Calculator
Rafter Length & Count Calculator
A rafter's cut length is not the slope distance you'd read off a section drawing with a scale ruler — it's the hypotenuse of a right triangle whose horizontal leg is half the building width, less half the ridge board's own thickness, and whose rise is set by the pitch. Get that run wrong by forgetting the ridge board eats into it, and every rafter in the roof is off by the same fixed amount, because the error is baked into the geometry before a single board is cut.
This calculator treats the rafter's body and its overhang as two separate triangles sharing the same pitch, because that's what they actually are — the tail continues the roof's slope past the wall plate over a different horizontal distance than the body covers. It reports the cut length in feet and inches, the way a rafter actually gets marked, and it works out the roof's true sloped surface area from the same pitch multiplier, which is the number the shingle and sheathing calculators downstream need, not the flat building footprint.
Calculate your quantity
Outside to outside, across the ridge.
Rafters needed
62 rafters
Rafter length
14.84 ft
- Pitch multiplier
- 1.118 x
- Horizontal run
- 11.94 ft
- Body length
- 13.35 ft
- Tail (overhang) length
- 1.49 ft
- Order length
- 16 ft
- Rafters per side
- 31 rafters
- Ridge height above plate
- 6 ft
- Roof surface area
- 1,073.3 sq ft
- Cut length along the rafter, ridge cut to tail cut: 14 ft 10-1/16 in.
Shopping summary
- • 62 rafters cut from 16 ft stock
- • 3 x 16 ft ridge boards
This is an estimate — confirm structural work with a professional.
How this calculation works
Everything here runs off one pitch multiplier: the ratio of sloped length to horizontal run for a roof rising a given number of inches per 12 inches of run, from plain Pythagorean geometry. The rafter's body run is half the building width minus half the ridge board's thickness — the ridge sits between two opposing rafter pairs and steals that thickness from each side's horizontal run before the rafters themselves begin. Multiply that run by the pitch multiplier and you have the body's sloped length.
The overhang tail is a second, separate calculation using the same multiplier: the overhang distance (given in inches, converted to feet) run through the identical pitch, because the tail continues the same slope past the wall plate rather than following a different angle. Body length plus tail length is the full cut length, and a half-foot cutting allowance gets added before rounding up to the next 2 foot stock increment — room for the ridge cut, the bird's mouth notch, and a mis-cut, on top of the stocking round-up itself.
- Overhang is entered in inches, not feet — it's a small number on a plan and it's easy to type 16 meaning inches when the field elsewhere on this page is reading feet.
- This page computes rafter length, count, and roof surface area from geometry. It does not check whether the rafter size you're planning to cut can actually carry your local snow and wind load over that span — that's a separate structural question a span table or an engineer answers.
- Roof surface area from the pitch multiplier is meaningfully larger than the building's flat footprint on anything steeper than a low-slope roof, and it's the number shingle and sheathing quantities should be figured against, not the footprint.
- Rafters per side comes from the roof length at the chosen on-center spacing; total rafter count doubles that for both sides of a standard gable roof.
The formula
rafterLength = run × multiplier + (overhang ÷ 12) × multiplier; multiplier = √(1 + (risePer12 ÷ 12)²); run = buildingWidth ÷ 2 − ridgeThickness ÷ 24
- multiplier (pitch multiplier)
- The ratio of sloped length to horizontal run for the stated pitch — how many feet of rafter one horizontal foot of run actually needs, from the geometry of the pitch alone.
- run
- The horizontal distance the rafter's body spans: half the building width, minus half the ridge board's thickness, because the ridge occupies that thickness on each side before either rafter starts.
- tail length
- The overhang's own horizontal distance, converted from inches to feet, run through the same pitch multiplier as the body — the tail is a continuation of the same slope, not a separate angle.
- orderLength
- The full cut length plus a half-foot cutting allowance, rounded up to the next 2 foot stock increment — margin for the ridge cut, the bird's mouth, and an honest mis-cut, on top of the stocking round-up.
- rafters per side / total
- The field count along the roof's length at the chosen on-center spacing, then doubled for the two sides of a standard gable roof.
Ridge thickness has a small effect on a narrow building and a larger one on a wide building, because it's subtracted once from a run that's already been halved — on a 24 foot building a 1.5 inch ridge board shifts the run by less than an inch, but the effect scales with how many rafter pairs are sharing that same ridge deduction across a longer roof.
Where these numbers come from
- Pitch multiplier = √(1 + (rise ÷ 12)²)
- Derived on this page from plain right-triangle geometry: a horizontal leg of 12 inches and a vertical leg equal to the stated rise. This is a mathematical identity, not a trade or published figure.
- Run reduced by half the ridge board's thickness
- Derived on this page from how a ridge actually sits between two opposing rafter pairs — a description of the framing geometry, not a formal citation.
- Half-foot cutting allowance before rounding to stock length
- Trade convention. Covers the material lost to the ridge cut and the bird's mouth notch, plus a small margin for an imperfect first cut, on top of rounding to the next 2 foot stocked length.
- 12, 16, and 24 in on-center rafter spacing options
- Trade convention for standard residential rafter spacing, chosen to align with common roof sheathing panel dimensions.
Worked examples
A 24 ft wide gable roof at a 6/12 pitch, 16 in overhang, 16 in o.c.
| Building width | 24 ft |
|---|---|
| Roof pitch (rise per 12) | 6 in |
| Eave overhang | 16 in |
| Rafter spacing | 16 in on-centre |
| Roof length (ridge) | 40 ft |
| Ridge board thickness | 1.5 in |
| Rafters needed | 62 rafters |
|---|---|
| Rafter length | 14.84 ft |
| Pitch multiplier | 1.118 x |
| Horizontal run | 11.94 ft |
| Body length | 13.35 ft |
| Tail (overhang) length | 1.49 ft |
| Order length | 16 ft |
| Rafters per side | 31 rafters |
| Ridge height above plate | 6 ft |
| Roof surface area | 1073.3 sq ft |
A 6/12 pitch multiplies horizontal run by 1.118 — every foot of run needs about 1 foot 1 3/8 inches of rafter. Half the 24 foot building, less half the 1.5 inch ridge board, gives an 11.94 foot run, which becomes a 13.35 foot body length; the 16 inch overhang adds another 1.49 feet of tail, for a 14.84 foot cut length that rounds up, with the cutting allowance, to a 16 foot stock order.
The 1,073 square foot roof surface figure is worth carrying forward rather than recomputing later — it's about 12% more than the 960 square foot building footprint (24 x 40 feet), and that difference is exactly what a footprint-based shingle or sheathing estimate would miss. Sixty-two total rafters is 31 per side at 16 inch spacing along the 40 foot roof length, doubled for both sides of the gable.
A steep 10/12 pitch, 32 ft wide roof, 24 in overhang, 24 in o.c.
| Building width | 32 ft |
|---|---|
| Roof pitch (rise per 12) | 10 in |
| Eave overhang | 24 in |
| Rafter spacing | 24 in on-centre |
| Roof length (ridge) | 50 ft |
| Ridge board thickness | 1.5 in |
| Rafters needed | 52 rafters |
|---|---|
| Rafter length | 23.35 ft |
| Pitch multiplier | 1.3017 x |
| Horizontal run | 15.94 ft |
| Body length | 20.75 ft |
| Tail (overhang) length | 2.6 ft |
| Order length | 24 ft |
| Rafters per side | 26 rafters |
| Ridge height above plate | 13.33 ft |
| Roof surface area | 2082.7 sq ft |
A 10/12 pitch is noticeably steeper than 6/12, and the multiplier shows it: 1.3017 against 1.118, meaning every horizontal foot now needs about 30% more rafter rather than 12% more. Combined with a wider building and a bigger overhang, the cut length nearly doubles to 23.35 feet — even though wider 24 inch spacing means fewer rafters per side (26 against 31) and a lower total count (52 against 62) than the first, gentler roof.
The roof surface area tells the sharper story: 2,083 square feet against a 1,600 square foot footprint (32 x 50 feet) is a 30% penalty from pitch alone, well past the first example's 12%. Anyone pricing shingles or underlayment by the building's footprint on a roof this steep will come up noticeably short, and it gets worse, not better, as the pitch climbs from here.
Common mistakes
- Taking the run as exactly half the building width and skipping the ridge board deduction — a small error per rafter that's identical on every rafter in the roof, so it doesn't average out.
- Entering the overhang in feet instead of inches, or the reverse. The field is inches; treating it as feet produces a tail length several times too long or too short.
- Ordering shingles, underlayment, or sheathing against the building's flat footprint instead of this page's pitch-adjusted roof surface area — the gap between the two grows with the pitch and is not a rounding error on a steep roof.
- Treating a computed rafter length and count as proof the rafter size is adequate. This page checks geometry, not load — a correctly cut rafter can still be too small for the snow and wind load it needs to carry over that span.
- Ordering only one side's worth of rafters because 'rafters per side' was the number that stuck, and forgetting the total for a full gable roof is double that.
- Cutting to the bare calculated length with no allowance, then finding there's no material left to correct a bird's mouth cut that came out a hair off.
Shopping summary
Order rafter stock at the rounded length this page gives, not the raw cut length — the cutting allowance is there so a small mistake on the first rafter doesn't cost you a whole board. Ridge board is a separate order sized to the roof length, not derived from this page's rafter count.
The roof surface area this page computes is exactly what the sheathing calculator and roofing calculator need for panel and shingle counts — carry that figure forward rather than re-measuring the footprint for either page. And since rafters bear on the walls framed by the wall stud calculator, confirm that wall's top plate and any header beneath a rafter bearing point are sized for the load coming down through it.
FAQ
Why does the ridge board's thickness matter to my rafter length at all?
Because the ridge board physically sits between the two opposing rafters at the peak, and it takes up horizontal space that would otherwise be part of each rafter's run. On a typical 1.5 inch ridge board the effect per rafter is under an inch, but it's the same small error repeated on every rafter in the roof, and it compounds if the ridge board is thicker than the default.
Why is overhang measured in inches when the rest of this page uses feet?
Because overhang is usually a small, specific dimension picked from a plan or an eave detail — 12, 16, or 24 inches — and inches is how it's actually specified on site. The calculation converts it to feet internally before applying the same pitch multiplier the body length uses.
Does a steeper pitch really add that much roof surface area over the footprint?
Yes, and it grows faster than the pitch number suggests — a 6/12 roof adds about 12% surface area over its footprint, while a 10/12 roof adds about 30%. That's not a small rounding difference on materials priced by the square: on a large roof it's easily an extra bundle or two of shingles that a footprint-based estimate would miss entirely.
Does this calculator check whether my rafter size can handle the snow load in my area?
No — it computes cut length, count, and roof geometry from the pitch and building dimensions you enter. It does not check bending capacity against snow or wind load for your span, which is a separate structural calculation your local span table or an engineer needs to confirm, especially at longer spans or heavier snow load zones.
Why do rafters per side change with spacing but the total always doubles that?
Rafters per side is the field count along the roof length at your chosen on-center spacing — tighter spacing means more rafters per side, wider spacing means fewer. The total simply doubles that figure because a standard gable roof frames both sides of the ridge identically; it isn't a separate calculation, just the same count applied twice.
How much does the cutting allowance actually add to the order?
Half a foot before the round-up to the next 2 foot stock length — sometimes that half-foot pushes the order into the next stock size and sometimes the stock rounding would have done that anyway. Either way it's cheap insurance against a bird's mouth cut or a ridge cut that doesn't come out clean on the first try.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.