Skylight Size Calculator
A skylight that is too small barely brightens a room; too large and it overheats and wastes energy. Designers size skylight glazing as a percentage of the room's floor area, often around 5 percent for a well-used, well-insulated space and more where extra daylight is wanted. Enter your room floor area, the daylight percentage you are targeting, and the glazing area of one skylight unit, and this calculator returns the recommended glazing area and the number of units to install. The percentage is yours to set, since the right value depends on climate, glazing, and code.
Skylight size formula
Recommended glazing = floor area * daylight% / 100
Units needed = ceil(recommended glazing / unit area)
Installed glazing = units * unit area
Effective daylight % = installed glazing / floor area * 100
The recommended glazing scales with floor area and your chosen percentage. Units are rounded up to whole skylights, so the installed glazing usually exceeds the minimum and the effective percentage is slightly higher.
Daylighting notes
- Around 5 percent of floor area is a common starting point for well-insulated, frequently used rooms.
- Higher percentages add daylight but increase heat gain or loss.
- Energy codes cap total fenestration area and set minimum glazing performance.
- Orientation and shading change the daylight a skylight delivers; this tool sizes by area only.
- Confirm requirements in the International Energy Conservation Code and local amendments.
Skylight size: frequently asked questions
How big should a skylight be?
A common daylighting guideline sizes skylight glazing as a percentage of the room's floor area. Many designers use about 5 percent for a well-insulated room with frequent use and up to 10 percent for spaces needing more light. This calculator multiplies your floor area by the percentage you choose, since the right figure depends on your climate, glazing, and code.
How is the recommended area calculated?
Recommended glazing area equals room floor area times the daylight percentage divided by 100. A 300 square foot room at 5 percent suggests 15 square feet of skylight glazing. Adjust the percentage to your goals and local energy code.
How many skylight units do I need?
Divide the recommended glazing area by the glazing area of one skylight unit, then round up to whole units. If one unit is 4 square feet and you need 15, that is four units. The calculator rounds up because you install whole skylights.
Why is the percentage a user input?
Because the correct ratio varies with climate zone, glazing performance, room use, and local energy code, there is no single universal figure. Rather than hardcode a number, the calculator lets you set the percentage your design or code requires.
Do skylights affect energy use?
Yes. Skylights add daylight but can increase heat gain or loss depending on glazing and orientation. Energy codes limit total fenestration area and require minimum performance ratings. Check the International Energy Conservation Code and local amendments before sizing.
Official sources
- U.S. Department of Energy: Energy Saver: Skylights.
- International Code Council: International Energy Conservation Code fenestration.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.