specifies.
- How many bags of concrete for a patio?
- Work out the volume in cubic feet, divide by the yield printed on your bag, and round up. A 320 square foot patio at 4 inches is 106.67 cubic feet, which is 178 bags at a 0.6 yield or 237 bags at a 0.45 yield.
- How thick should a concrete patio be?
- This calculator does not prescribe a thickness. It depends on what the patio will carry, the subgrade, the reinforcement, and any local requirements, so take it from your plan or your contractor.
- Is a round patio cheaper than a square one?
- For the same nominal dimension, yes, because a circle covers about 78.5 percent of the square that contains it. A 12 foot round patio is 113 square feet against 144 for a 12 by 12 square, so it uses about 21 percent less concrete and gives you less usable area.
- Should I use bags or order ready-mix for a patio?
- It depends on the size, your site access, and how many people are working. A small patio is comfortable with bags, while a large one can run to well over a hundred bags, which is a lot of mixing. The calculator prices both from your own figures so you can compare.
- Do I need to include the walkway?
- Yes, if it is being poured at the same time. Add it as its own section. In the worked example above a 15 by 4 foot walkway added nearly a fifth again to the patio volume.
Patio volume is calculated from your measured sections and the thickness you enter, converted at exactly 27 cubic feet per cubic yard. No thickness, mix, or code requirement is prescribed, and bag yields and prices are yours to supply. Concrete calculations estimate material quantity from geometry and user-entered values. Confirm mix design, thickness, reinforcement, code requirements, bag yield, delivery, and final order quantity with the appropriate supplier or professional.
Results are planning estimates. Last reviewed 2026-07-28.