Exterior calculator

Rafter Length Calculator

Calculate simple common-rafter geometry and total stock length without structural sizing. Switch between US customary and metric units, adjust the assumptions, and keep every step visible.

01 Measure02 Allow03 Order

Project inputs

Use one unit system throughout. Defaults show a realistic example; replace them with measurements from your project.

Unit system

Horizontal runExample input — replace with your project value.ftVertical riseExample input — replace with your project value.ftEave overhangExample input — replace with your project value.ftRafter pairsPlanning default — verify the selected product or project specification.pairsWaste allowanceEditable planning allowance for cuts, breakage, compaction, or site loss.%

Measurement guideRun is horizontal. Rise is vertical. Measure the building length separately.

Measure the finished shape. Geometry only. This does not select rafter size, grade, spacing, connections, ridge details, or prove structural adequacy.

Estimate matches the inputs shown.

ReviewedNovember 2, 2026

ByBuildGauge Editorial Team

ScopeFormula, defaults & sources

Report an issue ↗

The calculation

A visible formula, not a black box.

Length & geometry · Mathematical derivationone rafter = √(run² + rise²) + overhang × slope factor; total stock = rafter × 2 × pairs × waste

Inputs are converted to a common base unit before calculation. Purchase quantities are rounded only at the final buying boundary.

Example inputDefaultStatus

Horizontal run12 ftPlanning default — verify

Vertical rise6 ftPlanning default — verify

Eave overhang1 ftPlanning default — verify

Rafter pairs16 pairsPlanning default — verify

Waste allowance10 %Planning default — verify

Worked example

Example: planning rafter length

Using the editable example inputs—horizontal run 12 ft, vertical rise 6 ft, eave overhang 1 ft—the calculator returns 14.53 ft. Replace every example with measured dimensions and the selected product label before purchase.

Use this to understand the workflow—not as a recommendation for your project.

Sources & scope

What supports this calculation.

Industry guidanceOpenStax right-triangle trigonometry ↗

Supports the product rule or estimating basis used by the Rafter Length Calculator; geometry and unit conversion remain visibly derived in the calculator.

Mathematical derivationNIST SI conversion factors ↗

Exact length, area, and volume unit conversions; the project formula is derived from standard geometry.

Common mistakes

Check the job, not just the number.

01Leaving the example rafter length inputs unchanged instead of entering measured project values.
02Treating an editable coverage, size, density, or allowance as universal instead of checking the selected product label, project documents, and local requirements.
03Ordering from the rounded result without reviewing exclusions: Geometry only. This does not select rafter size, grade, spacing, connections, ridge details, or prove structural adequacy.

Questions, answered

Rafter length FAQ

What does the Rafter Length Calculator estimate?

Calculate simple common-rafter geometry and total stock length without structural sizing. The primary output is length & geometry, with the intermediate assumptions kept visible.

How is the rafter length result calculated?

The visible formula is: one rafter = √(run² + rise²) + overhang × slope factor; total stock = rafter × 2 × pairs × waste. Inputs are converted to a common base unit before the final purchase boundary is rounded.

Are the default values recommendations?

No. Dimensions are worked examples and product-dependent presets are planning defaults. Replace them with the selected product label, project documents, and local requirements.

Is the result ready to order from?

Use it as a checked planning estimate, then confirm packaging, minimum orders, installation instructions, site conditions, and any code or professional-design requirements.