Roof pitch, slope, rafter length and roof surface area for a gable roof.
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Roof Pitch Calculator: Free Online Tool
The Roof Pitch Calculator finds the pitch, slope, rafter length and roof surface area of a gable roof. You enter the half span (run), the rise, the building length and the eave overhang. It returns the pitch angle, the slope as rise per 12 of run, the slope percentage, rafter lengths with and without overhang, the area multiplier and the total roof surface area.
Carpenters, roofing contractors, architects, site engineers and students use it to cut rafters, order roof sheets or tiles and check drainage. A roof is always larger than the plan area below it, so ordering by plan area alone leaves you short. Knowing the real surface area and slope helps you choose the right material. The tool works in feet and inches or in metres and millimetres, so it suits both older site practice and metric drawings, and every result can be rechecked by hand.
When to use this calculator
Use this calculator when you plan a timber or steel roof for a house, a godown, a shed or a workshop. It is useful when you want the rafter length before cutting, when you need the roof surface area to order tiles, GI sheets or waterproofing, and when you must check the slope that your roofing material needs. Students can use it to practise right-triangle geometry. By changing the rise you can quickly see how a steeper roof increases rafter length and area.
How to use the Roof Pitch Calculator
- Open the calculator above on your phone, tablet or computer. Nothing needs to be installed and no sign-up is required.
- Choose the unit system first (feet / inches or metres / mm). The input labels change to match your choice.
- Enter your values in the input boxes: Half span / run (ft), Rise (ft), Building length (ft), Eave overhang (ft). The boxes already hold sample values, so overwrite them with your own.
- Press Calculate. The result appears straight away under the button.
- Change any value and press Calculate again to compare options. Press Reset to go back to the sample values.
- Check the result against your drawings, project specification and local code before you order material or approve work.
How the calculation works
The calculator treats half of the roof as a right triangle. Slope is rise divided by run. The pitch angle is the arctangent of the slope, in degrees. The slope in the 12 format is slope x 12, and the slope percent is slope x 100. The area multiplier is the square root of (1 + slope²). Rafter length without overhang is run x multiplier; with overhang it is (run + overhang) x multiplier, where the overhang is measured horizontally. Roof surface area for both sides is 2 x rafter length with overhang x (building length + 2 x overhang). For 12 ft run and 6 ft rise, the slope is 6 : 12, or 26.6 degrees.
Worked example
When you open the page the calculator is filled with sample values. Pressing Calculate with those values gives the result below, which you can use to check that you are reading the output correctly.
| Pitch | |
|---|---|
| Pitch angle | 26.6° |
| Slope (rise : 12 run) | 6.00 : 12 |
| Slope percent | 50.0 % |
| Rafter | |
| Rafter length (no overhang) | 13.42 ft (4.09 m) |
| Rafter length (with overhang) | 14.53 ft (4.43 m) |
| Roof area | |
| Roof area multiplier | 1.118 |
| Roof surface area (both sides) | 930.20 sq ft (86.42 m²) |
Tips and common mistakes
- Enter the run as half of the building width, measured to the outer face of the wall, not the full span of the roof.
- Measure overhang horizontally, as the calculator note requires. Do not enter the sloped length of the overhang along the rafter.
- The roof area is for a plain gable roof. Hips, valleys, dormers and chimneys change the area, and the calculator does not include them.
- Add waste for sheets, tiles and ridge pieces. The result is the net surface area, so you must add an allowance when ordering.
- Check the minimum slope recommended by the roofing material manufacturer. Low slopes may cause leakage with some sheets and tiles.
Frequently asked questions
What is the run in a roof?
The run is the horizontal distance from the wall to the ridge, which is half of the span for a symmetrical gable roof. The default run is 12 ft with a rise of 6 ft.
What does a slope of 6 : 12 mean?
It means the roof rises 6 units for every 12 units of horizontal run. It equals a slope of 50 percent and a pitch angle of about 26.6 degrees, as in the default result.
Why is the roof area larger than the plan area?
The roof surface is inclined, so it is longer than its horizontal projection. The calculator multiplies plan dimensions by the area multiplier, square root of (1 + slope²), which is 1.118 for the defaults.
Is the overhang included in the rafter length?
Both lengths are shown. One is rafter length without overhang, and one is with overhang. The overhang is a horizontal distance, and the calculator adds it to the run before applying the multiplier.
Can I use it for hip roofs?
No. The calculator is for a gable roof on a rectangular plan. The note says hips, valleys and waste are not included, so use a different method for those roof types.
Important note
This tool gives estimates for planning, study and quick checks. It does not replace design by a qualified engineer, site-specific testing, or the requirements of your project specification and local building code. Always confirm final quantities, test results and acceptance limits with the responsible engineer before using them for construction or payment.
