Skylight Sizing Calculator
How much skylight a room needs from its floor area and a target skylight-to-floor ratio — the number of units of a given size, and the daylight and heat implications.
Toplighting is far more effective per square metre than windows, because it sees the whole sky: a skylight-to-floor ratio of 2–4% gives good daylight in most climates, 5% or more in overcast ones, and above 6% the heat gain and glare outweigh the light.
How the skylight sizing calculator works
Toplighting is far more effective per square metre than windows, because it sees the whole sky: a skylight-to-floor ratio of 2–4% gives good daylight in most climates, 5% or more in overcast ones, and above 6% the heat gain and glare outweigh the light. Multiply the floor area by the ratio and divide by the unit size for the count; spread them evenly, no further apart than about 1.5 times the ceiling height.
Formula: skylight area = floor area × ratio; count = ceil(area ÷ unit area); max spacing ≈ 1.5 × ceiling height
Worked examples
| Inputs | Skylight area | Note |
|---|---|---|
| 120 m² at 3% with 1.2 × 0.8 m units | 3.6 m² | 3.6 m² — 4 units |
| A 500 m² warehouse at 4%, 1.5 × 1.5 m units | 20 m² | 20 m² — 9 units |
| A bathroom of 8 m² at 5% | 0.4 m² | 0.4 m² — 2 units |
FAQFrequently asked questions
Why are skylights so much more effective than windows?
A horizontal opening sees the whole sky dome, the brightest part of it, while a window sees a slice near the horizon. Per square metre a skylight admits roughly three times the daylight, which is what the equivalent-window row reflects.
What ratio do codes allow?
Energy codes limit skylights to about 3–5% of the roof area under the prescriptive path because of heat loss and gain; some daylighting provisions require a minimum in large single-storey spaces. Check both directions.
How do I control heat and glare?
Diffusing or prismatic glazing, low-e coatings, north-facing or splayed light wells, and in hot climates external shading or a smaller ratio. Vertical clerestories facing away from the sun are an alternative.
Why the spacing rule?
Light from a skylight spreads out in a cone; units further apart than about 1.5 times the ceiling height leave dim patches between them. Splayed light wells widen the spread.
Where these figures come from
- Sabine (1922) — Collected Papers on Acoustics — the reverberation-time equation
- CIBSE Guide A — Environmental design — daylight, comfort and ventilation criteria
- Neufert — Architects' Data — the standard reference for space and dimension
- International Code Council — International Building Code — occupant load and egress factors
- Building Regulations — Approved Documents — the UK building regulations
Last checked: September 2026. Formulas are the standard building-science and code relationships; default thresholds are common international values and should be replaced with the ones in your code.