Home › Calculators › Roof Pitch Calculator

Roof Pitch Calculator

Find a roof's pitch from its rise and run, or convert between pitch (x/12), angle in degrees and slope percentage. Add the horizontal run to get rafter length, ridge height and the multiplier for sloped roof area.

What do you know?

Rise & runRoof anglePitch (x/12)

Rise (vertical)

Use the same unit for rise and run (inches, feet, metres…).

Run (horizontal)

Roof angle

Pitch

/ 12

Inches of rise per 12 inches of run.

Optional: rafter length

Horizontal run of rafter

Usually half the building span, wall to ridge centre.

Overhang (horizontal)

Length unit

Enter your values to see the result.

How to use the Roof Pitch Calculator

Choose what you know: rise and run, the roof angle, or the pitch in x/12.
Enter the values. Rise and run can be in inches, feet or metres as long as both use the same unit.
Optionally add the horizontal run of a rafter (usually half the building width) and the overhang to get rafter length.
Press Calculate pitch to see the pitch, angle, slope %, multipliers and the working.
Use the pitch chart to compare common pitches or download it as CSV.

Roof pitch formulas

Pitch (x/12) = rise ÷ run × 12
Angle = arctan(rise ÷ run) = arctan(pitch ÷ 12)
Pitch from angle = tan(angle) × 12
Slope % = rise ÷ run × 100
Rafter multiplier = √(1 + (pitch ÷ 12)²); rafter length = horizontal run × multiplier
Hip/valley multiplier = √(pitch² + 288) ÷ 12 (equal-pitch roof planes)

Worked example: a roof rises 42 inches over a 7 ft (84 in) run. Pitch = 42 ÷ 84 × 12 = 6/12. The angle is arctan(0.5) = 26.57°, the slope 50% and the rafter multiplier √1.25 = 1.1180, so the rafter is 7 × 1.1180 = 7.83 ft long before overhang.

Roof pitch chart

Pitch	Angle	Slope	Rafter multiplier	Typical use
1/12	4.76°	8.3%	1.0035	Low-slope membrane roofs
2/12	9.46°	16.7%	1.0138	Low slope, sheds, porches
3/12	14.04°	25%	1.0308	Low slope, metal roofing
4/12	18.43°	33.3%	1.0541	Common lower limit for standard shingle application
5/12	22.62°	41.7%	1.0833	Conventional
6/12	26.57°	50%	1.1180	Conventional, very common
7/12	30.26°	58.3%	1.1577	Conventional
8/12	33.69°	66.7%	1.2019	Conventional
9/12	36.87°	75%	1.2500	Upper conventional
10/12	39.81°	83.3%	1.3017	Steep
12/12	45.00°	100%	1.4142	Steep
18/12	56.31°	150%	1.8028	Very steep, A-frames

Pitches below about 4/12 usually need special underlayment or a low-slope roofing system. Always follow the roofing manufacturer's minimum pitch and your local building code.

How to measure an existing roof

Place a level horizontally against the rafter or roof surface, mark 12 inches along the level, and measure straight up (or down) from that mark to the roof. That vertical distance in inches is the pitch: 5 inches means 5/12. From the attic you can also measure a rafter's rise and run directly and enter them here.

Frequently asked questions

How do you calculate roof pitch?

Measure the vertical rise over a known horizontal run, then scale it to a run of 12: pitch = rise ÷ run × 12. A roof that rises 3 ft over a 6 ft run has a pitch of 3 ÷ 6 × 12 = 6, written 6/12 or 6:12.

How do I convert roof pitch to degrees?

Angle = arctan(pitch ÷ 12). For example 6/12 gives arctan(0.5) = 26.57°, 4/12 gives 18.43° and 12/12 gives exactly 45°.

What is a rafter or pitch multiplier?

It is the length of the sloped rafter for each unit of horizontal run: √(1 + (pitch ÷ 12)²). A 6/12 roof has a multiplier of 1.1180, so a 14 ft run needs a 15.65 ft rafter before overhang and ridge adjustments. The same factor converts flat plan area to sloped roof area.

What is the hip and valley multiplier?

For a regular hip or valley where both roof planes have the same pitch, the hip rafter runs diagonally, so it covers √2 times the common run. Its length per unit of common run is √(pitch² + 288) ÷ 12, for example 1.5000 for a 6/12 roof.

What is the difference between pitch and slope percentage?

Pitch expresses rise per 12 units of run (6/12), while slope percentage is rise ÷ run × 100. A 6/12 pitch is a 50% slope and a 12/12 pitch is a 100% slope (45°).

What roof pitch is considered low or steep?

Roofs under about 2/12 are treated as flat or very low slope, 2/12 to 4/12 as low slope, roughly 4/12 to 9/12 as conventional, and above 9/12 as steep. Shingle manufacturers and building codes set minimum pitches for each roofing material, so check the product instructions and your local code.

Does the rafter length include the ridge board and plumb cuts?

No. The result is the theoretical line length along the slope from the wall plate to the ridge centreline, plus overhang if entered. Deduct half the ridge board thickness (measured horizontally) and allow for cuts when cutting real rafters.

Related tools