Patio Slope Calculator
Patio fall from run length at 1 to 2 percent.
What it does
Patios must look flat and act sloped. Two percent falls only a quarter inch per foot, invisible to the eye, yet it moves rain off the surface and away from the foundation. Skipping that fall invites ponding, algae, freeze damage, and wet basements.
The tool above turns run into drop. A 12 ft run at 2% needs 2.88 in of fall, or 7.32 cm, at a ratio of 1 in 50. The metric mirror at 3.7 m needs 7.4 cm, or 2.91 in. Set that with stakes and a string line, because visual checks miss grades under half a percent.
Code and trade guidance agree near the house. The residential code calls for about 2% on impervious surfaces within ten feet of the foundation, while paver guidance allows 1% minimum on open fields with 2% preferred. Shade, texture, and uphill runoff all push toward the full 2%.
This page sizes one straight fall. It does not design swales, drains, steps, or multi direction crowns. Where the yard drops a lot, break the patio into terraces rather than one steep plane.
How to use
- Pick imperial (ft) or metric (m).
- Measure the run where water should drain.
- Pick 1, 1.5, or 2% slope.
- Read the required drop first.
- Read the converted drop and the ratio.
- Set stakes and a string line to that fall before pouring.
Formula
- run
- Horizontal run in the drain direction, in ft or m.
- slope
- 1, 1.5, or 2; 2 is the standard near foundations.
- ratio
- 100 / slope, such as 1 in 50 at 2%.
Choices are 1%, 1.5%, and 2%, with 2% the standard near the house. At zero run there is no fall; above about 3 to 4% furniture and walking suffer, so terrace instead.
Worked example
Example: 12 ft run at 2%
Defaults: imperial 12 ft run at 2%. Metric mirror is 3.7 m. Slope choices are 1, 1.5, and 2%.
- Inputs (imperial default): 12 ft run at 2%.
- Fall: 12 times 12 times 0.02 is 2.88 in.
- Metric: 2.88 times 2.54 is 7.32 cm.
- Ratio: 100 / 2 is 1 in 50.
- Metric mirror (3.7 m at 2%) gives 7.4 cm and 2.91 in. Tool shows the same.
Reference table
| Run at slope | Fall | Ratio |
|---|---|---|
| 12 ft at 2% | 2.88 in | 1 in 50 |
| 16 ft at 1% | 1.92 in | 1 in 100 |
| 3.7 m at 2% | 7.4 cm | 1 in 50 |
| 5 m at 1.5% | 7.5 cm | 1 in 67 |
| 10 ft at 2% | 2.4 in | 1 in 50 |
Limits
- One straight fall. Crowns and valleys need layout.
- Near house rules assumed. Open yard may allow 1%.
- No drains or soils. Clay and springs need extra work.
- Steep grades excluded. Terrace past about 3 to 4%.
Common mistakes
- Building level because it looks better.
- Sloping toward the house instead of away.
- Eyeballing instead of pulling a line.
- Ignoring uphill water that crosses the patio.
- Pouring one steep plane where steps belong.
Try it yourself
Quick checks. Answers sit tucked under each question.
16 ft run at 1%?
Fall is 16 times 12 times 0.01 is 1.92 in, or 4.88 cm. Ratio is 1 in 100.
5 m run at 1.5%?
Fall is 5 times 1.5 is 7.5 cm, or 2.95 in. Ratio is 1 in 67.
Questions
How is the required fall figured?
Fall = run times slope. Here 12 ft times 12 times 0.02 is 2.88 in.
What slope should a patio use?
2% is the common minimum near the house (about 1/4 in per ft). ICPI allows 1% on open paver fields, but 2% drains better.
What does 1 in 50 mean?
Ratio is 100 / percent. At 2% it is 1 in 50; at 1% it is 1 in 100.
Can I use more than 2%?
Yes, within reason. Textured pavers, shade, and runoff from uphill all favor the full 2%.
Is level ever acceptable?
No. Flat ponds even when it looks level. Always pull a string line or level rather than eyeballing.
What if the yard falls a lot?
Break into steps or terraces with walls and drains. Steep single planes hurt furniture and footing.