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Slope and grade calculator

Enter rise and run in the same units and the calculator returns the percent grade, the angle in degrees, the run-to-rise ratio, and the equivalent inches per foot. Use it to check site drainage, a ramp, a swale, or a parking slope, then confirm the number against the governing code.

Worked example

An accessible ramp rises 1 in over 12 in of run. What is the grade and angle?

  • Rise1 in
  • Run12 in
  1. Percent grade = rise ÷ run × 100 = 1 ÷ 12 × 100 = 8.33%.
  2. Angle = arctan(1 ÷ 12) = 4.76°, a 12:1 ratio.

1:12 is 8.33% (4.76°) — the ADA maximum running slope for a ramp. Flatter is always easier to use.

Change the numbers in the calculator above to run your own.

anvilfield.com/calculators/slope-grade-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.

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Slope and grade FAQ

How much should the ground slope away from a house?

The ground should fall away from the foundation, with the common code rule a drop of at least 6 in within the first 10 ft, about 5 percent, per the IRC at R401.3. Impervious surfaces near the building can slope about 2 percent. Over-build the fall against settlement and confirm the adopted code edition.

What is the minimum slope for drainage?

Lawn and planted ground wants a minimum of about 2 percent, which is 1/4 in of fall per foot, to sheet water reliably. Hard paving can run flatter, around 1 percent, because it is smooth. Below those, soft ground ponds. These are common practice targets, so the civil drawings and local grading standard govern the job.

What is a French drain and how much slope does it need?

A French drain is a fabric-wrapped, gravel-filled trench with a perforated pipe that carries subsurface and surface water underground to an outlet. The pipe needs continuous fall, commonly at least 1 percent, which is 1/8 in per foot. Without slope, fabric, and a daylight outlet it is just a buried trench that holds water.

How do you fix a yard that ponds?

Find the true low and high points with a level, then re-grade to a positive slope of about 2 percent toward a lower outlet, or set a catch basin and pipe it out. Extend downspouts away from the low spot, and amend compacted clay so it drains. A French drain helps only with a real fall and outlet.

Why is my downspout causing a wet basement?

A downspout discharging at the foundation pours the whole concentrated roof load into the soil next to the wall, which is the top cause of a wet basement. Run an extension or a buried solid line to discharge at least 4 to 6 ft away, ideally near 10 ft, onto positive grade, and never tie it into the footing drain.

More in the Drainage, grading, and slope field guide.