Roof Slope Calculator

How Roof Slope Works

Roof slope describes how steep a roof is. It is usually written as a ratio of vertical rise to horizontal run — a slope of 0.50 means the roof rises 1 inch for every 2 inches of horizontal distance. The same roof can also be described as a 50% slope, a 26.57° angle, or a 6/12 pitch in standard roofing notation.

Roof Slope Formulas

Worked Example

A roof with a 6-inch rise over a 12-inch run has a slope ratio of 0.50, a slope of 50%, an angle of 26.57° and a pitch multiplier of 1.118.

What You Need

Just the rise and the run. The easiest place to measure them is the attic: hold a 12-inch level horizontally against a rafter, then measure the vertical distance from the end of the level up to the rafter. That vertical distance, in inches, is your rise per 12 inches of run.

Assumptions & Limitations

This calculator describes slope geometry only. Steeper roofs require more material for the same footprint — use the Roof Area Calculator to convert your footprint to actual roof surface area.

Roof Pitch vs Roof Slope

Slope and pitch measure the same steepness with different notation conventions. Slope is the raw ratio of rise to run, usually shown as a reduced ratio, a percentage or an angle. Pitch fixes the run at 12 inches and reports the rise — the convention US roofers use when ordering materials. Same roof, three ways to say it:

Converting either way: pitch = 12 × slope ratio; slope ratio = pitch ÷ 12. This page focuses on the slope notations above; the Roof Pitch Calculator covers pitch in depth, including how to measure it four different ways.