Roof Slope Calculator

Roof Slope Calculator

Wondering how steep your roof is? Enter rise and horizontal run, and the calculator returns the roof pitch, angle, slope percentage, area multiplier, and slope length in one pass.

Example values — replace these with your measurements.

Rise (vertical height)

in

Run (horizontal distance)

in

Pitch6:12 roof pitch

Angle from horizontal26.57°

Slope %50.0%

Multiplier1.118

Slope length for entered run13.42 in

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.

How to use this roof slope calculator

You only need two measurements. As you type, the calculator updates all five results at once: the roof pitch, angle, slope percentage, area multiplier, and slope length, so you can check them before moving on.

1
Measure the vertical rise
Find how far the roof rises over the section you are measuring.
2
Measure the horizontal run
Use the level horizontal distance, not the distance along the roof surface.
3
Use one unit system
Enter rise and run in the same unit, then switch between Imperial and Metric if needed.
4
Read the five results
Use the pitch, angle, slope percentage, multiplier, and slope length for the next decision. If run is measured along the roof surface instead of level, every one of those numbers shifts. The next section shows how to measure and prepare your inputs.

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
6:12 pitch is not 30 degrees. 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%. All three describe the same incline.
3
Use the derived values for rough geometry
The area multiplier is 1.118. For a roof plane with 600 square feet of horizontal plan area, 600 × 1.118 ≈ 671 square feet of sloped surface. Overhangs, openings, and waste are not included. The 6 by 12 triangle has a slope length of 13.42 inches, which 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. Want to compare your pitch with the common ones? The chart below lines up six typical roof pitches.

What do common roof pitches look like?

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 it for rough area checks only. The next section explains what the result can and cannot do.

Can I order materials from this result?

This calculator works out the geometry of a single roof plane: one flat section of roof. Use the result to prepare questions and estimates, then check 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 .

Which roof tool should you use next?

Choose the page that matches the information you already have.

Known information	Use this page	Why
Rise and horizontal run	Roof Slope Calculator	Calculate the roof pitch and supporting geometry.
Angle, percentage, or 1:N ratio	Pitch Converter	Convert between pitch formats without measuring rise and run.
Span, overhang, or ridge deduction	Rafter Length Calculator	Include framing details that a slope-only result cannot know.
Low-slope drainage questions	Flat Roof Slope Calculator	Check fall, ponding, and low-slope material limits.

Measure rise and run without guessing

A correct input matters more than extra decimal places. Use a safe method that gives you a level run.

Correct measurement — Rise is vertical. Run is horizontal. Together they describe one roof plane as a right triangle.

Common measurement mistake — Measuring along the roof surface gives slope length, not horizontal run, and makes the pitch look flatter than it is.

What should I calculate next?

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.

Quick answers about roof slope

Is this calculator trustworthy?

This tool is built and maintained by an independent developer. The math is standard rise/run geometry, and automated tests cover normal inputs plus edge cases like very small runs. Where practical, the code and safety sources are linked so you can check them.

Results describe roof-plane geometry. They do not replace a site measurement, structural design, local code review, or manufacturer installation instructions.

Method and source notes last reviewed July 2026.