How do I calculate rafter length if I only know the span and pitch?

You can use the span and pitch to find the rise, then apply the Pythagorean theorem: rafter length equals the square root of (run squared plus rise squared), where run is half the span.

What is the difference between rafter length and overhang?

Rafter length is the sloped distance from the wall top plate to the ridge board, while overhang is the horizontal extension of the rafter past the wall line, often measured as a horizontal run or a sloped tail length.

Do I need to account for ridge board thickness when measuring rafters?

Yes, the ridge board thickness is typically deducted from the horizontal run on each side, so the rafter length measured to the wall line will be slightly shorter than the total rafter length if the ridge is not accounted for.

Can I use a rafter length calculator for hip roofs?

Most rafter length calculators are designed for gable roofs; hip roofs require additional geometry for the hip rafter angle and length, so a specialized hip roof calculator or trigonometric inputs are needed.

Why does my calculated rafter length not match my actual measured rafter?

Discrepancies often come from rounding, missing ridge thickness allowances, or the calculator providing geometry-only results without accounting for actual lumber dimensions or site-specific cuts.

Construction2 options compared

Rafter Length Calculator

Calculates rafter lengths for accurate roof truss dimensions. Enter known measurements like span, rise, and run to get precise rafter length results. Architects, builders, and DIY enthusiasts need this tool to ensure proper roof frame construction. Accurate rafter lengths prevent structural issues and save time on site adjustments.

Roofing calculators

Editors’ Top PickBased on community votes

Omni Calculator

omnicalculator.com

Use this rafter length calculator to make sure you got your roof truss dimensions right.

Open Omni Calculator Read review →

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02
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GQ Calc
gqcalc.com
Calculate common-rafter line length from building span, x-in-12 roof pitch, ridge thickness, and horizontal eave over...
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Side by side

Rafter Length Calculator options compared

Tool	Best for	Strengths	Limitations
Omni Calculator omnicalculator.com	Quick rafter length from basic roof dimensions	Simple input of span, rise, or run Covers core roof geometry needs	Fewer detailed cut-length options No ridge thickness or overhang breakdown
GQ Calc gqcalc.com	Detailed rafter cut lengths and overhangs	Separates ridge-to-wall from sloped tail lengths Supports US and metric units with editable assumptions	Requires ridge thickness and eave overhang inputs Result is geometry only, not code-verified lumber sizing

Buyer's guide

How to choose a rafter length calculator

When picking a rafter length calculator, decide whether you need a quick overall rafter size or detailed cut lengths for each roof component. If you just need the total rafter length from basic span, rise, or run, a simple tool will save time. If you are cutting rafters on site and need to know the exact ridge-to-fascia length, the tail length beyond the wall, and how ridge thickness or overhang changes those numbers, choose a tool that asks for those extra measurements.

Questions

Rafter Length Calculator FAQ

How do I calculate rafter length if I only know the span and pitch?: You can use the span and pitch to find the rise, then apply the Pythagorean theorem: rafter length equals the square root of (run squared plus rise squared), where run is half the span.
What is the difference between rafter length and overhang?: Rafter length is the sloped distance from the wall top plate to the ridge board, while overhang is the horizontal extension of the rafter past the wall line, often measured as a horizontal run or a sloped tail length.
Do I need to account for ridge board thickness when measuring rafters?: Yes, the ridge board thickness is typically deducted from the horizontal run on each side, so the rafter length measured to the wall line will be slightly shorter than the total rafter length if the ridge is not accounted for.
Can I use a rafter length calculator for hip roofs?: Most rafter length calculators are designed for gable roofs; hip roofs require additional geometry for the hip rafter angle and length, so a specialized hip roof calculator or trigonometric inputs are needed.
Why does my calculated rafter length not match my actual measured rafter?: Discrepancies often come from rounding, missing ridge thickness allowances, or the calculator providing geometry-only results without accounting for actual lumber dimensions or site-specific cuts.

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