Roof Slope Calculator

Roof Slope Calculator

Enter rise and horizontal run to calculate roof pitch, angle, slope percentage, roof area multiplier, and the slope length for the measurements entered.

Rise (vertical height)

in

Run (horizontal distance)

in

Pitch6:12 roof pitch

The roof rises this many units for every 12 horizontal units. Use it when comparing roofing plans and references.

Angle from horizontal26.57°

This is the angle between the roof surface and the horizontal plane.

Slope %50.0%

This is the same rise-to-run relationship written as a percentage.

Multiplier1.118

Multiply one roof plane's plan area by this value for a rough sloped-area estimate. It excludes overhang, waste, planes with different slopes, and complex roof geometry.

Slope length for entered run13.42 in

This is the hypotenuse of this rise/run triangle. It is not a framing cut length with construction details.

Formulas: pitch = (rise ÷ run) × 12; angle = arctan(rise ÷ run) × (180 / π); slope = (rise ÷ run) × 100; multiplier = √(1 + (rise ÷ run)²); slope length = √(rise² + run²).

Only know an angle or slope percentage? Use the pitch converter.

Unit System

Quick Presets

Roof slope diagram

This diagram updates as you change the rise and run.

Prepare your rise and run

You only need two measurements from the same roof plane.

Use the same unit for rise and run. Inches, feet, centimeters, and meters all work when both values use the same unit.
Run is the horizontal distance, not the sloped distance along the roof surface.
Do not climb onto a roof just to use this calculator. Choose a safer measuring method when needed.

See safe roof-pitch measurement methods

How a 6:12 roof slope calculation works

Follow one set of measurements from the inputs to the five results shown by the calculator.

Example: 6 inches of rise over 12 inches of run

1
Convert the ratio to X:12 pitch
Divide 6 by 12 to get 0.5, then multiply by 12. The result is a 6:12 pitch. Pitch is a ratio, so 50 centimeters of rise over 100 centimeters of horizontal run gives the same answer.
2
Read the angle and slope percentage
The angle is arctan(6 divided by 12), or 26.57 degrees above horizontal. The slope percentage is (6 divided by 12) times 100, or 50%. These are equivalent ways to describe the same incline. A 6:12 pitch does not mean 30 degrees.
3
Use the derived values for rough geometry
The area multiplier is 1.118. If one roof plane has 600 square feet of horizontal plan area, its sloped surface is about 671 square feet before overhangs, openings, and waste. The 6 by 12 triangle has a slope length of 13.42 inches. That length describes this triangle, not a framing-ready rafter.

Mistakes that change the answer

Most incorrect results come from the measurements or from treating a geometric result as a construction quantity.

Measuring run along the roof surface
Run is level horizontal distance. Measuring up the sloped roof gives a longer dimension and produces the wrong pitch.
Mixing measurement units
Rise and run must use the same unit before you enter them. A rise in inches and a run in feet cannot be compared directly.
Converting pitch to degrees as a linear scale
Each pitch uses an arctangent calculation. Doubling the rise does not double the angle, so fixed degrees-per-inch shortcuts give incorrect results.
Ordering materials or cutting rafters from one result
The multiplier and slope length describe simple roof-plane geometry. Final quantities still depend on roof shape, waste, overhangs, openings, and framing details.

Common Roof Pitch Reference

Compare six common pitches by angle, slope percentage, and area multiplier. Select a row to load it into the calculator.

View the full roof pitch chart

Pitch	Angle	Slope %	Multiplier	Action
2 : 12	9.5°	16.7%	1.014
3 : 12	14.0°	25.0%	1.031
4 : 12	18.4°	33.3%	1.054
6 : 12	26.6°	50.0%	1.118
8 : 12	33.7°	66.7%	1.202
12 : 12	45.0°	100.0%	1.414

The area multiplier is √(1 + (pitch/12)²).

Use the result as a starting point

This calculator handles roof-plane geometry. Use the result to prepare questions and estimates, then verify the details that depend on the building, product, and job site.

Plan roof area, not an order

The area multiplier converts horizontal plan area to the sloped surface area of one roof plane.

It does not account for roof shape, overhangs, openings, waste, product coverage, or the final field measurement. Do not treat it as a purchase quantity or final roof area.

Check the material system

In 2021 IRC R905.2.2, asphalt shingles are allowed on slopes of 2:12 or greater. From 2:12 up to 4:12, the model code requires double underlayment.

This is a model-code example, checked July 23, 2026. Your local adopted edition, amendments, and the manufacturer's installation instructions control the work.

Source: 2021 IRC R905.2.2

Do not infer safe access

Pitch alone cannot determine whether a roof is safe to enter or work on. Site conditions, weather, roof condition, edges, access, and the planned work all matter.

For covered construction work, OSHA uses 4:12 or less as low-slope and greater than 4:12 as steep; its roof fall-protection rules include 6-foot unprotected-edge conditions. These classifications do not make a roof safe to walk.

Sources: OSHA 1926.500 definitions and OSHA 1926.501 fall-protection duty

For a framing estimate, use the Rafter Length Calculator . For low-slope drainage questions, use the Flat Roof Slope Calculator .

Choose your next roof task

Use a dedicated page when your question goes beyond a rise and run calculation.

Measure your roof pitch

Find rise and horizontal run from the roof, attic, ground, or a phone.

Convert pitch, angle, or slope

Start with degrees, a percentage, or a 1:N ratio instead of rise and run.

Look up a roof pitch

Compare the full pitch, angle, slope, and multiplier reference table.

Plan a rafter

Account for framing details such as span, overhang, ridge deductions, and birdsmouth cuts.

Check a flat or low-slope roof

Work through drainage and low-slope material questions separately.

Calculate a lean-to roof

Solve for wall height, height difference, pitch, or slope length on a lean-to.

Frequently Asked Questions