What is the Rafter Calculator?

The Rafter Calculator is a free web-based construction tool designed to help carpenters, roofers, builders, and DIYers determine accurate rafter lengths, roof pitch angles, birdsmouth cut dimensions, and material requirements directly in their web browser.

How do you calculate rafter length with a calculator?

To calculate rafter length with a calculator, measure your roof's effective horizontal run and vertical rise, then apply the right-triangle Pythagorean formula: Rafter Length = √(Run² + Rise²). You then adjust the result for half the ridge board thickness and add any desired eave overhang.

How do you work out rafter length using a Rafter Calculator?

Working out rafter length using our Rafter Calculator is simple: enter your total roof span or half run, select your roof pitch ratio or slope angle in degrees, and input your overhang distance. The tool automatically computes the exact rafter length, total rise, plumb cut angle, and seat cut width in real time.

How does a Rafter Calculator work with a diagram?

Our Rafter Calculator features an interactive 2D vector schematic canvas that updates dynamically as you change your inputs. The visual diagram illustrates the angled rafter board, ridge beam intersection, wall plate, birdsmouth notch, and overhang with clear dimension callout lines, allowing you to visually verify all cuts before making your first saw mark.

Is there a free Rafter Calculator app?

Yes, Raftercalculator.org is a completely free web application accessible on any desktop, tablet, or smartphone without registration, subscriptions, or app store installation required.

Does the Rafter Calculator support metric measurements?

Yes, the Rafter Calculator provides full support for metric measurements including Centimeters (cm) and Meters (m), as well as Imperial Inches (in) and Feet-Inches (ft-in). Changing the unit selector automatically updates all input fields, labels, and output specifications.

How do I use this Rafter Calculator?

To use this Rafter Calculator, select your preferred measurement unit, choose whether you are entering Total Roof Span or Half Run, enter your horizontal distance, select your roof pitch, and specify your eave overhang. Use the advanced settings to fine-tune ridge board thickness, wall plate width, and rafter lumber depth.

When should I use a Rafter Calculator?

You should use a Rafter Calculator during the planning, estimation, and cutting phases of any roof framing project. It is ideal for estimating lumber quantities, preparing job-site cut lists, verifying birdsmouth notch depths, and checking slope angles for sheds, garages, home additions, and residential roofs.

What is the difference between "Span" and "Run"?

Total Roof Span is the horizontal distance between the outside edges of opposite exterior wall plates. Roof Run is the horizontal distance a single rafter covers from the wall plate to the center of the building, equal to half the total span minus half the thickness of the ridge board.

How do you calculate the standard length of a common rafter?

The standard length of a common rafter from the ridge plumb cut line to the wall plate plumb cut line is calculated using Rafter Length = √(Run² + Rise²), where Run is half span minus half ridge thickness, and Rise is Run multiplied by the pitch ratio. Eave overhang is added separately.

What does a "6/12 roof pitch" mean?

A 6/12 roof pitch means the roof rises 6 units vertically for every 12 units of horizontal run. This represents a 50% slope grade with a pitch angle of 26.57 degrees, which is one of the most common standard roof slopes in residential construction.

How does the ridge board affect the rafter calculation?

The ridge board runs horizontally along the peak where opposing rafters meet. Because rafters abut the ridge board rather than each other, half of the actual ridge board thickness (e.g., 0.75 inches for a nominal 1.5-inch 2x ridge board) must be subtracted from the total building run before calculating rafter length.

Is there a Rafter Calculator app available for free download?

This Rafter Calculator runs directly in your web browser as a responsive progressive web application, eliminating the need to download or install software from an app store. You can bookmark or save the page to your mobile home screen for instant offline-ready access on the job site.

Legal & Transparency

Privacy Policy

At Rafter Calculator, we believe privacy is a fundamental standard. Our framing estimation tool is built to process all calculations directly inside your browser without uploading your project dimensions to external servers.

Last Updated: September 2026

1. Zero Personal Data Collection

Rafter Calculator (raftercalculator.org) operates as a free, static web utility. You are never required to create an account, log in, or provide personal identifiable information (PII) such as your name, phone number, physical jobsite address, or billing details to use our calculator.

All inputs—including roof span, rise, pitch ratio, eave overhang, ridge thickness, and lumber prices—remain strictly local to your computing device's memory.

2. Client-Side Execution & Local Storage

Our calculation engine and interactive 2D SVG schematic canvas execute 100% in your client web browser using standard Vanilla JavaScript routines. When you adjust units (such as switching from Imperial Feet-Inches to Metric Centimeters) or save project presets:

Browser Storage: Preferences may be stored in your browser's localStorage strictly for your convenience during repeat visits.
No Remote Database: We do not transmit, index, or store your custom rafter cut dimensions on any remote database or cloud server.

3. Server Logs & Web Analytics

Like almost all web servers, our hosting infrastructure automatically collects standard non-identifying technical metadata when requests are served. This may include:

Anonymized IP addresses for DDoS mitigation and server security.
Browser type, device classification, operating system, and referral headers.
Time stamps and aggregate page view counts.

This technical log data is used solely to maintain platform stability, ensure fast page load performance, and protect the site against cyber threats.

4. Cookies & Third-Party Services

We do not sell, rent, or trade user data. We do not use intrusive tracking cookies or cross-site behavioral profiling.

If Google Fonts or CDN assets are loaded to render Geist typography or iconography, those service providers receive standard web requests necessary to deliver visual static assets efficiently.

5. Contacting Us About Privacy

If you have questions, concerns, or feedback regarding our privacy practices or security standards, please contact us via our Contact Us page or email us directly at:

support@raftercalculator.org

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