Cross Slope Calculator

Cross slope is the sideways tilt of a road, sidewalk, ramp or parking lot, measured perpendicular to the direction of travel. It is what keeps rainwater sheeting off the surface instead of ponding, and it is one of the first things an inspector checks on an accessible route.

This calculator works in both directions. Enter an elevation change and a surface width and it returns the cross slope as a percentage, a ratio and an angle, with a pass or fail check against the 2% accessible-route limit. Or switch modes, give it a target slope and a width, and it tells you the elevation difference to build.

Quick Answer

How to use the cross slope calculator

Start in Find cross slope mode when you have measurements from an existing surface. Enter the elevation change, how much higher one edge sits than the other, and the surface width across which you measured it. Each value has its own unit selector, and that matters: field measurements almost always mix inches of drop with feet of width, and converting each input separately keeps the classic inches-divided-by-feet error out of the result. Press Calculate and you get the percentage, the 1:X ratio, the angle, and the verdict against the 2% accessible-route limit, with the full substitution printed under the result.

Switch to Find elevation change mode when you are designing rather than inspecting. Enter the slope you want to build, in percent, and the width you have to work with. The answer comes back in the same unit as the width, ready to mark on a form board. A common design move is to run this mode at 1.5% rather than the full 2%: the margin absorbs normal construction tolerance, so the finished surface still passes inspection against the legal maximum.

Measure the inputs the way an inspector would. Stretch a level or a straightedge across the full width, measure the gap at the low edge, and repeat at several points along the run. Grading is rarely uniform, and a compliant reading in the middle does not rescue a steep patch at a driveway crossing. Run the worst reading through the calculator; that is the number an accessibility survey will record.

The cross slope formula, in percent, ratio and degrees

Cross slope is a rise-over-run calculation where the run is the width of the surface, measured sideways, perpendicular to travel.

The three output forms are the same tilt described in the languages different trades speak. Plans and inspectors often work in ratios, grading crews think in percent, and digital levels read in degrees or percent depending on their setting. The table converts the values that come up most often, and every row is one the calculator reproduces exactly.

Note the small angle throughout. Even a failing cross slope is barely more than a degree, which is why eyeballing a sidewalk tells you almost nothing and a measured elevation difference over a known width is the only reliable route to the number.

Worked examples, verified against the tool

Checking an existing sidewalk. One edge sits 1 inch above the other across a 5-foot width. Convert the width first: 5 x 12 = 60 inches. Cross slope = (1 / 60) x 100 = 1.67%. The calculator reports the same figure with a 1: 60.0 ratio, a 0.95 degree angle, and a passing verdict, and its steps line shows the substitution in inches: (1.000 in / 60.000 in) x 100 = 1.67%.

Designing a new walk. The walk is 48 inches wide and the target is the full 2%. Elevation change = (2 / 100) x 48 = 0.96 inches. That is the drop to form between the edges, roughly 15/16 of an inch, and hitting it to the nearest eighth still lands between about 1.8% and 2.1%, which is exactly why the 1.5% design target in the how-to section exists.

The bars below scale that same 2% target across common widths. On a 72-inch walk the required drop is already 1.44 inches, a difference a wheelchair user feels as a constant sideways pull.

The 2 percent line, and the 1:48 line behind it

Two numbers govern accessible cross slope in the United States, and honest guidance has to name both. For pedestrian facilities in the public right of way, such as sidewalks along a street, the Public Rights-of-Way Accessibility Guidelines (PROWAG) cap cross slope at 2 percent maximum, recorded in the FHWA summary of PROWAG sections R302.5-6 as generally no more than 2%. For buildings and facilities, the 2010 ADA Standards, Chapter 4 set the maximum at 1:48 in sections 403.3 and 405.3, which is 2.08 percent.

This calculator checks against the stricter 2% line, and that choice is deliberate. A surface built to exactly 2.08% has no room for the small humps and settlement real grading leaves behind, so part of it will measure over the limit on inspection day. Designing and checking at 2%, or better 1.5%, keeps the as-built surface legal under either standard. If your result lands between 2.00% and 2.08%, the calculator flags it for re-grading; treat that band as a warning to verify which standard your project falls under, not as an automatic legal failure everywhere.

Vehicle surfaces follow different rules, and mixing the two is a common specification error. FHWA guidance drawing on the AASHTO Green Book puts normal roadway cross slopes at roughly 1% to 2% on straight sections, with more on curves, because the goal there is shedding water at speed rather than wheelchair comfort. The same percent math serves a concrete slope calculator for a driveway, patio or slab, where 1% to 2% drainage targets are typical and no accessibility maximum applies unless the slab is part of an accessible route.

Cross slope is not running slope, and neither is a roof pitch

Cross slope runs sideways, perpendicular to the direction of travel. Running slope, also called longitudinal slope or grade, runs along the direction of travel. An accessible route has limits on both: running slope up to 5% (1:20) is allowed on a walking surface before the surface counts as a ramp, and a ramp itself is capped at 8.33% (1:12) with its own landing and handrail rules in section 405, as the Access Board's guide to ramps and curb ramps walks through. A general slope tool averages rise over a run in whatever direction you measure; this calculator exists because the sideways direction has its own limit, its own drainage job, and its own inspection habit.

Once you start measuring, the same rise-over-run pattern shows up across the trades: the overall grade of a route in the average slope calculator, the tilt of a vaulted ceiling in the ceiling slope calculator, and the fall along a yard drain. The formulas rhyme; the limits do not, which is why each surface gets its own check.

Too flat ponds, too steep pulls: the drainage trade-off

Cross slope exists because a perfectly level exterior surface is a puddle waiting for rain. The design problem is a narrow window between two failure modes.

Where a walk meets a lawn or a bed, the water the slope sheds still needs somewhere to go. A swale or a drain run at its own gentle fall finishes the job, and the french drain cost calculator covers what that buried half of the plan typically runs. FHWA's sidewalk design guidance makes the same point: landings and crossings are detailed at a 2.0% slope so drainage works without breaking the accessibility limit.

What competing calculators miss

The tools ranking for this keyword mostly stop at a single percentage. That number is the easy part; everything a builder or inspector actually needs comes after it.

Miss 1: no verdict. The most detailed competing page works an example that lands at 2.08%, notes in prose that it exceeds the standard, and still leaves its own tool silent: no pass, no fail, just a number to interpret. Here the verdict is part of the result, worded against the 2% design limit, with the 1:48 background explained in the standards section rather than hidden.

Miss 2: no reverse mode. Inspectors ask what the slope is; builders ask what drop to build. Only one top result offers the slope-to-elevation direction at all. This calculator keeps both modes on one set of tabs, sharing the same unit system, so a design figure and its inspection check never drift apart.

Miss 3: units you must reconcile yourself. Several tools demand both inputs in the same unit and leave the inches-to-feet conversion to you, the most common cross slope error in the field. Here each input converts independently, and the steps line prints the converted values in inches so the arithmetic can be audited at a glance.

Miss 4: percent only. Plans speak in ratios and levels speak in degrees. Printing percent, ratio and angle together means the number on the screen matches whatever document or instrument sits next to it, with no second conversion in between.

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