Drainage Slope Calculator

What slope do I need for drainage?

A drainage slope calculator compares how far the ground or pipe drops with how far it travels. Divide the rise in inches by 12 to get feet, divide by the run in feet, and multiply by 100 for a percent grade. A 6-inch drop over 25 ft is a 2% slope, or 1.15°. Drain pipe usually needs at least 1/8 inch of fall per foot (1.04%), and soil next to a house is commonly graded at 5% for the first 10 ft.

Enter the rise in inches and the run in feet. The calculator returns percent slope and the angle in degrees.

The formula in plain words

• Rise in feet = rise in inches ÷ 12
• Percent slope = rise in feet ÷ run × 100
• Angle = arctangent(rise in feet ÷ run)

Percent slope is the number in most drainage guidance. Degrees are small at drainage grades: 2% is barely more than 1°. Builders often think in inches per foot instead. Divide the rise in inches by the run in feet to get that.

Worked examples

All three match the calculator.

1. Patio pitched away from the house, 6 in over 25 ft

• Rise: 6 ÷ 12 = 0.5 ft
• Slope: 0.5 ÷ 25 × 100 = 2.00%
• Angle: 1.15°
• In inches per foot: 6 ÷ 25 = 0.24 in/ft, close to 1/4 inch

2. French drain pipe, 100 ft long at 1/8 in per foot

• Rise: 100 × 0.125 = 12.5 in, or 1.04 ft
• Slope: 1.04 ÷ 100 × 100 = 1.04%
• Angle: 0.60°

Dig the trench 12.5 inches deeper at the outlet than at the start, on top of the depth needed for gravel cover.

3. Grading at the foundation, 6 in over 10 ft

• Slope: 0.5 ÷ 10 × 100 = 5.00%
• Angle: 2.86°

Slope conversion table

Fall	Percent	Degrees	Common use
1/8 in per ft	1.04%	0.60°	Minimum for drain pipe
1/4 in per ft	2.08%	1.19°	Preferred drain pipe, patios
1/2 in per ft	4.17%	2.39°	Steeper swales
0.6 in per ft	5.00%	2.86°	Soil within 10 ft of a foundation
1 in per ft	8.33%	4.76°	Steep lawn
1.5 in per ft	12.50%	7.13°	Needs erosion control
2.4 in per ft	20.00%	11.31°	Hard to mow; plant or terrace

Turning a target slope into a drop

Planning works the other way too. To find how much fall you need, multiply the run in feet by the percent and divide by 100, then multiply by 12 for inches. A 40 ft swale at 2% needs 40 × 2 ÷ 100 = 0.8 ft, or 9.6 inches, of drop from end to end. Enter 9.6 and 40 into the calculator to confirm it reads 2.00%. If the site does not have that much fall, the water needs a drain or a different route.

How to measure rise and run on site

1. Set two stakes, one at the high point and one at the low point.
2. Run a string between them and level it with a line level.
3. Measure the run along the level string, in feet.
4. Measure the rise as the difference between the string-to-ground distances at each stake, in inches.
5. For a pipe, measure the trench bottom, not the ground surface. Ground and trench rarely slope the same.

A laser level or a builder’s level and rod is faster over long distances and avoids string sag.

Common mistakes

• Mixing inches and feet. Enter rise in inches. Entering 0.5 for a 6-inch drop gives a slope 12 times too small.
• Grading to the soil but not the finished surface. Mulch and sod change the final grade.
• Back-pitch at the outlet. A drain that slopes the right way for 90 ft and the wrong way for the last 10 ft holds water.
• Assuming steeper is always better. Steep bare soil erodes and clogs the drain it feeds.
• Ignoring where the water goes. The slope must end at a safe outlet, not a neighbor’s lot.

Related calculators

Budget the trench with the French drain cost calculator and the stone with the French drain gravel calculator. For a full yard regrade, the land grading cost calculator prices the dirt work. Where drainage affects a foundation, retaining wall, or neighbor, confirm the plan with a civil engineer or your local building department.