Roof Pitch Calculator

Last updated: June 2026

Mode

Horizontal span

Vertical rise

Pitch

18.4°

Ratio

4:12

Rafter length

3.16 m

Roof multiplier

1.054

Formula

Pitch degrees = atan(rise / span) x 180 / pi. Rafter length = square root of rise² + span². Multiplier = rafter length / span.

Pitch reference table

Pitch	Degrees	Description
3:12	14°	Low pitch
4:12	18°	Standard
6:12	27°	Medium
8:12	34°	Steep
12:12	45°	Very steep

About this calculator

Calculate roof pitch angle, pitch ratio, rafter length, and roof multiplier from rise and span, or estimate rafter length from a known pitch. It is intended for planning and checking roof geometry, not structural design.

Roof pitch method

The calculator uses metric dimensions and practical UK planning assumptions. Results are estimates, so check product labels, site conditions, and supplier guidance before ordering materials.

pitch degrees = atan(rise / span) x 180 / pi
pitch ratio = (rise / span) x 12
rafter length = square root of rise² + span²

How to use the roof pitch calculator

Choose the project type or measurement mode where the calculator offers one.
Enter all dimensions in metric units.
Add allowances such as coats, waste, pattern repeat, roll width, or bag size where relevant.
Review the main highlighted result first.
Check secondary results such as packs, bags, rolls, posts, or tin combinations.
Round up to supplier pack sizes and confirm the label coverage before buying.

Worked examples

Pitch from rise and span

Input: 3m horizontal span, 1m rise

Calculation: atan(1 / 3) x 180 / pi = 18.4°.

Result: Pitch is about 18.4°, close to a 4:12 roof.

Rafter from pitch

Input: 3m span at 27°

Calculation: Rise = tan(27°) x 3. Rafter = square root of rise² + span².

Result: The calculator returns rafter length and multiplier.

What is roof pitch?

Roof pitch describes how steep a roof is. It can be expressed as an angle in degrees or as a rise-to-run ratio such as 4:12, meaning 4 units of rise for every 12 units of horizontal run.

Minimum pitch for roofing materials

Different roofing products have different minimum pitch requirements. Tiles, slates, sheets, and membranes may also have manufacturer-specific rules, so always check the product guidance before choosing a covering.

Common mistakes

Mixing centimetres, millimetres, and metres without converting first.
Ordering exact quantities with no allowance for cuts, waste, damage, or uneven surfaces.
Forgetting that products are sold in tins, packs, bags, rolls, or boards rather than exact calculated quantities.
Using average deductions or coverage rates when the project has unusual shapes, textures, slopes, or access limits.
Not checking manufacturer instructions before final ordering.

Home improvement estimate disclaimer

This calculator is for general DIY planning only. It is not a substitute for a professional survey, structural design, building control advice, manufacturer instructions, or installer measurement.

Actual product coverage and pack sizes vary by supplier.
Site conditions can change material quantities.
Structural, roofing, electrical, gas, and regulated building work may need qualified advice.

Frequently asked questions

What is horizontal span?

For pitch calculations, horizontal span is the run from the wall plate or roof edge to the ridge line, not the full roof slope length.

What does a 4:12 pitch mean?

It means the roof rises 4 units for every 12 units of horizontal run. That is about 18.4 degrees.

Is rafter length the same as roof width?

No. Rafter length is the sloping length, which is longer than the horizontal span.

Can I use this for structural sizing?

No. It calculates geometry only. Rafter sizes, loads, fixings, and building control requirements need proper design checks.

Why is pitch important?

Pitch affects drainage, usable loft space, roofing material choice, visual appearance, and rafter length.

Related calculators

Triangle Calculator
Area Calculator
Decking Calculator

What does this mean?

This calculator is designed to help you understand the likely number before you make a decision or start an application.

Your result should be checked against official UK guidance, especially if your circumstances include dependants, exemptions, prior leave, or a complex immigration history.

Treat the figure as a planning tool rather than legal advice. Where the answer affects an application deadline or major payment, speak to an authorised adviser.