Rafter Calculator 

Every common rafter is the hypotenuse of a right triangle built from the run and the rise, and getting that triangle right on paper is what makes the lumber order, the cuts and the ridge line up on site. This calculator works the triangle from your span and pitch, or from a known total rise, then adjusts for eave overhang and ridge board thickness, suggests the stock board length to order, and counts rafters at 24 inch on-center spacing.

Quick Answer

How to use the rafter calculator

Pick the mode that matches what you know, pitch or total rise, fill in the span, and click Calculate Rafter. Results stay hidden until you calculate.

1. Choose a mode: “From Roof Pitch” if you know the pitch (6 for 6:12), or “From Known Rise” if you measured the height and want the pitch worked out for you.
2. Enter the roof span in feet: the full outside-to-outside building width. The pre-filled example uses 20 ft.
3. Add the eave overhang and ridge thickness in inches: both default to 0. Leave them at 0 for the pure textbook triangle; enter real values for a cut-ready length.
4. Optionally enter the roof length in feet: the ridge length, for the rafter count at 24 inch on-center spacing. Leave it blank and the count row stays hidden.
5. Read the panel: rafter length first, then run, rise, pitch angle with the pitch restated as x:12, the order length, and the count when requested. Copy or share buttons save the whole result as one line of text.

Bad entries stop with a plain message instead of a number: a missing or zero span, a zero pitch or rise, a negative overhang or ridge thickness, and a ridge so thick it eats the whole run (“Ridge thickness is too large for this span.”). Blank overhang, ridge and roof length fields are treated as zero or skipped, never as errors.

The rafter formulas, with the unit conversions done honestly

A common rafter is the hypotenuse of a right triangle. The run is the horizontal leg, the rise is the vertical leg, and Pythagoras does the rest. The part most write-ups gloss over is that the span is entered in feet while the overhang and ridge are entered in inches, so the inches are divided by 12 before they touch the run.

Two consequences follow from doing the overhang inside the triangle instead of tacking it on afterwards. First, the rise in pitch mode is figured on the full run including the overhang, so the tail of the rafter sits at the same pitch as the rest of the roof, which is how a real rafter is laid out. Second, the length grows along the slope: each foot of horizontal overhang adds the rafter length factor for your pitch, 1.118 ft at 6:12, 1.202 ft at 8:12. Tools that add the overhang flat, foot for foot, quietly short every rafter.

Worked example: a 20 ft span at 6:12

The pre-filled example is the textbook case: 20 ft span, 6:12 pitch, no overhang, no ridge adjustment, 30 ft roof length. Running it gives a 10.00 ft run, 5.00 ft rise, 11.18 ft rafter and 16 rafter positions.

Now the same roof framed for real: 24 ft span, 8:12 pitch, a 12 inch eave overhang, a 1.5 inch ridge board and a 40 ft roof length. The run becomes 12 + 1 − 0.0625 = 12.94 ft, the rise 8.63 ft, the rafter 15.55 ft at 33.69 degrees, the order length 16 ft boards, and the count 21 positions per slope. The ridge adjustment alone shortened the run by three quarters of an inch, small on one rafter and the difference between a tight ridge and a gaping one across forty feet of roof. Rise mode checks out against the same triangle: enter a 28 ft span with a 7 ft rise, an 18 inch overhang and the same ridge, and the tool returns a 5.44:12 pitch at 24.39 degrees with a 16.95 ft rafter.

Rafter lengths by pitch for a 20 ft span

Steeper pitches buy attic headroom with board length. Every row below is a run of this calculator at a 20 ft span with no overhang and no ridge adjustment, so the run is 10.00 ft throughout.

Read the order column with a lumber yard in mind: yards stock even lengths (8, 10, 12, 14, 16 ft and up), so an 11 ft order length usually means buying 12 ft boards and trimming. The calculator reports the honest next whole foot rather than pretending every length exists on the rack.

Birdsmouth cuts, the ridge, and what the length really measures

The length this tool returns is the rafter’s line length: the straight slope distance from the ridge centerline to the tail, measured along the top edge line of the rafter. Two site details sit on top of that number.

The birdsmouth is the notch that seats the rafter on the wall: a horizontal seat cut resting on the top plate and a vertical plumb cut against the wall face. It removes material below the line, so it does not shorten the line length, but the seat cut depth is capped in practice because cutting too deep weakens the rafter at exactly the point carrying the wall load. Layout marks for the seat and plumb cuts come from the same pitch angle the tool reports, and test-fitting one pattern rafter before cutting the set is standard practice. Converting the tool’s decimal feet to the fractions on a tape, 11.18 ft is 11 ft 2 3/16 in near enough, is where an inches to fraction calculator earns its keep on layout day.

The ridge adjustment is the opposite case: it changes the line itself. Rafters on both slopes butt against the same ridge board, so each rafter’s run ends at the board’s centerline, half a thickness short of the theoretical peak. Enter the board’s actual thickness, 1.5 in for a nominal 2x ridge, which the USDA Forest Products Laboratory’s Wood Handbook chapter on commercial lumber confirms is the dressed size of 2 inch dimension stock. Leaving the ridge at zero returns the theoretical peak-to-tail length, which is the right answer for a ridge-less layout exercise and the wrong one for cutting.

Length is not sizing: spans, spacing and codes

This calculator answers “how long,” never “how big.” Whether a 2×6 may legally span your run at your snow load is a different question with a code table behind it.

Allowable rafter spans depend on lumber species and grade, spacing, roof live and dead load, and whether a ceiling is attached. The International Code Council’s 2021 IRC Chapter 8, Roof-Ceiling Construction tabulates those spans in the R802.4.1 tables, and the American Wood Council’s tutorial on loads and span tables walks through how the load side of the lookup works. The count here is fixed at 24 inch on-center spacing; drop to 16 inch spacing for heavier loads and the count formula changes with it, floor(length × 12 ÷ 16) + 1 per slope. A roof joist calculator covers the horizontal members that tie the rafter feet together, and once the frame is sized, a gable roof area calculator turns the same span, pitch and length into the surface area for sheathing and shingles.

What competing calculators miss

Rafter tools are plentiful, and most of them get the central square root right. The differences live at the edges, which is where rafters get cut wrong:

• Flat-overhang tools and charts (CalculoOnline among them): the worked example adds a 2 ft overhang as a flat 2 ft of length, and its reference chart lists 10.77 ft for a 20 ft span at 6:12, where the triangle gives 11.18 ft. Both errors point the same way: boards ordered short.
• Angle-method guides with broken trig: one widely copied tutorial finds the rafter by dividing the run by sin(angle), which turns a 10 ft run at 25 degrees into a 23.66 ft “rafter” for a house 20 ft wide. The run is the adjacent side: the divisor is cos(angle), or skip the angle and use Pythagoras.
• Run-only calculators (Brave Calculator’s common rafter page): clean and correct for run plus pitch, exactly the 11.18 ft answer for the standard example, but there is no span input, no overhang, no ridge adjustment and no count; the framer does three of the four framing inputs by hand.
• Full framing suites (Simulations4All, Billdr): genuinely broader, with hip and valley geometry, lumber sizing and seat-cut inputs. The cost is a control panel a first-time user has to learn before the first number appears, and spacing defaults (16 inch) that silently differ from the 24 inch assumption many simple roofs are framed at.
• The gap this page fills: span-first entry in either pitch or rise mode, overhang and ridge handled inside the triangle with the inch conversion stated, a stock length and a per-slope count in the same run, and the count’s one-slope basis printed on the result instead of left ambiguous.

Frequently asked questions

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