Slope Calculator: Percent Gradient to Degrees and 1 in N Ratio (Road, Ramp, Drain)

Slope & Gradient Calculator

Convert between percent, degrees and 1 in n, or find slope from rise and run.

Rise (vertical)

Run (horizontal distance)

Unit

Gradient

Angle

1 in

Horizontal length to apply it to (optional)

Free tool by EngineeringNepal.com.np · Results are for guidance; check critical figures with a qualified engineer.

Slope, gradient and ratio: the same thing written three ways

Engineers in Nepal meet slopes on almost every job: road profiles, drain and sewer falls, roof pitch, ramps at school and hospital entrances, and cut slopes on hill roads. Drawings may give the slope as a percentage, an angle in degrees, or a ratio such as 1 in 40 or 1.5H : 1V. This calculator converts between all of them and shows how much the ground rises over a given distance.

Formulas

Gradient (%) = rise ÷ horizontal run × 100
Angle = tan⁻¹(rise ÷ run), and rise ÷ run = tan(angle)
1 in n: n = run ÷ rise. A 1 in 12 ramp rises 1 m for every 12 m along the ground.
H : V is used for embankment and cut slopes. 1.5 : 1 means 1.5 m horizontal for every 1 m vertical.
Length along the slope = run × √(1 + gradient²)

Percent is not the same as degrees

A 100% slope is 45 degrees, not 90. At small slopes the numbers look similar in shape but differ: 10% is about 5.7 degrees. Mixing the two is a common mistake when reading road and roof drawings.

Typical values to keep in mind

House drains and sewers are usually laid with a small fall such as 1 in 40 to 1 in 100, depending on pipe size.
Wheelchair ramps are commonly limited to about 1 in 12 (8.3%).
Hill roads often have gradients of several percent, with steeper short sections where the terrain forces it. Use the design standard that applies to your road class.
Cut and fill slopes are chosen from the soil or rock type; loose soil needs flatter slopes than rock.

These figures are general guidance. Always follow the standard or code that applies to your project.

Frequently asked questions

How do I find the slope between two levels?

Subtract the two reduced levels to get the rise, and divide by the horizontal distance between the points. The levelling calculator gives the reduced levels from your staff readings.