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Skylight Size Calculator

Enter the room's floor area, choose how much other daylight it has, give the rafter spacing and thickness, and the calculator returns the glazing area to aim for, the clear width between rafters and the standard sizes and counts that come closest.

Fitting a skylight between existing rafters avoids cutting the structure, which is the expensive and engineered part; the calculator filters the common sizes to the ones that fit.

Updated · See the formula

Skylight Size Calculator

Units
sq ft
in

Skylights that fit between rafters avoid cutting structure

in

Example result for the starting values. Enter your own and press Calculate.

Skylight glazing to aim for

10 sq ft

closest fit between 24 in rafters: 2 × 21 × 38 in

Rule
5%
Clear width
22.5 in
Best fit
2 × 21×38
How the skylight is sized
Rule5% of the floor area
Glazing area200 sq ft × 5% = 10 sq ft
Clear width between rafters24 in − 1.5 in = 22.5 in
2 × 21 × 38 in11.1 sq ft (closest)
4 × 14 × 30 in11.7 sq ft
3 × 21 × 27 in11.8 sq ft
2 × 21 × 46 in13.4 sq ft
Show the arithmetic
  1. 200 sq ft × 5% = 10 sq ft
  2. 24 in − 1.5 in = 22.5 in
  • The percentages are daylighting rules of thumb used by skylight makers, not a code requirement; too much glazing overheats a room in summer, especially on a south or west slope.
  • Sizes are common nominal sizes; makers' exact rough openings differ by an inch or so. Fitting between rafters avoids cutting structure; wider units need headers designed for the load.
  • Choose low-e, laminated or tempered glazing as the code requires for overhead glass (IRC R308.6), and a flashing kit matched to your roofing.

Understanding your result

The headline is the glazing area to aim for, with the closest standard combination that fits between the rafters. The stats give the rule, the clear width and the best fit; the table lists up to four combinations near the target.

The flashing around the skylight follows the same rules as the step flashing calculator; the rafter spacing and size come from the house or the rafter length calculator. A skylight in a roof face planned for solar uses space the solar fit calculator would otherwise count.

How we calculate this

glazing = floor area × rule % clear = rafter spacing − rafter thickness fits = standard sizes whose width ≤ clear count = round to the nearest number of units giving the glazing best = the combination closest to the glazing area

The assumptions behind the numbers

Assumption Default Where it comes from
Rules 3%, 5%, 10–15% of floor area Skylight makers’ daylighting guidance
Sizes 14 × 30 to 44 × 46 in nominal Common residential skylight sizes
Rafter 24 in spacing, 1½ in thick Common framing
Overhead glazing IRC R308.6 International Residential Code

Assumptions last reviewed October 7, 2026.

The calculator does not size tubular skylights, model heat gain or design headers. The flashing guide covers the details around any roof opening.

Two worked examples

A 200 square foot room with some windows

5% rule, 24-inch rafters.

  • Glazing: 10 sq ft; clear width 22.5 in
  • Closest: 2 × 21 × 38 in = 11.1 sq ft

A 180 square foot windowless room with 16-inch rafters

12.5% rule.

  • Glazing: 22.5 sq ft; clear width 14.5 in
  • Closest: 5 × 14 × 46 in = 22.4 sq ft

Where to find your inputs

Floor area. Length times width of the room.

Rafter spacing. In the attic, centre to centre.

Rule. How much other daylight the room gets.

Common mistakes

  • Oversizing. Heat and glare.
  • Cutting rafters casually. Headers need design.
  • Wrong flashing kit. Match the roofing.
  • Ignoring the shaft. A deep attic needs a light shaft that changes the effect.
  • South and west faces. Use low-e and shades.
  • Forgetting the membrane. Seal around the curb.

Questions people ask

How big should a skylight be?
Skylight makers' rules of thumb: about 5 percent of the floor area in a room with some windows, 10 to 15 percent in a room with none, and about 3 percent as accent light in a well-lit room. A 200 square foot room with some windows wants about 10 square feet of skylight.
What size skylight fits between 24-inch rafters?
The clear space between 24-inch-centre rafters is about 22½ inches, which fits the common 21-inch-wide (nominal 22½-inch rough opening) and 14-inch units without cutting a rafter. Between 16-inch rafters only 14-inch-wide units fit.
Can a skylight be wider than the rafter spacing?
Yes, but a rafter must be cut and the opening framed with headers and doubled rafters, designed for the roof's loads. That is structural work for an engineer or an experienced framer.
Can too much skylight be a problem?
Yes: overheating in summer, heat loss in winter and glare, especially on south and west slopes. Low-e glazing, shades and modest sizes help.
What glass is required for skylights?
The International Residential Code (R308.6) sets requirements for overhead glazing: laminated or tempered glass with screens in some cases, or approved plastics. Manufacturers' residential units comply.
How is a skylight flashed?
With the manufacturer's flashing kit matched to the roofing: step flashing up the sides, a head flashing above and an apron below, over self-adhering membrane around the curb. The step flashing calculator explains the pieces.
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