Rafter Length Calculator

Estimate a common rafter’s sloping length, including its overhang. Define the horizontal reference points first; the result is geometry, not an approved cut list or structural design.

Horizontal measurement available

Dimension unitsSwitching units converts the entered dimensions.

Effective horizontal runftHorizontal distance from the chosen wall reference to the ridge face. Do not subtract ridge thickness again in this mode.

Reference span across symmetric roofftDistance between matching wall reference lines. Run = (span − horizontal ridge thickness) ÷ 2.

Horizontal ridge-board thicknessinSpan mode only. Zero gives a centerline geometry example; enter the actual horizontal thickness when adjusting to the ridge face.

Roof slope input

Rise per 12 units of run: 126 means a 6:12 slope, not a 6° angle.

Angle above horizontaldegrees

Horizontal overhang beyond wall referenceftThis is horizontal projection, not the sloping tail length. Use the intended tail endpoint consistently.

Extra stock length (optional)ftAdded along the board after the geometric length. Choose your own trim or stock allowance.

Calculation results

Common-rafter geometric length

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How to use this calculator

Define the run

Use the horizontal wall-to-ridge-face distance, or use a symmetric span and enter the horizontal ridge thickness. Do not confuse the full building width with a single rafter’s run.

Enter the roof slope

Choose a rise-per-12 ratio or an angle from horizontal. These describe the same straight slope; neither specifies rafter strength or allowable span.

Measure the overhang horizontally

The calculator converts the overhang projection into a sloping tail using the same roof slope. It keeps this tail separate from the wall-to-ridge length.

Check the actual layout

If needed, add extra stock along the board. Verify cuts, birdsmouth, fascia, connection details and available stock independently before ordering or cutting.

Formula and worked example

Span mode: effective run = (reference span − horizontal ridge thickness) ÷ 2
Slope = rise per 12 ÷ 12, or tan(angle)
Slope multiplier = √(1 + slope²)
Wall-to-ridge length = effective run × slope multiplier
Sloping tail = horizontal overhang × slope multiplier
Geometric total = wall-to-ridge length + sloping tail
Length with entered stock allowance = geometric total + extra stock

For an effective run of 12 ft, a 6:12 slope and a 1 ft horizontal overhang, the slope multiplier is √1.25 ≈ 1.118034. The wall-to-ridge length is 13.4164 ft and the tail is 1.1180 ft, giving 14.5344 ft before any extra stock. A 24-ft symmetric span with zero ridge thickness gives the same example. A real ridge thickness changes the effective run.

Common-rafter geometry for 12 ft run plus 1 ft overhang

Same effective run and horizontal overhang; no cut geometry or extra stock. These are not recommended roof slopes.
Slope	Wall-to-ridge length	Total including tail
4:12	12.6491 ft	13.7032 ft
6:12	13.4164 ft	14.5344 ft
12:12	16.9706 ft	18.3848 ft

Frequently asked questions

Is this different from the Roof Pitch Calculator?

Yes. This page focuses on the rafter’s length components: effective run, optional symmetric-span ridge adjustment, sloping overhang and extra stock. The Roof Pitch Calculator focuses on expressing and converting slope.

Does this give the final board cut length?

Not necessarily. The result follows the reference line and endpoints defined by your inputs. Birdsmouth layout, plumb cuts, fascia details, board depth and trimming are not modeled. Check an actual rafter layout.

How is ridge thickness deducted?

In span mode it is deducted from the horizontal span before dividing by two. The shorter effective run is then converted to sloping length. Run mode assumes you already measured to the intended ridge face and makes no further deduction.

Can I calculate hip or valley rafters?

No. The equations here are for a common rafter in one vertical roof section. Hip, valley, jack, unequal-pitch and skewed-roof geometry need different inputs and methods.

Will a longer result tell me which lumber size is safe?

No. Species, grade, spacing, loads, connections and governing requirements affect structural sizing. This calculator does not approve a rafter size, span or cut.

Methodology and sources

Straight common-rafter geometry. Span mode assumes a symmetric roof and a centered ridge. Ridge thickness is a horizontal adjustment, not a fixed deduction along the sloping board. No birdsmouth or structural calculations are included. Angles are limited to 0–85°; pitch inputs to 0–120 per 12. Defaults are illustrative.

Johnson Level — rafter run and pitch — Manufacturer guidance distinguishing span, run and roof slope. Our tail and stock results are explicit geometric calculations.
Swanson Tool — common-rafter geometry — The run/rise right-triangle relationship. Do not treat our simplified geometry as a replacement for the manufacturer’s layout and cutting instructions.
NIST — unit conversion factors — International foot and metric length conventions.

Sources checked September 1, 2026. Referenced organizations do not endorse Efficienco. Lengths retain full precision until display. Extra stock is entered explicitly; no standard board size or cut allowance is assumed.