Ramp gradient calculator

What a ramp slope calculator does

A ramp slope calculator turns a ramp's rise and horizontal run into its gradient, expressed as a ratio (1:12), a percentage (8.33%) and an angle (4.76°), plus the length measured along the ramp surface. It also works backwards: give it the rise and a target ratio or percentage and it tells you how much horizontal run you need.

The most common question it answers is how long a wheelchair ramp needs to be. Under the 2010 ADA Standards for Accessible Design, a ramp's running slope can be no steeper than 1:12, so every inch of rise needs at least 12 in (1 ft) of horizontal run. A 24 in rise needs 24 ft of run before you add landings.

The calculator does the geometry, then compares it with a reference you choose: the 2010 ADA Standards §405 (1:12, 30 in rise per run), UK Approved Document M (1:12 to 1:20 by going length), NZBC D1/AS1 (1:12) or your own limits. It schedules intermediate landings at that maximum and totals the handrail length, but it does not check clear width, cross slope, edge protection, top and bottom landings or local amendments, and it does not tell you whether a ramp complies. Use it to size a ramp, then check the design against the adopted code.

How to use the ramp gradient calculator

1. Choose Imperial or Metric. In imperial, rise is entered in inches and run in feet.
2. Choose what to Solve from: Rise and run (you know both), Rise and ratio (you know the target 1:x) or Rise and percent.
3. Enter the rise, measured vertically from the finished surface at the bottom to the finished surface at the top. This is not the length of the ramp.
4. Enter the horizontal run, the ratio denominator (12 for 1:12) or the percent slope, depending on the mode.
5. Pick an Accessibility reference: ADA 2010 Standards §405, UK Approved Document M, NZBC D1/AS1 or Custom limits. Custom and NZBC show fields for the maximum gradient and the maximum rise between landings; enter the intermediate landing length for all of them.
6. Read the horizontal run, ratio, percent, angle and sloping length, then whether the gradient is within the reference, the number of runs and intermediate landings, the total length including landings and the handrail length for both sides, with a section diagram and the cited source. Export PDF, CSV or SVG, or copy the results.

Worked example

Imperial: A 30 in rise over 30 ft of horizontal run is a 1:12 gradient, 8.33% slope and 30.10 ft sloping length. At the ADA 30 in maximum rise per run it needs one run and no intermediate landing.

Metric: A 0.5 m rise over 6 m horizontal run has a 1:12 gradient, 8.33% slope and 6.021 m sloping length.

ADA ramp slope requirements: the 1:12 rule

Section 405 of the 2010 ADA Standards for Accessible Design sets the geometry for ramps on an accessible route. The key numbers are summarized below for orientation. Read the full text and confirm which standard applies to your project; state accessibility codes and the IBC (which references ICC A117.1) can differ in detail.

For ramps that are part of a means of egress, the 2021 IBC also limits running slope to 1:12 (1012.2), allows up to 1:8 for other pedestrian ramps, and limits the rise of a single ramp run to 30 in (1012.4).

• Running slope: not steeper than 1:12, or 8.33% (405.2). Walking surfaces steeper than 1:20 (5%) must be designed as ramps (403.3).
• Rise: no more than 30 in for any single ramp run (405.6). A greater rise needs intermediate landings.
• Cross slope: not steeper than 1:48 (405.3).
• Clear width: at least 36 in between handrails (405.5).
• Landings: at the top and bottom of each run, at least 60 in long and at least as wide as the widest run leading to them. A landing where the ramp changes direction must be at least 60 × 60 in (405.7).
• Handrails: required where a ramp run rises more than 6 in (405.8).
• Existing sites: where space is limited, Table 405.2 allows slopes steeper than 1:12 but no steeper than 1:10 for a rise of up to 6 in, and steeper than 1:10 but no steeper than 1:8 for a rise of up to 3 in.

Ramp slope chart: ratio, percent and degrees

Ratios, percentages and angles are not interchangeable at a glance. A 1:12 ramp is 8.33%, not 12%, and less than 5°. Accessibility limits are written as ratios or percentages, so work in those and treat the angle as a check.

How long does a wheelchair ramp need to be?

At 1:12, the minimum horizontal run in feet equals the rise in inches. The sloping length is slightly longer than the run. Add 60 in (5 ft) landings at the top and bottom of every run, and plan an intermediate landing for every 30 in of rise.

Worked example: a ramp to a 42 in high porch

A front porch sits 42 in above the path. At 1:12 the ramp needs 42 ft of horizontal run, and because no single run may rise more than 30 in, it must be split into at least two runs. Two equal runs of 21 in rise each need 21 ft of run apiece. In the calculator, choose Rise and ratio, enter 21 in and 12, and it returns 21 ft of run and a sloping length of 21.07 ft.

Add a 60 in landing at the bottom, a 60 in landing between the runs and a 60 in landing at the top (the porch can serve if it is level and large enough). Laid out in a straight line, that is 42 + 5 + 5 + 5 = 57 ft overall. Turning the ramp back on itself at the middle landing (a switchback) shortens the footprint, and a landing where the direction changes must be at least 60 × 60 in.

That is the geometry. Handrails, edge protection, cross slope, surface and clear width all still apply, and many states and cities have their own accessibility codes.

Who it is for and when to use it

Method, formulas & assumptions

Gradient (%) = rise/run × 100. Ratio denominator = run/rise. Sloping length = √(rise² + run²). The selected reference sets a maximum gradient and either a maximum rise per run (ADA: 30 in) or a maximum going per run (UK Part M: 2 m at 1:12, 5 m at 1:15, 10 m at 1:20, interpolated). Runs = ceil(run ÷ maximum going) or ceil(rise ÷ maximum rise); landings = runs − 1; total length = run + landings × landing length; handrails = 2 × (sloping length + landings × landing length).

Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.

Common mistakes to avoid

• Entering the ramp's sloping length as the horizontal run. Measure the run in plan, level, from the bottom to the top of the ramp.
• Reading 1:12 as 12%. It is 8.33%; a 12% slope is about 1:8.3.
• Forgetting landings. Every run needs a landing at each end, and each landing adds at least 5 ft of length.
• Measuring the rise at the wrong points. Take it from the finished surface of the lower landing to the finished surface of the upper landing, including any threshold.
• Designing exactly to 1:12. Construction tolerances can make a ramp built to the limit slightly steeper than allowed, so a little shallower gives some margin.
• Assuming the geometry proves compliance. Width, handrails, edge protection, cross slope and surface are separate requirements.

Scope & limits

Geometry against entered or cited limits only (2010 ADA Standards §405, Approved Document M Volume 2 2015, NZBC D1/AS1). Top and bottom landings, handrail extensions, edge protection, crossfall and surface requirements are not evaluated; confirm with the adopted code.

Frequently asked questions

Further reading

• Accessible parking requirements
• ADA restroom dimensions
• Egress and corridor widths
• All drawing & geometry tools
• All free architecture & construction tools
• Harth resource library

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• Roof pitch & roof area calculator
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• Occupant load & egress worksheet
• Feet, inches & construction units
• Floor & wall area calculator