Ramp Slope Calculator

How to use the ramp slope calculator

Three tabs, three starting points. All of them return the same set of answers.

Rise & Run. The height you have to climb and the flat distance you can give it. Use this when the space is already decided and you need to know what slope it produces.

Ratio 1:n. Name the slope you want, add the rise, get the run it demands. Use this when you are still choosing where the ramp goes.

Length & Angle. For a ramp you already own. Measure the board and the angle it sits at, and get the rise and run it actually delivers.

Two optional boxes sit under every tab: ground clearance and wheelbase. Fill in both and the calculator checks whether your machine grounds out at the top of the ramp, which most slope calculators do not do. Leave them blank and you still get every slope number. There is a section below on why that check matters more than the grade does.

Measure the rise as one number, straight down from the top surface to the ground. Measure the run flat, not along the ramp. Those two mistakes account for most wrong answers.

What you get back

• The slope, four ways. Ratio written 1:n, percent grade, angle in degrees, and the decimal.
• The geometry. Rise, run, and slope length, which is the board you cut rather than the ground it covers.
• The clearance check. With a ground clearance and a wheelbase entered: the steepest vertex your machine clears, your actual ramp angle, and whether it passes or grounds out and by how much.

What you do not get here is an ADA verdict. Landings, handrails and the 1:12 maximum belong to a ramp somebody will be pushed up in a chair, and applying them to a mower ramp or a loading dock is how a page ends up telling a man with a trailer that he needs a five foot landing at both ends. If accessibility is what you are building, the wheelchair ramp slope calculator runs the same maths with the ADA checks attached. And if you are building a fixed ramp at a house rather than loading something onto a trailer, the code that governs it is residential: IRC R311.8.1 holds the ramp serving your required exit door to 1:12 and allows any other ramp up to 1:8.

How precise the numbers are

Grade and slope carry three decimals, the angle three, slope length four, and nothing rounds until it prints. The clearance check prints two decimals because a tenth of a degree is already finer than you can build to.

About units

Inches, feet, centimetres and metres. Rise and run only have to match each other, because slope is a ratio and the units cancel. Ground clearance and wheelbase only have to match each other, not the ramp. The check is a ratio, so inches over inches gives the same answer as feet over feet. Measure both in inches even when the ramp is in feet, which is what most people do anyway.

Ramps get written as a ratio, roads and driveways as a percent. Same number, different convention, which is why a driveway slope is quoted at 12 percent rather than 1:8.

The ramp slope formula

Slope is rise divided by run. Everything else is that one division written differently.

• ratio = 1 : (run ÷ rise)
• grade = (rise ÷ run) × 100
• angle = arctan(rise ÷ run)
• slope length = √(rise² + run²)

Run is flat distance. Slope length is the ramp itself. At shallow slopes they are nearly the same and the difference does not matter; at 1:3 the board is 5.4% longer than the ground it covers, and if you buy timber to the run you will be short.

How to calculate ramp length by hand

A trailer bed 24 inches off the ground, and you want the gentlest ramp that will fit in a 10 foot driveway.

1. Pick the ratio. 1:4 is the common rule of thumb for loading equipment. 24 × 4 = 96 inches, so 8 feet of run.
2. Grade. 24 ÷ 96 = 0.25, so 25%.
3. Angle. arctan(0.25) = 14.04°.
4. Slope length. √(24² + 96²) = 98.95 inches, so 8 feet 3 inches of board.
5. Check the clearance. This is the step everyone skips, and the next section is about it.

Ramp ratio, percent and angle

The same slope, in every convention you will meet on a drawing, a spec sheet or a product listing.

Ratio	Percent	Angle	Run per 12 in of rise	Where it turns up
1:20	5.0%	2.86°	20 ft	Gentlest walking surface
1:12	8.333%	4.76°	12 ft	The accessibility maximum
1:10	10.0%	5.71°	10 ft	Forklift load-orientation trigger
1:8	12.5%	7.13°	8 ft	Non-egress pedestrian ramps in buildings under the IBC, which does not cover houses
1:6	16.667%	9.46°	6 ft	Portable and threshold ramps
1:4	25.0%	14.04°	4 ft	The equipment-loading rule of thumb
1:3	33.333%	18.43°	3 ft	Construction scaffold access only, not a trailer ramp limit
1:2	50.0%	26.57°	2 ft	Steeper than almost anything drives

One conversion trap. A 1:3 slope is 18.43°, not 20°, even though OSHA’s own scaffold rule writes it as “one (1) vertical to three (3) horizontal (20 degrees above the horizontal)”. That parenthetical is the regulator rounding, not geometry. Quote the ratio, not the degrees, if you are working to that rule.

What a 1:12 slope ramp is, and when it does not apply to you

1:12 means one unit of rise for every twelve of run. Twelve inches of height needs twelve feet of ramp. It is 8.333% and 4.76°.

It is also, on the open internet, the answer to almost every ramp question regardless of what was asked. Search for a ramp slope and you will be handed 1:12 whether you are building a wheelchair ramp, loading a zero-turn into a trailer or pouring a dock apron. That is worth untangling, because 1:12 is a comfort standard for a person moving themselves in a chair, not an engineering limit for a machine.

Where 1:12 genuinely binds: a ramp on an accessible route or in a means of egress under the ADA Standards and the building code. Everywhere else it is a choice, and often a wasteful one. A 24 inch trailer bed at 1:12 is a 24 foot ramp. Nobody carries a 24 foot ramp.

What the rules actually say, and what they do not

Two OSHA rulebooks get quoted on this subject and they do not cover the same people. 29 CFR 1910 is general industry, which is where a loading dock sits. 29 CFR 1926 is construction. Loading a mower onto your own trailer is under neither, because both are written for employers. Keeping them apart matters, because the only published slope figure in either one comes from the construction book and it is about scaffolds. OSHA sets no maximum ramp slope in general industry. This surprises people. 29 CFR 1910.22 covers surface conditions, loads, access and egress, and inspection, and says nothing about slope. 1910.25 covers stairways and has no ramp section. Subpart D does define a ramp, and 1910.28(b)(1) makes the employer protect anyone working four feet or more above a lower level, with 1910.29(b)(14) setting how a guardrail on a ramp is built. What it never sets is a ramp slope. It is not shy about slope elsewhere: 1910.25 pins standard stairs between 30 and 50 degrees and ship stairs between 50 and 70. It specifies stairs to the degree and leaves ramps alone.

Construction scaffolding is the exception. 29 CFR 1926.451(e)(5) caps a scaffold ramp or walkway at one vertical in three horizontal, and requires cleats no more than 14 inches apart if it is steeper than one in eight. That is a scaffold rule, not a general construction rule, and it gets quoted far outside its scope.

The forklift figure is not a limit either. 29 CFR 1910.178(n)(7)(i) says that “when ascending or descending grades in excess of 10 percent, loaded trucks shall be driven with the load upgrade”. That is an instruction about which way to point the load, not a ceiling on the grade. OSHA’s own guidance defers the actual limit to the machine: do not travel on ramps that exceed the manufacturer’s recommendation. The number you want is on the truck’s data plate, not in the regulation.

The building code has two tiers, and almost no calculator mentions the second one. The 2021 International Building Code, section 1012.2: ramps used as part of a means of egress are capped at 1:12, and “the slope of other pedestrian ramps shall not be steeper than 1 unit vertical in 8 units horizontal (12.5-percent slope)”. So the code itself allows 1:8 for a pedestrian ramp that is not an egress route. Check which edition your jurisdiction has adopted before leaning on it, because adoption lags publication by years.

Two things that changed recently

If your ramp is a curb ramp in the public right of way, the rules moved and most pages have not caught up. The Access Board published PROWAG as a final rule on 8 August 2023. It drops the term “counter slope” entirely. The old 2010 ADA rule capped the gutter counter slope at 1:20; PROWAG replaces that with R304.5.2 Change of Grade, which caps the change of grade at 13.3 percent, or else requires a 24 inch transitional space at 1:48. That is an arithmetic difference between two surfaces rather than a cap on one of them, and an 8.3% ramp meeting a 5% gutter lands exactly on the limit.

The catch is enforceability, and this is where nearly everyone gets it wrong. PROWAG is enforceable today only for federal rights of way, adopted by GSA effective 3 September 2024, and for transit stops, adopted by the Department of Transportation effective 17 January 2025. The Department of Justice has not adopted it. For an ordinary municipal sidewalk or curb ramp the enforceable federal standard is still the 2010 ADA Standards. Many state DOTs use PROWAG as design policy anyway.

Also worth knowing if you are specifying anything this autumn: ICC A117.1-2026, the referenced accessibility standard, is listed by ICC as available late September 2026, the first new edition since 2017. Its ramp section changes only editorially, but it adds a dedicated EV charging station section.

Why the grade is not what stops you

Here is the thing none of the ten pages ranking for this term asks about, and that a great many ramp questions are really about.

Your machine does not get stuck because the ramp is steep. It gets stuck at the vertex, the point where the ramp meets the flat surface at the top. For a moment the front wheels are on the flat and the back wheels are still on the slope, and the lowest point underneath, a mower deck or an exhaust or a frame rail, has to pass over the corner between them. That is breakover, and it is decided by ground clearance and wheelbase, not by the grade.

It explains what looks like a contradiction in a lot of forum threads on this subject. A trailer that “is pretty close to sitting level” still hangs the deck up. A ramp at a perfectly reasonable angle still bottoms a touring bike. The grade was never the problem.

For a ramp meeting flat ground, with the low point roughly at the middle of the wheelbase, the steepest vertex a machine clears is:

maximum ramp angle = 2 × arctan(2 × clearance ÷ wheelbase)

Treat the answer as a ceiling rather than a target. Tyres squat under load, suspension moves, and a ramp lip is never the clean corner the geometry assumes, so a result that only just clears will not clear in the yard. Leave yourself a degree or two.

A zero-turn with 3 inches under the deck and a 48 inch wheelbase: 2 × arctan(6 ÷ 48) = 2 × arctan(0.125) = 14.25°. Put that mower on a 6 foot ramp up to a 24 inch trailer bed, which is 18.43°, and it grounds out by just over four degrees. Which is what the lawn care forums are full of.

The 4:1 rule, decoded

Toro and Exmark both print the same two instructions in their operator manuals: do not exceed a 15 degree angle between the ramp and the ground, and make the ramp “at least 4 times as long as the height of the trailer or truck bed to the ground”.

Nobody explains why 4:1. Here is why. A 4:1 ramp is arctan(0.25), which is 14.04°, just under their 15°. And a machine whose clearance is exactly one sixteenth of its wheelbase clears up to 14.25°. Three inches under a 48 inch wheelbase is one sixteenth. The 4:1 rule is the breakover answer for a typical mower, written as a ratio so you do not have to do trigonometry in a car park. Note how little is left over. The ramp is 14.04° and the mower clears 14.25°, so the famous rule of thumb passes by around two tenths of a degree. It is a limit, not a comfortable margin.

Which also tells you when the rule of thumb fails. If your machine has less than a sixteenth of its wheelbase in clearance, and plenty of lowered cars and touring bikes do, 4:1 is not enough and you need to work the actual number. A Harley Street Glide Ultra has 5.3 inches of clearance over a 64 inch wheelbase. That is a twelfth, so it is fine at 4:1. A car with 4 inches under a 106 inch wheelbase is a twenty-sixth, clears 8.63°, and needs 7:1 or gentler. At 6:1 it grounds.

Why your spec sheet number beats this one

If your machine has a published breakover angle, use it instead of the formula. SAE J1100 defines it as dimension H147, taken between two lines tangent to the front and rear tyres at their static loaded radius and meeting at the lowest point underneath. Solve the same geometry for a ramp meeting flat ground and you get the same expression, so the two are directly comparable. No conversion needed.

What is not the same is how well each one is measured. The formula has to assume the low point sits halfway between the axles, and on most machines it does not. Ford publishes 17.1° of breakover for a 2026 F-150 SuperCrew 4×2 with 8.2 inches of clearance on a 145.4 inch wheelbase. Put those two numbers into the formula and you get 12.87°. Over four degrees of difference, and the formula is the pessimistic one, because the real low point on that truck is nowhere near the middle.

So read the calculator’s answer as a floor. If it says you clear, you clear. If it says you ground out and the margin is small, find the manufacturer’s figure before you buy a longer ramp. Plenty of makers publish nothing at all, which is exactly why this goes wrong so often on cars and so rarely on trucks.

One industry has quietly worked all this out and sells around it. Race Ramps put the approach angle in the product name: a 5 inch trailer ramp at 4.3°, the same 5 inch lift at 8.6° in a shorter version, and an extender product whose entire purpose is to drop a 67 inch ramp to 6.6°. Their range runs from 4.3° to 16.7°. Customers are buying degrees, and paying for length to get them.

Six reasons a ramp slope comes out wrong

1. The clearance check never happened. Covered above. The grade can be perfect and the machine still grounds at the top.
2. The run was measured along the ramp. Run is flat distance. Pull the tape level. Measuring down the surface gives a slope that looks gentler than it is.
3. 1:12 got applied to something it does not govern. It is an accessibility figure. A dock ramp built to it is three times longer than it needs to be.
4. The ground at the bottom is not level. A downhill approach makes the bottom transition easier and the top vertex worse. If the ramp foot sits on a slope, add that slope to the angle your machine has to break over.
5. A ratio got read as a degree. 1:3 is 18.43°, not 20°. 1:4 is 14.04°, not 15°. Regulators and manuals round; the ratio is the exact thing.
6. Two narrow ramps instead of one wide one. Toro’s manual specifically warns against this for mowers. Individual ramps let one wheel drop and change the geometry mid-climb.

Frequently asked questions

How steep can a ramp be?

It depends entirely on what is going up it, and there is no single legal answer for a general ramp. For a person in a wheelchair on an accessible route, 1:12. For other pedestrian ramps the building code allows 1:8. For a scaffold walkway OSHA caps it at 1:3. For a machine, the limit is usually not the grade at all but whether it clears the top vertex, which is set by ground clearance and wheelbase. OSHA sets no general maximum for industrial ramps.

How long should a ramp be for a 24 inch rise?

At the 4:1 rule of thumb used for loading equipment, 8 feet. At 1:12 it would be 24 feet, which is the accessibility figure and almost certainly not what you need. Check the clearance of whatever is driving up it before you settle on a length: a machine with less than a sixteenth of its wheelbase in ground clearance needs more than 4:1 regardless of how strong the ramp is.

Why does my mower deck scrape at the top of the ramp when the angle is fine?

Because the angle was never the constraint. At the top of the ramp your front wheels are on the trailer bed and the rears are still on the slope, and the deck has to clear the corner between the two. That is breakover, and it is set by ground clearance and wheelbase. The fixes that work all attack the corner rather than the slope: a longer ramp, a block under the ramp foot, raising or lowering the hitch, or a board laid across the trailer lip to round the transition off.

What is the maximum ramp grade for a forklift?

OSHA does not publish one. 29 CFR 1910.178(n)(7)(i) requires that above 10 percent a loaded truck is driven with the load pointing upgrade, but that is an instruction about orientation, not a ceiling. OSHA’s own guidance says not to exceed the manufacturer’s recommendation, so the real figure is the gradeability on your truck’s data plate. Worth knowing that clearance is often the binding constraint anyway: a Toyota electric counterbalance publishes about 3 inches of clearance at the centre of the wheelbase, and a hand pallet truck’s load wheels are under 3 inches in diameter, which is why they cannot climb a lip at all.

Is a 15 degree ramp too steep?

15° is about 1:3.7, or 27 percent. For a riding mower it is right at the manufacturer’s stated ceiling: Toro and Exmark both print “do not exceed a 15-degree angle between the ramp and the ground”. For a car it is far too steep, since most will ground out somewhere between 8 and 14 degrees. For a person on foot with a load it is hard work but possible. For a wheelchair it is three times the legal maximum.

How do you convert a ramp slope to degrees?

Take the arctangent of rise divided by run. A 1:8 ramp is arctan(0.125), which is 7.13°. From a percent, divide by 100 first: a 20 percent grade is arctan(0.20), or 11.31°. Degrees always look like the smaller number, which is why a slope quoted in degrees sounds gentler than the same slope quoted as a percent.

Does the ground at the bottom of the ramp matter?

Yes, and it works in two directions at once. If the ground falls away from the trailer, the ramp sits at a shallower angle relative to the vehicle coming up it, which makes the bottom edge easier to get onto but makes the vertex at the top worse. If the ground rises towards the trailer, the reverse. Nothing on the market takes ground slope as an input, so add it by hand: a 3 degree downhill approach makes the top vertex 3 degrees harder.

What is a 1:12 slope ramp in percent and degrees?

8.333 percent and 4.76°. One inch of rise for every twelve inches of run, so a foot of height needs twelve feet of ramp. It is the maximum running slope for a ramp on an accessible route, and it is a comfort standard for someone propelling themselves in a chair rather than a structural or mechanical limit. For loading a machine it is unnecessarily shallow.