Ramp slope calculator
Show the working — with your numbers
This solves the triangle — rise, run, slope and length — against a slope ratio you enter. Maximum slopes, landing lengths, handrails and edge protection are set by the accessibility standard or building code that applies to your job; the verdict here only compares your ramp to the number you typed.
Type the rise and the slope ratio your job requires, and you get the run the ramp needs and the length of the sloped surface; type the run you actually have instead, and you get the slope you would build — as a 1:n ratio, a percentage and an angle — with a plain verdict on whether it meets the ratio you entered. Run is rise × n; the surface is the hypotenuse, √(rise² + run²).
Worked example
A porch 30 inches above the walk, and the job calls for 1:12. The run the ramp needs is 30 × 12 = 360 in, which is 30 feet. The surface along the slope is √(30² + 360²) = √(900 + 129,600) = 361.2 in — 30 ft 1 1/4 in of decking. The slope reads 1 ÷ 12 = 8.33%, or 4.76°. If you only have 24 feet (288 in) of yard, the ramp you can build is 288 ÷ 30 = 1:9.6 — run divided by rise, never the other way — steeper than the 1:12 you typed, so the tool says it does not meet it, and the answer is a switchback, a longer run or a different door.
In metric: a 750 mm rise at 1:12 needs 750 × 12 = 9,000 mm of run — 9 m. The surface is √(750² + 9,000²) = √81,562,500 ≈ 9,031 mm. With only 7.2 m (7,200 mm) of yard, the ramp you can build is 7,200 ÷ 750 = 1:9.6 — steeper than 1:12, so it does not meet it.
On the job
Run first, then argue about the yard. The rise is fixed by the building; the ratio is fixed by the code; the run is what those two multiply to. Work that number out before anyone draws a ramp on the ground — most ramps that fail are laid out to fit the space and checked afterwards.
The surface is longer than the run. At 1:12 it is only a third of a percent longer, and nobody notices. At a 1:4 loading ramp it is 3.1% longer — √(1² + 4²) ÷ 4 = 1.031, so a 30 ft run wants 30 ft 11 in of decking (a 9 m run wants about 9.28 m). Order the material off the length line, not the run line.
Verdict, not permission. "Meets the ratio you entered" means your triangle is no steeper than the number you typed. It does not know whether that was the right number, whether a landing is due, or what the inspector will want at the top — read the standard, then type from it.
What went wrong?
Ramp layouts go wrong the same three ways: the ratio applied the wrong way round, the run confused with the surface length, and feet and inches mixed.
Quick referencefor ramps
- Run
- rise × n (for 1:n)
- Surface length
- √(rise² + run²)
- 1:n as a percent
- 100 ÷ n — 1:12 = 8.33% · 1:20 = 5% · 1:8 = 12.5%
- 1:n as degrees
- arctan(1 ÷ n) — 1:12 = 4.76° · 1:20 = 2.86° · 1:8 = 7.13°
- Run per inch of rise at 1:12
- 1 ft — a 30 in rise wants 30 ft
- Run per 100 mm of rise at 1:12
- 1.2 m — a 750 mm rise wants 9 m
- Steeper or flatter?
- a smaller n is steeper: 1:8 is steeper than 1:12
- Metric
- 1:12 is 1:12 in any unit — 600 mm rise wants 7,200 mm run
Mistakes to catchbefore you trust the number
- Measuring the rise from the ground to the top of the deck when the ramp lands on a threshold — rise is landing surface to landing surface.
- Typing 12 for a 1:12 ratio in a field that wants a percent, or 8.33 in a field that wants the ratio. One field takes both here: a bare number is read as the n in 1:n, and the page says under the box which way it read you and prints the percent beside it — check that line before you trust the verdict.
- Reading the ramp length as the run; the ramp surface is the hypotenuse and is a little longer than the horizontal run.
What an inspector looks forthe building inspector and, on a public building, the accessibility reviewer
On this part of the job
- The running slope of every segment, measured on the built ramp, against the maximum ratio your jurisdiction adopts. The tool works to the ratio you type in — that number comes from the code, and it is not printed here.
- Cross-slope, which is measured just as strictly and is usually what fails a poured ramp.
- Landings at the top, the bottom and at any turn: length, width, and slope in every direction.
- Maximum rise between landings, and the total run that follows from it.
- Handrails both sides where required — height, extensions past the ends, continuity and graspability — and edge protection along the ramp.
- The surface: firm, stable and slip-resistant, and a transition at each end without a lip.
Have this ready
- The ratio, maximum rise and landing dimensions your code requires, written down before you set the forms.
- A digital level long enough to read the running slope and the cross-slope in one place.
- The drawing showing landings, handrails and the level area at each door.
Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.
The formula
run = rise × n · length = √(rise² + run²)
Rise and run in the same unit. The ratio 1:n turns into run by multiplying; percent is rise ÷ run × 100; the angle is arctan(rise ÷ run). With a run typed instead of derived, n is run ÷ rise, and the ramp meets the required ratio when its n is the same or larger — larger n, flatter ramp.
What this tool assumes
That the ramp is one straight sloped run between two level surfaces, that the rise is measured finished to finished, and that the ratio you typed is the one your job is governed by. It has no built-in maximum, no landing rule and no handrail rule — those belong to the standard you are building to.
Questions
- How long does a ramp need to be?
- Multiply the rise by the "n" in the slope ratio your job requires. A 30-inch rise at 1:12 needs 30 × 12 = 360 inches of run — 30 feet — and the ramp surface itself is a touch longer, √(30² + 360²) = 361.2 inches. In metric, a 750 mm rise at 1:12 needs 9,000 mm (9 m) of run. The ratio is set by whichever accessibility or building code applies to your job; type it and the tool gives the run and the length.
- What is a 1:12 slope?
- One unit of rise for every twelve units of run — the same steepness whether you measure in inches or millimetres. As a percentage it is 1 ÷ 12 = 8.33%, and as an angle it is arctan(1 ÷ 12) ≈ 4.76°. The tool shows all three so you can talk to a surveyor, a code inspector or a concrete finisher in their own language.
- Does the tool tell me the maximum slope allowed?
- No — on purpose. Accessibility standards and building codes set maximum ramp slopes, landing lengths and rise-per-run limits, and those numbers vary by country, province, occupancy and whether the ramp is public or private. You type the ratio your job requires, and the tool tells you whether the ramp you have room for meets it.
- What about landings, handrails and the top and bottom?
- Out of scope. This is the triangle only: rise, run and the slope between them. Landings break a long ramp into runs and add length; handrails, edge protection and the transitions at each end are all code items. Work each sloped run through the tool on its own and add the landings by hand.
- Can I use this for a driveway, a walkway or a loading ramp?
- Yes — it is the same right triangle. Type the rise and either the run you have or the grade you want, and read the length, the percent and the angle. For a driveway or a pipe fall in percent, the slope and grade calculator does the same job with a grade-first layout.
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