Roof pitch is the steepness of a roof, expressed as a ratio of rise to run or as an angle in degrees. A higher pitch means a steeper roof. In the UK, pitch is commonly expressed in degrees or as a ratio such as 1:2.5.

How do I calculate roof pitch from rise and run?

Divide the rise by the run to get the pitch ratio. To convert to degrees, use the arctangent: angle = arctan(rise divided by run). For example, a rise of 1200 mm and a run of 3000 mm gives a pitch of 0.4 and an angle of approximately 21.8 degrees.

What does 4:12 roof pitch mean?

A 4:12 pitch means the roof rises 4 units for every 12 units of horizontal run. This works out at approximately 18.4 degrees. The X:12 format is more common in the US; UK roofing more often uses degrees or a simple rise-to-run ratio.

Is roof pitch the same as roof angle?

They describe the same thing in different formats. Pitch is usually a ratio (rise over run), while roof angle is expressed in degrees. A pitch of 1:2.5 is equivalent to approximately 21.8 degrees.

What is a low-pitch roof in the UK?

In the UK, a low-pitch roof is generally considered to be below 20 to 22.5 degrees, depending on the roof covering. Most tile and slate manufacturers specify a minimum pitch — typically 17.5 to 22.5 degrees for interlocking concrete tiles and 20 to 25 degrees for natural slate. Always check the manufacturer's data sheet.

Can I use this calculator with millimetres?

Yes. As long as you use the same unit for both rise and run, the result is the same. Enter both in millimetres, centimetres, metres, inches, or feet — the pitch ratio and angle do not change because the units cancel out.

Why does roof pitch matter for tiles and slates?

Different roof coverings have minimum pitch requirements to remain weather-tight. Below the minimum, water can back up under tiles or slates and cause leaks. The minimum varies by product, so always check the manufacturer's fixing instructions before specifying a roof covering.

Can this calculator tell me if my roof covering is suitable?

No. This calculator gives you the pitch angle and ratio, but suitability of a roof covering depends on the specific product, the full roof build-up, and site exposure. Always check the manufacturer's data sheet and consult a qualified roofer for final confirmation.

Roof Pitch Calculator UK

Use this roof pitch calculator to convert rise and run into roof angle, pitch ratio, and slope. It is useful for roofing estimates, extension planning, rafter setting out, and quick site checks where you need to sense-check the shape of a pitched roof. Many UK users measure in millimetres, but the maths works the same in mm, cm, metres, inches, or feet as long as both inputs use the same unit. This tool gives you a fast answer before you move on to rafter length, roof covering choice, or batten spacing.

Units

Rise

Run

Display Pitch Ratio using Common Run:

How to use this calculator

1.Enter the rise and run of the roof, or switch to the angle tab if you already know the roof angle.
2.Choose metric or imperial units depending on how the roof has been measured.
3.Read the result as a roof angle in degrees, a pitch ratio, and a slope value.
4.Use the result as a quick planning check before moving on to rafters, battens, tiles, slates, or other roofing details.

Formulas used

Pitch ratio

pitch = rise / run

Angle in degrees

angle = arctan(rise / run)

Slope percentage

slope % = (rise / run) × 100

X in 12 pitch

X = 12 × (rise / run)

Slope factor / pitch multiplier

slope factor = √(1 + (rise / run)²)

Rafter length from rise and run

rafter length = √(rise² + run²)

Use the same unit for rise and run. The result stays correct whether you measure in mm, cm, metres, inches, or feet.

Worked example

Take a small extension roof with a rise of 1200 mm and a run of 3000 mm. Divide the rise by the run and you get 0.4. That converts to a roof angle of about 21.8 degrees. In practical terms, that is a moderate pitch rather than a very shallow one. It may suit many pitched-roof applications, but the final covering still depends on the specific tile, slate, or roofing system being used and the manufacturer's minimum pitch guidance.

Pitch, angle and minimum roof pitch

Roof pitch and roof angle describe the same roof shape in different formats. Pitch is often shown as a ratio, such as 4:12, while roof angle is shown in degrees. A 4:12 pitch works out at roughly 18.4 degrees. That matters because roof coverings are not interchangeable at every slope. In the UK, the minimum pitch depends on the exact roofing product and the full roof build-up, not one universal number. As a quick rule of thumb, lower-pitch roofs need more care with product selection and detailing.

Why roof pitch matters

Getting the pitch right affects more than appearance. It influences runoff, tile or slate suitability, felt and batten layout, and how easily the roof sheds water in exposed conditions. It also affects follow-on calculations such as rafter length and true roof area. If you are pricing work, ordering materials, or checking whether a roof covering is suitable, pitch is usually one of the first numbers you need.

Frequently asked questions

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Roof Tile Calculator — work out gauge, tiles per m² and total tiles for your roof
How to Calculate Roof Pitch UK — four measuring methods, the formula, and when precision matters

Disclaimer: This calculator is for estimation and planning only. Final roof design, structure, and covering choice should always be checked against project requirements, manufacturer guidance, and professional advice where needed.