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Trades & Professional Tools · Carpentry & Framing

Birdsmouth Cut Calculator

Lay out common-rafter birdsmouth geometry from building width, ridge thickness, pitch, seat length and overhang.

Trades & Professional Tools · Carpentry & Framing

Enter your project measurements

Measured inputs · visitor-entered product and design assumptions
  1. 1EnterProvide the known values
  2. 2CalculateResults update automatically
  3. 3VerifyReview the details and units
Try an example

Use actual measured dimensions and keep one unit system throughout. Enter locally approved structural limits, product data, stock sizes, kerf, waste, fastener schedules, and finished-surface allowances where requested. Values stay in this browser.

Trade calculator result

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Understand the result

What the Birdsmouth Cut is for

Lay out common-rafter birdsmouth geometry from building width, ridge thickness, pitch, seat length and overhang.

Roof-framing geometry converts run, rise, pitch, plan shape, and entered allowances into centerline lengths and layout dimensions.

At a glance

See the field workflow

1Run, rise, and plan
2Rafter geometry
3Layout dimension
Read from measured conditions to the calculated field check. Each stage remains a separate assumption you can verify.
Outside building width (m or ft)Ridge-board thickness (mm or in)Pitch risePitch run

The relationship

The formula used here

Rafter run=(building width−ridge thickness)/2; vertical heel rise=seat×pitch; notch depth perpendicular to the rafter=seat×sin(roof angle). Line lengths use the rafter factor.

Apply this relationship only after confirming that the measurements, product basis, allowances, and units describe the same project condition.

Field reading

Use the result as a checked next step

1. Measure the real work

Use field dimensions, approved drawings, and actual product or stock information.

2. Reconcile the result

Verify the plan geometry and pitch first, then distinguish theoretical line length from actual cut length, bearing, overhang, and fitting allowances.

3. Keep the boundary clear

It does not size rafters, hips, valleys, ridges, beams, ties, connectors, bracing, bearings, or approve a roof structure.

Quick guide

How to use this calculator

  1. Choose the calculation mode or measurement system that matches the values you actually have.
  2. Enter measured project dimensions and use product-specific coverage, density, yield, resistance, or waste data where requested.
  3. Review the intermediate results before ordering material or applying the result on site.

Calculation method

Calculation method

Rafter run=(building width−ridge thickness)/2; vertical heel rise=seat×pitch; notch depth perpendicular to the rafter=seat×sin(roof angle). Line lengths use the rafter factor.

The calculator preserves full internal precision and rounds only displayed results. Quantity outputs round up when whole purchasable pieces are required.

Worked example

Worked example

An 8 m building with a 38 mm ridge has a 3.981 m horizontal run; a 90 mm seat at 6:12 cuts about 40.25 mm perpendicular to the rafter.

Rafter run=(building width−ridge thickness)/2; vertical heel rise=seat×pitch; notch depth perpendicular to the rafter=seat×sin(roof angle). Line lengths use the rafter factor.

Supported inputs

Precision and limits

Carpentry geometry and structural boundary

Carpentry results calculate measured geometry, layout marks, cut dimensions, quantities, stock planning, or comparisons with limits you enter. They do not select joists, rafters, beams, headers, posts, footings, sheathing schedules, connectors, fasteners, bracing, guards, stairs, or lifting methods; verify species, grade, moisture, loads, bearing, deflection, notches, connections, code, manufacturer data, approved drawings, and site conditions with the responsible professional.

Planning scope

This calculator supports measurement, estimating, and scenario planning. It does not inspect a site, select a product, certify a design, or replace drawings, specifications, manufacturer instructions, local codes, permits, or a qualified trade professional.

Measured and product-specific inputs

Nominal dimensions can differ from actual dimensions. Verify inside pipe diameters, actual lumber sizes, product coverage, bag yield, material density, conductor resistance, waste, and packaging against the materials used on the project.

Safety and regulatory limits

Electrical, HVAC, structural, excavation, roofing, pressure, lifting, and life-safety work may require jurisdiction-specific calculations and professional approval. Do not use a generic planning result as a code-compliance or safety determination.

Precision and privacy

Inputs accept bounded plain decimals. Calculations remain in this browser and raw entered project values are not submitted to 365CALCS.COM.

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