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Slope Calculator for Mowing

The single most important number for mowing a hill is the maximum slope in your mower’s manual. To compare against it, you need your own slope in the same unit, and manuals use degrees, percent or both. Measure the steepest part you mow, enter it below, and the calculator converts it and shows how it compares with typical published ratings.

How the calculation works

  1. Rise and run. Measure along the steepest line of the slope. Run is the horizontal distance (use a level board or a string line), rise is how far the ground drops over that distance. Use the same unit for both.
  2. Degrees. degrees = arctan(rise / run).
  3. Percent. percent = rise / run x 100. With an angle from a phone level app, percent = tan(degrees) x 100.
  4. Comparison. The result is placed in a general band: gentle (up to 5 degrees), moderate (5 to 10), steep for a rider (10 to 15), too steep for most riders (15 to 20) and very steep (over 20).

Worked example

A bank drops 2 ft over a 10 ft level run: 2 / 10 = 0.2, so the slope is 20% or 11.3 degrees. That is close to the limit many riding mower and zero-turn manuals list, so check the figure in yours before riding it. A gentler section that drops 1 ft over 10 ft is 10% or 5.7 degrees.

Note that percent and degrees are not the same: a 45 degree slope is 100%, and a 15 degree slope is about 27%.

Typical published ratings

These ranges are general and vary by model. Treat them as a reason to look up your manual, not as a rating for your machine:

  • Lawn tractors: often about 10 to 15 degrees.
  • Residential zero-turns: often about 10 to 15 degrees.
  • Garden tractors: often around 15 degrees.
  • Robotic mowers: usually given in percent, often 20 to 45% (about 11 to 24 degrees).
Zero-turn safety

Zero-turns steer and brake through the rear wheels, so they can lose traction on slopes, wet grass and near ponds, ditches and drop-offs. Stay within the slope rating in your manual, keep well back from water and edges, use the ROPS and seat belt, never carry passengers, keep children and pets out of the yard, and disengage the blades before reversing. Refuel only with the engine off and cool. See CPSC riding lawnmower safety. Read our Safety Disclaimer.

Frequently Asked Questions

What slope is too steep for a riding mower?

Whatever your manual says. Many riding mowers and zero-turns list a maximum of about 10 to 15 degrees, and some list less. If the manual lists no figure, ask the dealer before you ride any slope.

How do I measure a slope with my phone?

Lay a straight board along the steepest line of the slope, set the phone on it and read the angle in a level or measure app. Take a few readings and use the highest.

Is a zero-turn or a lawn tractor better on hills?

Lawn and garden tractors steer with the front wheels and usually handle side hills and descents more predictably. Zero-turns steer and brake through the rear wheels and can slide on slopes and wet grass. See riding mowers for hills.

Should I mow a slope up and down or across?

With a riding mower or zero-turn, up and down. With a walk-behind, across, so the mower cannot roll back onto you.

What can I use on slopes too steep to ride?

A self-propelled walk-behind mowed across the slope, a string trimmer or brush cutter, or a professional crew with slope-rated equipment. Some owners replace very steep banks with ground cover.

Guides for hills and rough ground

See also the Mower Matcher, which takes your slope into account.