Skylight Size Calculator
Enter the room's length and width and how much window it already has. The calculator gives the skylight glass area to aim for and how many skylights you would need in common sizes.
- Skylight area
- How many skylights
- Share of floor
Common skylight sizes are given by their rough opening in inches.
Skylights are sized as a share of the floor area. Aim for about 5% of the floor in a room with plenty of windows, 10% with few and 15% with almost none. A 15 × 12 ft room with few windows needs about 18 sq ft, which is three 22.5 × 46.5 in skylights.
Skylight size chart
Few windows, 10%. Search for a room.
| Room (sq ft) | Skylight (sq ft) | 22.5 × 46.5 in | 30.5 × 30.5 in |
|---|---|---|---|
| 80 | 8 | 2 | 2 |
| 100 | 10 | 2 | 2 |
| 150 | 15 | 3 | 3 |
| 180 | 18 | 3 | 3 |
| 200 | 20 | 3 | 4 |
| 250 | 25 | 4 | 4 |
| 300 | 30 | 5 | 5 |
| 400 | 40 | 6 | 7 |
22.5 × 46.5 in skylights.
Room and skylights
Few windows. Slide the floor area.
The section shows skylights lighting the room.
How to size skylights
Floor to glass. Step through it.
Floor
Area.
Space several skylights evenly for even light rather than one big one.
Fixed, vented or tubular
Skylight types. Compare them.
Light only.
Opens.
Light tube.
Tubular skylights bring daylight to small rooms with no direct roof space.
Before you fit
Skylight checks. Tick each item.
Tick each item as you go.
Check the rafters. Most standard sizes fit between rafters at 24 in centres.
What you get
Skylight sizing. Each tile explains one result.
Glass.
By size.
Of floor.
Area.
Low-e.
A skylight lets in up to three times the light of a wall window of the same size.
Frequently Asked Questions
No question matches that search.
How big should a skylight be?
About five to fifteen per cent of the floor area, depending on how many windows the room has.
How many skylights do I need?
Divide the skylight area by the area of the size you want, and round up.
Can a skylight be too big?
Yes. Too much overheats the room in summer and loses heat in winter.
What sizes do skylights come in?
Common sizes include twenty-two and a half by forty-six and a half inches.
Do skylights fit between rafters?
Many standard sizes are made to fit rafters at twenty-four inch centres.
Which rooms suit a skylight?
Kitchens, bathrooms, halls and loft rooms.
Should a skylight open?
In kitchens and bathrooms, a vented skylight helps clear steam.
Daylight from above
Size the skylights for the room and they fill it with even daylight without overheating it.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.