Project measurements

Change the defaults to match your project and the product you plan to buy.

Start with

Horizontal run to ridge centerline

ft

Roof pitch rise per 12 run

in/12

Horizontal overhang beyond wall line (optional)

ft

Project result

Common-rafter geometry and line length

Review horizontal run, rise, pitch multiplier and the geometric sloped line to the wall and through the optional overhang before any cut-detail or structural adjustment.

Planning estimate only. Verify product coverage, package size, supplier specifications and project requirements before purchasing.

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Budget scenario

Add your current local costs

Use a current supplier subtotal or reuse a priced calculator result, then add optional delivery, tax and contingency. Home Measure Lab does not fetch or guess live prices.

Currency

Supplier / material subtotal

local

Delivery / project fees

optional

Tax

%

Contingency

%

This calculator does not currently expose a priced result. Enter a current supplier subtotal manually.

Material subtotal$0.00

Delivery / fees$0.00

Tax$0.00

Contingency$0.00

Planning total$0.00

Enter a current supplier subtotal or reuse a priced calculator result.

Budget values are local planning inputs, not live quotes. Verify tax treatment, delivery, labor and supplier pricing before purchasing.

Use the result

From measurement to purchase

Measure the project area carefully.
Replace editable defaults with product-label or supplier values.
Review the waste or extra allowance.
Use the rounded purchase quantity for whole packages.

Assumptions & notes

This is a geometry calculator for a common rafter on one uniform roof slope. It does not size lumber, verify span capacity, choose spacing, calculate loads or design connections.
In span mode, the calculator assumes a simple symmetrical gable and uses half the entered outside-wall span as the horizontal run to the ridge centerline. Ridge-board thickness, seat/birdsmouth geometry and plumb-cut deductions are not included.
Overhang is entered as horizontal projection beyond the wall line, not as a measurement taken along the sloped rafter tail. Use plans or safe measurements instead of climbing onto a roof merely to obtain an input.

Choose the right tool

When to use this calculator

You need geometric common-rafter line length from a verified horizontal run or simple centered-gable span plus pitch and optional horizontal overhang.

Use a different calculator when

Use Roof Pitch for inclination conversion, Roof Area for sloped surface area, or Roofing for material quantities. This tool does not size rafters or create cut/structural details.

Roof Pitch Calculator →Roof Area Calculator →Roofing Calculator →

Methodology

Formula, geometry boundary & assumptions

Horizontal run

entered run, or building span ÷ 2

Span mode assumes a simple symmetrical gable and uses the ridge centerline as the geometric midpoint.

Rise to ridge

horizontal run × (pitch rise ÷ 12)

X/12 pitch is vertical rise for 12 units of horizontal run.

Pitch multiplier

√(1 + (pitch rise ÷ 12)²)

The same right-triangle multiplier converts a horizontal run into its sloped line length.

Common-rafter line to wall

horizontal run × pitch multiplier

Geometric line only; ridge, birdsmouth and cut details remain outside this result.

Total line with overhang

(horizontal run + horizontal overhang) × pitch multiplier

Overhang must be entered as horizontal projection.

How to measure this project

Use drawings, a verified span record, safe ground/attic measurements or a qualified measurement source. Run is horizontal distance from the wall reference to the ridge centerline, not the sloped roof surface. If you enter the full span of a simple symmetrical gable, verify that the two roof sides share one pitch and that the ridge is centered. Treat additions, offset ridges, shed roofs, hips, valleys and mixed pitches as separate geometry rather than forcing them into span mode.

Before you buy materials

Use this result as a geometric planning line, not as a lumber order or cut list. Actual stock length can require allowances for ridge details, birdsmouth/seat geometry, fascia or tail treatment, field trimming and project-specific detailing. Lumber species, grade, member size, spacing, loads, allowable spans, connections and code compliance require the applicable project design and requirements.

Reference

Common pitch multipliers for rafter geometry

Pitch	Angle	Multiplier	12 ft run line
3/12	14.04°	1.0308	12.37 ft
4/12	18.43°	1.0541	12.65 ft
5/12	22.62°	1.0833	13.00 ft
6/12	26.57°	1.1180	13.42 ft
8/12	33.69°	1.2019	14.42 ft
10/12	39.81°	1.3017	15.62 ft
12/12	45.00°	1.4142	16.97 ft

Source evidence

Primary references behind key assumptions

These references document specific assumptions used or discussed on this page. The exact product label, technical data sheet, project drawings and applicable requirements take priority.

OSHA — Fall Protection in Residential Construction

Supports: Residential roof work, including installing ridge poles and rafters, presents fall hazards and requires appropriate fall protection planning.

Boundary: This source supports the safety boundary only; it does not provide or validate HomeMeasureLab rafter geometry, lumber sizing or structural design.

Questions

Frequently asked questions

Is rafter run the same as rafter length?

No. Run is horizontal. Rafter length is the sloped hypotenuse created by the run and roof rise.

Can I enter the full building width as run?

Only in the calculator’s building-span mode. For a centered simple gable, the geometric run for one side is half the full span.

Does the result include the birdsmouth or ridge-board deduction?

No. It reports geometric line length to a ridge centerline and through an optional horizontal overhang. Cut details and ridge/seat adjustments depend on the framing detail.

Does this calculator tell me what rafter size or spacing to use?

No. Structural member size, species/grade, spacing, loads, connections and allowable span are separate design questions.

Is overhang measured along the rafter?

No. Enter the horizontal projection beyond the wall line; the calculator converts that projection into sloped line length using the pitch multiplier.

Next step

Keep this project moving

Follow the measurement into the most useful related reference, calculator or project view.

Understand the inputHow to Measure Roof Run for Common Rafter LengthContinue →Compare a nearby toolRoof Pitch CalculatorContinue →Browse this project typeRoofing & Exterior calculatorsContinue →

Planning guides

Understand the inputs behind this calculator

All guides →

Roofing & Exterior

How to Measure Roof Run for Common Rafter Length

Measure horizontal roof run or simple-gable span correctly before calculating common-rafter length, pitch rise and horizontal overhang.

Read guide →

Roofing & Exterior

Rafter Length vs Roof Pitch: Geometry, Multiplier & Limits

Understand how roof pitch, horizontal run, rise and pitch multiplier relate to common-rafter line length without confusing geometry with structural sizing.

Read guide →

Related calculators

All calculators →

Roofing & Exterior

Common Rafter Length Calculator: Run, Pitch & Overhang

Project measurements

Common-rafter geometry and line length

Add your current local costs

When to use this calculator

Formula, geometry boundary & assumptions

Horizontal run

Rise to ridge

Pitch multiplier

Common-rafter line to wall

Total line with overhang

How to measure this project

Before you buy materials

Common pitch multipliers for rafter geometry

Primary references behind key assumptions

Frequently asked questions

Keep this project moving

Understand the inputs behind this calculator

How to Measure Roof Run for Common Rafter Length

Rafter Length vs Roof Pitch: Geometry, Multiplier & Limits

Related calculators

Roof Pitch Calculator

Roof Area Calculator

Roofing Calculator