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Skylight Rough Opening Calculator

Calculate the framed rough opening dimensions for a skylight from the unit size and manufacturer clearance.

  • Answers as you type
  • Every formula cited
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SettingsSettings for this calculationUS
Market
Imperial · sales tax
The manufactured width of the skylight unit itself.

Use the unit's overall frame width from the manufacturer's spec sheet, not the glazed opening size.

The manufactured height of the skylight unit itself.

The UNIT's height from the manufacturer's data sheet. Three numbers get confused on a skylight and they are all different: the unit, the rough opening it needs, and the daylight opening you can actually see through. This field wants the first. If the unit sits on a curb, the curb has its own dimensions and its own tolerance, and the opening has to suit the curb rather than the unit.

The manufacturer's required installation clearance on each side of the unit.

A common clearance is 0.5in (13mm) per side, but always confirm the exact figure on your specific skylight's installation instructions.

The centre-to-centre spacing of the rafters the opening is being cut through.

Measure it on the roof rather than taking it off the drawing, and measure across several bays: a roof framed at 600 mm centres routinely has one bay at 540 and another at 660 where the setting out met a valley or a chimney. It is the figure that decides whether this opening misses the rafters or cuts one, and that is the difference between a fixing job and a trimmed opening with a header at the head and the foot.

The thickness of one rafter, measured across the face the opening runs past.

The narrow way, not the depth. A US 2x rafter is 1.5 in (38 mm) and a UK regularised or CLS rafter is 47 mm, and the nine millimetres between them come straight off the gap an opening has to fit into. Where the roof is doubled at a valley, a hip or an existing opening, two rafters together are the obstruction and the thickness to enter is both of them.

Rough opening width

2.083 ft

High confidence
Rough opening height
3.08 ft
Clear gap between two rafters
1.87 ft
Widest unit that misses the rafters
1.79 ft
Rafters interrupted, at fewest
1
Rafters interrupted, at most
2
Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • Rough opening dimension = skylight unit dimension + 2 x manufacturer-specified installation clearance (commonly 0.5in / 13mm per side)
  • Clear gap between two rafters = spacing on centres - rafter thickness, so the widest unit that misses the rafters entirely is that gap less twice the clearance
  • A rafter is interrupted when its own thickness overlaps the opening, so the rafter centres that fall in an open span of (opening width + rafter thickness) are the ones cut: either ceil(span / spacing) - 1 or ceil(span / spacing) of them, depending where the opening lands

Inputs used

Skylight Unit Width
2 ft
Skylight Unit Height
3 ft
Clearance per Side
0.5 in
Rafter Spacing, On Centres
2 ft
Rafter Thickness
1.5 in

Intermediate steps

Rough opening height
3.08 ft
Clear gap between two rafters
1.87 ft
Widest unit that misses the rafters
1.79 ft
Rafters interrupted, at fewest
1
Rafters interrupted, at most
2
Final result2.08 ft

What this calculation does not cover

  • The single clearance figure is added twice to each dimension, so the same gap is assumed at both jambs and at the head and sill alike; a unit whose instructions ask for more room at the head than down the sides needs two passes and the two answers read off separately.
  • The rafter count says how many members the opening interrupts, not what to do about them. Every cut rafter wants a header across the opening at the head and the foot, the rafters each side of it doubled to carry those headers, and the whole of that sized by somebody for the load it now takes, because a trimmed opening is a short beam and nothing on this page sizes one.
  • Where the opening lands across the roof is not asked, which is why the rafter count comes back as a range rather than a figure. The same opening slid half a bay either way can catch one more rafter or one fewer, and on a roof where the setting out has already been disturbed by a valley or a chimney the bays are not equal anyway. Mark the rafters before ordering the unit, not after.
  • No pitch term enters either sum. Both dimensions are taken as lying in the plane of the roof, so an up-slope measurement lifted off a plan-view drawing will set the opening out short.
  • The input ranges cap the clearance between 6 mm and 25 mm, roughly a quarter-inch to an inch per side, and the unit itself at 2 m wide by 2.5 m high, which puts oversized rooflights and any unusually generous shimming gap outside what can be entered.
  • The answer comes back as the exact sum rather than a dimension anyone would mark on a rafter, and the direction of any rounding is left to you — a surplus millimetre can be shimmed out, whereas an opening rounded down has to be cut again.
2 ft0.61 m3 ft0.914 m6 sq ft0.557 m²12 in0.2 m

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-05 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations3
  1. Rough opening dimension = skylight unit dimension + 2 x manufacturer-specified installation clearance (commonly 0.5in / 13mm per side)
  2. Clear gap between two rafters = spacing on centres - rafter thickness, so the widest unit that misses the rafters entirely is that gap less twice the clearance
  3. A rafter is interrupted when its own thickness overlaps the opening, so the rafter centres that fall in an open span of (opening width + rafter thickness) are the ones cut: either ceil(span / spacing) - 1 or ceil(span / spacing) of them, depending where the opening lands
Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Called something else where you work? Window opening types — the term in each market, how close the equivalence really is, and the standard that governs it.

How to calculate skylight rough opening in 6 steps

  1. Skylight Unit WidthThe manufactured width of the skylight unit itself.
  2. Skylight Unit HeightThe manufactured height of the skylight unit itself.
  3. Clearance per SideThe manufacturer's required installation clearance on each side of the unit.
  4. Rafter Spacing, On CentresThe centre-to-centre spacing of the rafters the opening is being cut through.
  5. Rafter ThicknessThe thickness of one rafter, measured across the face the opening runs past.
  6. Rough opening widthThe tool computes the rough opening width from those figures and shows the formula, its sources, and a confidence rating alongside it.

Rough opening width by skylight unit width

Page defaults, not your figures above.

Skylight Unit WidthRough opening width (ft)
1 ft1.09
1.5 ft1.59
2 ft2.09
2.5 ft2.59
3 ft3.09
3.5 ft3.59

Frequently asked questions

Why is the opening bigger than the skylight?
Because a rough opening is a framing dimension, not a product one. It is sized to the unit's outside frame plus the clearance the maker asks for so the unit can be squared, shimmed and fixed without being forced — force a skylight into a tight opening and you rack the frame, which is how a brand-new unit ends up with a seal that leaks at one corner. The number that matters more than the clearance, though, is where the opening LANDS. A skylight between two rafters costs nothing structurally; one that crosses a rafter means cutting it and heading the ends, which turns a trimming job into a structural one and often decides the size of unit actually bought. Measure the rafter spacing before choosing a skylight, not afterwards.
Will the skylight fit between two rafters without cutting one?
Almost never, at the spacings roofs are framed at. Take the rafter spacing, subtract one rafter thickness for the clear gap, then subtract the clearance twice: 600 mm centres on a 38 mm rafter leave a 562 mm gap and room for a 536 mm unit, and 400 mm centres leave 342 mm. The smallest rooflights sold are wider than either, so on a normally framed roof the answer is no, and the rows above say how many rafters the unit you are looking at would interrupt.
Should I use the same clearance for width and height?
This calculator applies the same clearance value to both dimensions, which matches most manufacturers' installation instructions — but always check your specific unit's spec sheet in case it calls for different clearances per side.
Does this account for curb-mounted skylights?
No — this calculates the rough opening for the skylight unit itself. A curb-mounted skylight also needs a separate curb sized per the manufacturer's curb detail, which is not covered here.
Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.