Roof Pitch Calculator: Angle, Slope Percentage and Roof Length

Use this Roof Pitch Calculator to convert a slope such as 4:12 into an angle and a percentage, then calculate its inclined length from a horizontal run. It works in feet, inches, meters, and millimeters.

How to Use the Calculator

1. Select whether you know the pitch ratio, angle, or slope percentage.
2. Enter the slope. For a 4:12 roof, type 4:12 or simply 4 in Pitch mode.
3. Enter the horizontal run between the endpoints you want to compare and choose its length unit.
4. Select Calculate. Read the slope length, vertical rise, and converted slope values.

Switching the slope format converts your existing slope into the new format. Switching the run unit converts the run as well. The results show both imperial and metric length equivalents, so you can check a drawing in either system.

Understand Run, Rise, and Slope Length

Run is measured horizontally. Rise is the vertical difference between the same two endpoints. Slope length follows the inclined line. A tape measurement taken along a sloping member is therefore a different input from a horizontal plan dimension.

Does 4:12 work with metric measurements?

Yes. The ratio has no units as long as rise and run use the same unit. A 4:12 slope rises 1 m over a 3 m horizontal run, just as it rises 4 in over 12 in. Choosing millimeters changes the length units, not the shape of the triangle.

Why does this page say “pitch” and “slope”?

In common searches, “roof pitch” often means the rise per 12 units of run. This calculator uses that convention. Traditional roofing terminology can distinguish slope (rise/run) from pitch (rise/full span). Always check which dimension a drawing or reference uses.

Roof Pitch, Angle, and Slope Chart

The multiplier converts horizontal run to inclined length. Multiply your run by the listed factor for a quick estimate; the calculator uses unrounded values. These are geometric conversions, not roofing-product approval ranges.

Use One-Side Run, Not the Entire Building Width

If a symmetrical gable has a centered ridge and a 24 ft span, one side has a 12 ft horizontal run. At 4:12, enter 12 ft to calculate one side. Entering 24 ft would calculate an inclined line across twice the intended run.

For an offset ridge or unequal roof slopes, take each side’s actual horizontal run from the drawing. A shed roof uses the horizontal distance between its low and high endpoints; there is no automatic division by two.

Three Worked Examples

1. A 4:12 roof over a 12 ft run

The rise/run ratio is 4 ÷ 12 = 1/3. Rise = 12 × 1/3 = 4 ft. Slope length = √(12² + 4²) = 12.649111 ft, approximately 3.855449 m. This is 151.789328 total inches; it is not 12 ft 6.49 in.

2. A 6:12 roof over a 3 m run

Rise = 3 × 6/12 = 1.5 m. Slope length = √(3² + 1.5²) = 3.354102 m. The angle is 26.565°, and the slope is 50%. Notice that 50% is not 50 degrees.

3. A 30-degree roof over a 4 m run

Choose Angle and enter 30. This converts to about 6.928:12 and 57.735%. Rise = 4 × tan(30°) = 2.309401 m. Slope length = 4 ÷ cos(30°) = 4.618802 m. A 30-degree roof is therefore slightly shallower than 7:12, rather than exactly 7:12.

What About an Overhang?

Choose the endpoints first. If the desired line extends from the ridge to the outer eave, include the overhang’s horizontal projection in the run. Do not add a horizontal overhang directly to an already calculated inclined length.

For example, a 4:12 roof with a 12 ft wall-to-ridge run and a 1 ft horizontal overhang has a combined run of 13 ft. Its geometric ridge-to-eave length is 13 × √(1 + (4/12)²) = 13.703203 ft. The vertical rise shown for that input is measured from the lower eave endpoint, not from the wall support.

Roof Pitch Formulas

Let p be rise per 12 units of run, R the horizontal run, H the vertical rise, and L the inclined length. Use the same length unit for R, H, and L.

• Rise/run ratio: q = p ÷ 12
• Angle above horizontal: θ = arctan(q)
• Slope percentage: 100 × q
• Vertical rise: H = R × q
• Slope length: L = √(R² + H²) = R × √(1 + q²)
• Pitch from angle: p = 12 × tan(θ)

Set a handheld calculator to degree mode when working with the angles shown here. The online tool handles the degree conversion automatically.

Frequently Asked Questions

Is a 12:12 roof vertical?

No. Equal rise and run give a 45-degree angle and a 100% slope. A vertical line is 90 degrees and has zero horizontal run, so it is outside this calculator’s model.

Can I enter a low slope such as 1/4 inch per foot?

Yes. Enter 0.25:12 or 0.25 in Pitch mode. Do not enter 1/4 by itself: that denotes a rise/run ratio of 1 to 4 in this tool, equivalent to 3:12. A 0.25:12 slope is about 1.193 degrees and 2.083%.

Does the calculator choose an acceptable roofing material?

No. A geometric result does not establish drainage performance, structural capacity, or a roofing product’s suitability. Check the selected product’s installation requirements and the project’s applicable code.

Can this calculate total roof area?

This tool calculates a line length. For one flat roof plane at constant slope, its horizontal projected area can be multiplied by the same slope factor. A complete roof takeoff still needs the correct plan boundaries, openings, separate slopes, laps, and waste allowances.

Why do I see a small difference after changing units?

Displayed conversions use a maximum of six decimal places. Calculations use the available unrounded input values, but editing a converted number uses the digits currently shown. Keep the original drawing dimensions for precise work.

Related Tools

For mixed feet-and-inches measurements, use the Feet and Inches Calculator. For fractional-inch equivalents, use the Fraction to Decimal Inches Chart.

Reference Basis

Calculations use right-triangle geometry and trigonometry, as explained in OpenStax, Algebra and Trigonometry 2e, Section 7.2. Terminology is cross-checked against GAF’s Pro Field Guide for Steep-Slope Roofs, Version 2.1. Unit definitions follow NIST: 1 inch = 25.4 mm and 1 international foot = 0.3048 m, exactly. This is a geometry tool, not a code-based roof design calculator.