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Roof Pitch Calculator

How to Calculate Roof Pitch: Degrees, Ratio, Rafter Length and Slope Area

Roof pitch ties together the angle, the ratio, the rafter length and the roof area. Learn how to turn rise and run (or span) into degrees, a pitch ratio, slope percentage and common rafter length, and how to estimate slope area, with worked examples and a free calculator.

Last updated 7 November 2026

Why roof pitch matters

Pitch affects almost every roofing decision. It decides which coverings you can use, how long the rafters are, how much material you need and how the roof sheds rain and snow. It also affects the look of the building and the volume of the roof space.

Drawings give the pitch in different ways: degrees, a ratio, a percentage or an “X in 12” figure. KaelSite’s free Roof Pitch Calculator converts between them from rise and run (or full span), and gives the common rafter length and a simple slope area.

Key terms

  • Rise: the vertical height from wall-plate level to the ridge.
  • Run: the horizontal distance from the wall plate to the ridge centre. On a symmetrical gable it is half the span.
  • Span: the full width from eaves to eaves. Run = span ÷ 2.
  • Rafter length: the sloping length from the wall plate to the ridge.

The formulas

Use the same units for rise and run.

  • Pitch angle = arctan(rise ÷ run)
  • Slope percentage = (rise ÷ run) × 100
  • Pitch ratio = rise : run, simplified
  • X in 12 = (rise ÷ run) × 12
  • Common rafter length = √(rise² + run²)
  • One slope area ≈ rafter length × roof length along the ridge

Quick conversion table

Angle Rise per 1 m run Slope % X in 12
15° 0.268 m 26.8% 3.2
22.5° 0.414 m 41.4% 5.0
30° 0.577 m 57.7% 6.9
35° 0.700 m 70.0% 8.4
40° 0.839 m 83.9% 10.1
45° 1.000 m 100% 12.0

 

Worked examples

1. Rise and run
Rise 2.0 m, run 4.0 m:
Angle = arctan(2.0 ÷ 4.0) = arctan(0.5) = 26.6°
Slope = 50%, ratio 1:2, which is 6 in 12
Rafter length = √(2.0² + 4.0²) = √20 = 4.47 m

2. Starting from the full span
Span 8 m, rise 2.4 m:
Run = 8 ÷ 2 = 4.0 m
Angle = arctan(2.4 ÷ 4.0) = arctan(0.6) = 31.0°
Slope = 60%, ratio 3:5, which is 7.2 in 12
Rafter length = √(2.4² + 4.0²) = √21.76 = 4.67 m

3. Rafter length and slope area for a gable
Rise 1.8 m, run 3.5 m, roof length 9 m:
Angle = arctan(1.8 ÷ 3.5) = 27.2°, slope 51.4%
Rafter length = √(1.8² + 3.5²) = √15.49 = 3.94 m
One slope = 3.94 × 9 = 35.4 m²
Both slopes of a simple gable = 70.8 m²

What the calculator does not include

The common rafter length is the theoretical length from wall plate to ridge. It does not include eaves overhang, birdsmouth cuts, ridge board thickness, hips or valleys. For an overhang, you can add it separately:

Overhang along the rafter = horizontal overhang ÷ cos(pitch angle)

For example, in worked example 1 a horizontal overhang of 0.4 m on a 26.6° pitch adds 0.4 ÷ 0.894 = 0.45 m, giving a total rafter length of about 4.92 m before cuts. Slope area for hips, dormers and valleys needs a proper take-off, not the simple gable formula.

Common mistakes

  • Using the span instead of the run. The run is half the span on a symmetrical gable.
  • Measuring rise from the wrong level. Rise is from wall-plate level to the ridge.
  • Mixing units. Entering rise in millimetres and run in metres gives nonsense.
  • Confusing ratio and angle. A 1:2 ratio is 26.6°, not 45°.
  • Forgetting the overhang. The calculated rafter will be too short to cut from.
  • Using plan area for covering. Roof coverings are measured on the slope, not on the flat plan.

Is a steeper pitch better?

Not necessarily. Steeper roofs shed water and snow more readily, but they use more material, cost more to build and make roof work harder. Coverings such as tiles, slates and sheet materials have minimum pitches set by the manufacturer. A lower pitch may need a different covering or special detailing. Always follow the manufacturer’s minimum pitch and the structural design for your building.

UK points to know

The structural design of a roof, including rafter sizes and spans, comes from the designer, span tables or the manufacturer of trussed rafters, and must meet the Building Regulations. Fixing and wind resistance for tiles and slates are covered by BS 5534. Pitch also changes how snow and wind loads act on the roof. Roof work is high risk, so plan edge protection, access and the work at height arrangements before anyone climbs up.

Frequently asked questions

How do I calculate roof pitch in degrees?
Take the arctan of rise divided by run. Rise is the height from wall-plate level to the ridge, and run is the horizontal distance to the ridge centre.

What is the difference between run and span?
Run is half the span on a symmetrical gable. Span is the full eaves-to-eaves width.

How is rafter length calculated?
It is √(rise² + run²). Add eaves overhang separately. Hips, valleys and birdsmouth cuts are not included.

What does pitch ratio or “X in 12” mean?
Pitch ratio is rise:run in simplest form. “X in 12” is the inches of rise for every 12 inches of run. Slope percentage is rise ÷ run × 100.

How do I work out roof slope area?
Multiply the rafter length by the roof length along the ridge for one slope. Double it for a simple gable with two equal slopes.

Is a steeper pitch always better?
No. It sheds water and snow more easily but uses more material. Follow the covering manufacturer’s minimum pitch and the structural design.

Related calculators

Roof work involves cutting, access, time and cost. These tools help:

See all of them in our complete guide to free construction calculators.

Keep your roofing records in one place

Roof inspections, photos of hidden structure before covering and sign-offs all need recording. KaelSite helps UK site teams manage inspections, safety and reporting from one mobile-first app. Explore KaelSite features.

References