Rafter Length Calculator

Use this rafter length calculator to estimate the length of a common roof rafter from the rise, run, and overhang. It is useful when you need a quick framing check for sheds, porches, and simple pitched roofs.

What This Rafter Length Calculator Helps You Do

How to Calculate Rafter Length Calculator

Measure the rafter length inputs - Enter the dimensions, spacing, density, or thickness values that define the roof rafter length and roof angle.
Check the units - Keep every input in the same unit system so the output stays consistent and easy to compare.
Choose the solve mode - Select the output you want, such as length, weight, volume, posts, or cost.
Read the output - Use the main result and the extra detail rows to verify the estimate before you order materials or build.

Rafter Length Calculator Formula

L = sqrt(run^2 + rise^2) + overhang

Symbol	Definition	Unit
run	Horizontal run	ft
rise	Vertical rise	ft
overhang	Overhang allowance	ft
L	Rafter length	ft

Worked Examples

USA - Porch roof

run: 8.0
rise: 4.0
overhang: 1.0
rafterLength: 9.0
roofAngle: 0

Result: Rafter length = 9.94 ft

A simple porch roof gives a fast framing check before cutting lumber. The estimate is 9.94 ft.

UK - Garden shed

run: 6.0
rise: 3.0
overhang: 0.75
rafterLength: 7.0
roofAngle: 0

Result: Roof angle = 26.57 deg

A modest shed roof helps you confirm pitch before ordering rafters. The estimate is 26.57 deg.

EU - Lean-to canopy

run: 7.0
rise: 3.5
overhang: 1.0
rafterLength: 9.0
roofAngle: 0

Result: Run = 7.19 ft

Run is easy to back-calculate when the rafters are already specified. The estimate is 7.19 ft.

GCC - Covered walkway

run: 10.0
rise: 5.0
overhang: 1.25
rafterLength: 12.0
roofAngle: 0

Result: Rise = 3.94 ft

A covered walkway needs the rise back-calculated before you cut the framing. The estimate is 3.94 ft.

How to Interpret Your Results

Range	Meaning	Action
< 6 ft	Short rafter	Usually a small porch or lean-to.
6–12 ft	Typical rafter	Common for sheds, carports, and simple roofs.
> 12 ft	Long rafter	Check lumber size, span limits, and code requirements.

Frequently Asked Questions

It estimates the main planning quantity for rafter length work using the formula shown on the page. That gives you a practical number before you order materials, compare suppliers, or talk to a contractor. Use this as a layout check, not as a substitute for stamped structural drawings.

Enter the values that match the unit labels beside the fields. If the page expects feet, inches, gallons, pounds, or watts, keep everything in that unit family so the result stays reliable.

The calculator multiplies or divides the main quantity by the values you enter, so every measurement feeds directly into the final answer. A small change in depth, area, density, or factor can make a large difference on a bigger project.

Yes, as long as the units stay consistent within the calculation. If the page expects feet, inches, gallons, or pounds, convert first so the final result is accurate and easy to interpret.

Treat the result as a planning estimate. Use the main output for sizing or ordering, then review the detail rows for waste, weight, cost, or conversion notes before you finalize the purchase.

Yes if the job involves cut losses, uneven ground, spill risk, or irregular shapes. A small allowance is usually safer than ordering exactly to the bare math, especially for rafter length projects that are hard to top up later.

It is exact for the numbers you enter, but real-world projects can still vary because of compaction, tolerances, site conditions, and product differences. Use the result as a solid working estimate, not a final structural or procurement check.

Yes. That is one of its main uses. The result helps you estimate how much to buy, what it may weigh, and what the budget might look like before you place an order or request a quote.

Disclaimer: This calculator provides planning estimates only. Verify local framing code, snow load, and structural requirements before cutting rafters.

Sources

Last reviewed: March 2026

Rafter Length Calculator

Result

What This Rafter Length Calculator Helps You Do

How to Calculate Rafter Length Calculator

Rafter Length Calculator Formula

Worked Examples

How to Interpret Your Results

Frequently Asked Questions

What does the Rafter Length calculator estimate?

Which measurements should I enter for the Rafter Length calculator?

Why does the Rafter Length result change when I change one input?

Can I use metric and imperial units with the Rafter Length calculator?

How should I read the Rafter Length result?

Should I add waste or allowance to the Rafter Length calculation?

Is the Rafter Length calculator exact?

Can I use the Rafter Length calculator for ordering materials?

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