Roof Pitch Calculator
Convert an explicitly measured rise and horizontal run, or a known angle, into roof-pitch degrees, percentage slope and rise-to-run ratios.
How the estimate works
Own formula from right-triangle trigonometry: p = rise / horizontal run; degrees = atan(p) × 180 / π; percent = 100 × p; rise:horizontal-run ratios = p:1 and (12 × p):12. Known degrees use p = tan(degrees × π / 180). Explicit horizontal/flat mode gives p = 0 and degrees = 0. No intermediate rounding is applied.
Horizontal run is the horizontal projection, not the sloping roof length, full building span or surface area. Supplied rise must be nonnegative and run strictly positive; supplied degrees must satisfy 0 ≤ degrees < 90. Blank inputs never imply a flat roof. This tool converts one straight slope and does not measure a whole roof, produce a Materials List, select products or provide access, structural, installation or permission advice.
Worked example
A supplied rise of 1 m over a horizontal run of 1 m gives a 45° pitch, 100% slope and a rise:horizontal-run ratio of 1:1, equivalently 12:12. Entering a known 45° angle gives the same conversions but cannot recover either actual length.
What to measure and check
Start with a rise and its corresponding horizontal run from an existing safe measurement or reliable drawing. Alternatively, convert an already-known angle. Use the explicit horizontal/flat option only when you intend to calculate a mathematically horizontal slope; it is not an assessment of a roof described as “flat”. Do not access a roof to obtain inputs for this calculator.
What does horizontal run mean?
It is the horizontal distance corresponding to the vertical rise of the same straight slope. It is not the sloping roof or rafter length. For a simple symmetric gable, a full building span is not the run of one slope; use only the already-known corresponding run.
How are degrees, percent and ratios related?
Divide rise by horizontal run to obtain p. The angle is atan(p) in degrees, percentage slope is 100p, and the normalised rise:run ratio is p:1. The equivalent twelve-unit form is (12p):12. A 100% slope is 45°, not 90°.
Can I enter a zero rise or 0°?
Yes, when explicitly supplied. A zero rise still needs a positive horizontal run. The separate horizontal/flat method explicitly selects a mathematical 0° slope without inventing measurements. Blank rise or angle fields never become zero. A roof called “flat” may have a fall and should not automatically be entered as 0°.
Can a known angle give me roof dimensions or material quantities?
No. An angle alone provides only slope conversions, not run, rise, sloping length, roof area, purchase quantities or a cutting plan. This calculator does not produce a Materials List or recommend products.
Why is 90° rejected?
A vertical line has no positive horizontal run and its rise-to-run ratio is not defined. Angles outside 0 ≤ degrees < 90 are outside this calculator’s nonnegative roof-slope convention. Extreme numerical values that cannot produce finite, representable conversions are also rejected.
Can I use the answer to choose a roof covering or access the roof?
No. These geometric results do not establish covering suitability, minimum permitted pitch, drainage, loading, structural adequacy, roof-access safety, permissions or an installation method. Obtain inputs from existing safe measurements or reliable drawings and seek appropriate professional guidance separately.
Does the tool round the calculation?
No intermediate rounding is used. Numeric results retain floating-point precision and the shared site formats the final display. Very small slopes may look like zero when displayed, so do not use display rounding as a specification or tolerance.
Where the numbers come from
Formula version v1.0. Implemented and tested 10 November 2026. Product-dependent figures remain your supplied facts; a cited example is not a selected product or a universal yield.