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Slope and Grade Calculator

Slope as percentage, degrees, ratio and fall per foot.

Your measurementsEvery assumption editable01

The vertical change.

The horizontal distance over which it changes.

SlopeCalculated live02

Result

5% grade

5% — 2.86°

Rise
12 in
Run240 in
20 ft
Grade
5%
Angle
2.862°
Ratio
1 : 20
Fall per foot
0.6 in/ft

Drainage is usually specified as a fall per foot rather than a percentage. A quarter inch per foot is roughly two per cent and is the common minimum for a surface intended to shed water.

Accessible ramps are conventionally limited to about 8.33 per cent, which is a 1:12 ratio. Steeper than that and a ramp stops being usable by the people it was built for, regardless of what it is called.

Percentage and degrees diverge as the slope steepens. They are close below about ten per cent and noticeably different above twenty, so it is worth being explicit which one a specification means.

The same slope gets specified four different ways depending on the trade. This converts between all of them — grade for site work, degrees for setting out, ratio for accessibility, fall per foot for drainage.

Why use this tool?What it does differently03

Why use this tool?

All four notations

Percentage, degrees, ratio and fall per foot — whichever your specification happens to use.

Drainage figures

Fall per foot is how drainage is actually specified, and a quarter inch per foot is the usual minimum.

Ramp guidance

Accessible ramps run around 8.33%, or 1:12. Steeper stops being usable.

Divergence noted

Percentage and degrees track closely below 10% and separate sharply above 20%.

How this worksThe method04

How this slope and grade calculator works

Rise divided by run gives the tangent of the slope angle. Multiplying by a hundred gives percentage grade; taking the arctangent gives degrees; inverting gives the run-per-unit-rise ratio.

Fall per foot is the rise divided by the run in feet, which is how drainage is specified on drawings — a quarter inch per foot rather than two per cent.

Percentage and degrees are close at shallow slopes and diverge as it steepens, because one is a ratio and the other an angle. At 10% they differ by about half a degree; at 50% by nearly five.

How to use itStep by step05

How to use it

  1. Step 1: Enter the rise

    The vertical change, in inches. Negative for a fall.

  2. Step 2: Enter the run

    The horizontal distance over which it happens, in feet.

  3. Step 3: Read the notation you need

    All four are shown; take whichever your specification uses.

  4. Step 4: Check against the requirement

    Drainage minimums and ramp maximums are both common constraints.

Example usageWorked figures06

Example usage

Minimum drainage fall
5 in over 20 ft is a 2.08% grade, 1.19°, roughly 1:48, and a quarter inch per foot — the standard minimum for shedding water.
An accessible ramp
10 in over 10 ft is 8.33%, 4.76°, exactly 1:12 — the conventional maximum for a ramp people actually use.
A steep driveway
36 in over 20 ft is 15%, 8.53°, about 1:6.7. Steep enough that long vehicles start grounding at the transitions.
Frequently asked questionsCommon questions07

Frequently asked questions

How do I convert percent grade to degrees?

Take the arctangent of the grade expressed as a decimal. A 10% grade is 5.71 degrees, not 10 — the two are close at shallow slopes and diverge as it steepens.

What slope do I need for drainage?

A quarter inch per foot, about two per cent, is the usual minimum for a surface intended to shed water. Less than that and small settlements create ponding.

What is the maximum slope for a ramp?

Around 8.33 per cent, or 1:12, is the conventional accessibility limit. Steeper ramps exist but stop being usable by many of the people they were built for.

What does a 1:12 slope mean?

One unit of rise for every twelve units of run. It is the same as 8.33 per cent, or 4.76 degrees.

Is grade the same as pitch?

The same underlying slope, different notation. Roofing uses pitch over a twelve inch run; site work uses percentage grade. This converts between them.

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