Slope calculator: rise over run, percent and degrees
This slope calculator turns a rise and a run into every way a slope is written: percent grade, degrees, a 1:n ratio, a roof-style pitch in 12, fall per metre and per foot, and the sloped length along the surface. You can also start from a percentage, an angle or a ratio and it works out the rise over your run. A small diagram redraws as you type.
Builders, landscapers, plumbers and anyone planning a wheelchair ramp use it as a slope calculator for a ramp or a drain. Pick a check and it compares your slope with the US 2010 ADA Standards (1:12 ramps, 30 in rise per run, handrails over 6 in), England's Approved Document M ramp lengths, the ADA 1:20 walking-surface limit, or the minimum falls for drain pipes in the International Plumbing Code.
The maths runs in your browser. Code checks are a guide to the main numeric limits only — local amendments and the full text of each standard apply, so confirm with your building official.
How to use the Slope Calculator (Ramps & Drains)
- Choose what you know: Rise & run, Percent, Degrees or Ratio 1:n.
- Enter the rise and its unit (or the percent, angle or n), and the horizontal run with its unit. Rise and run may use different units — inches over feet is fine.
- Pick a check: an ADA ramp (new or existing site), an Approved Document M ramp, an ADA walking surface, a drain pipe with its size, or none.
- Read the percent grade, angle, ratio, pitch and sloped length, and the pass or fail note under the diagram.
- Copy the summary for your drawings or permit notes.
Slope Calculator (Ramps & Drains) features
Four ways in
Start from rise and run, a percentage, an angle or a 1 in n ratio.
Every slope format
Percent, degrees, 1:n, pitch in 12, mm per metre and inches per foot in one table.
Ramp checks
ADA 405 running slope, rise per run, landings and handrails; the existing-site table; Approved Document M flight lengths.
Drain falls
Minimum slope by pipe size from IPC Table 704.1, shown as inches per foot, percent and millimetres over your run.
Mixed units
mm, cm, m, inches and feet for rise and run independently.
Slope formulas
Slope = rise ÷ run. Percent grade = rise ÷ run × 100. Angle = arctan(rise ÷ run). Ratio 1:n has n = run ÷ rise. Pitch is the rise for 12 units of run. The sloped length is √(rise² + run²).
Worked example — a ramp rising 24 in over 24 ft (288 in): slope = 24 ÷ 288 = 0.0833, so the grade is 8.33%, the angle is arctan(0.0833) = 4.76° and the ratio is 1:12. The surface itself is 24.01 ft long. Note that percent and degrees are not the same thing: 100% is 45°, not 90°.
| Ratio | Percent | Degrees | Typical use |
|---|---|---|---|
| 1:48 | 2.08% | 1.19° | ADA maximum cross slope; ¼ in per ft drain fall |
| 1:20 | 5% | 2.86° | Steepest ADA walking surface; Approved Document M gentle slope |
| 1:15 | 6.67% | 3.81° | Approved Document M ramp up to 5 m long |
| 1:12 | 8.33% | 4.76° | Steepest new ADA ramp; Approved Document M up to 2 m |
| 1:8 | 12.5% | 7.13° | ADA limit even for existing sites (3 in rise max) |
| 1:1 | 100% | 45° | Slope equal to run |
Ramp rules this tool checks
- ADA 2010 (US): running slope no steeper than 1:12 (405.2); cross slope 1:48 (405.3); clear width 36 in (405.5); rise per run 30 in maximum (405.6); landings 60 in long (405.7.3); handrails where the rise exceeds 6 in (405.8). On existing sites with no space, 1:12–1:10 is allowed for a 6 in rise and 1:10–1:8 for a 3 in rise (Table 405.2).
- Approved Document M Volume 1 (England): ramp gradients between 1:20 and 1:12, with each flight no longer than 10 m at 1:20, 5 m at 1:15 and 2 m at 1:12 — the calculator uses the straight line in Diagram 2.1, which works out as (n − 10) metres.
- Drain pipes (IPC Table 704.1): at least ¼ in per foot (2.08%) for pipes 2½ in and smaller, ⅛ in per foot (1.04%) for 3–6 in, and 1/16 in per foot (0.52%) for 8 in and larger.
- Rules were checked against the US Access Board text, the gov.uk Approved Document M PDF and published IPC table text on 19 October 2026. For steps rather than a ramp use the stair calculator; for roof pitch and area use the roofing calculator; for trench and cut-and-fill volumes use the excavation calculator.
Slope Calculator (Ramps & Drains) FAQ
How do I calculate slope from rise over run?
Divide the rise by the run, using the same units for both. Multiply by 100 for percent, or take the arctangent for degrees. This slope calculator lets you enter rise and run in different units and converts them for you.
What slope does a wheelchair ramp need?
Under the ADA, a new ramp can be no steeper than 1:12 — 1 in of rise for every 12 in of run, or 8.33%. That means a 24 in rise needs 24 ft of ramp, split into runs of no more than 30 in rise each with level landings.
How do I convert slope percentage to degrees?
Degrees = arctan(percent ÷ 100). A 10% slope is 5.71°, 25% is 14.04° and 100% is 45°.
What is a 2% slope?
A rise of 2 units per 100 units of run — 20 mm per metre or about ¼ in per foot. It is a common fall for paving, patios and small drain pipes.
What slope should a drain pipe have?
The International Plumbing Code asks for at least ¼ in per foot for pipes up to 2½ in, ⅛ in per foot for 3–6 in pipes and 1/16 in per foot for 8 in and larger. Local codes may differ.
Is a 1 in 12 slope the same as 12%?
No. 1 in 12 is 1 ÷ 12 = 8.33%. A 12% slope is about 1 in 8.3.