A skylight brings in far more light than a wall window of the same size, because it faces the open sky. That also means a skylight that is too large can overheat a room in summer and lose heat in winter. This calculator applies the usual rule of thumb to size skylights for a room.
The percentage rule
Skylight glazing is usually sized as a share of the room’s floor area:
- about 5% for a room that already has plenty of windows;
- about 10% for a room with some windows;
- up to 15% for a room with few or no other windows.
For a 12 × 15 ft room (180 sq ft) with some windows: 180 × 10% = 18 sq ft of skylight.
The calculator divides that area by the size of one unit and rounds up. With 2 × 4 ft skylights (8 sq ft each), you need 3, which actually gives 24 sq ft, or 13% of the floor. It warns you above about 17%.
Choosing size and placement
- Rafter spacing matters: many skylights are sized to fit between rafters 16 or 24 in on center, which avoids structural work. Wider units need the framing to be headed off.
- Several smaller units spread light more evenly than one large one.
- North-facing slopes give soft, steady light with little heat gain; south- and west-facing skylights bring the most heat.
- In rooms with a ceiling below an attic, a light shaft is needed; a flared shaft spreads light better than a straight one.
- Tubular skylights (sun tunnels) are a cheaper option for hallways, closets and bathrooms.
Glazing and comfort
Choose low-e, insulated glazing, and consider shades or blinds, especially for bedrooms. Venting skylights let hot air escape at the top of the room and help in kitchens and bathrooms. In hot climates, look for a low solar heat gain coefficient on the NFRC label.
Cost and permits
Installed costs vary widely, from about $1,000 to $3,000 or more per unit depending on size, type and roof work. A new roof opening usually requires a building permit, and the installation must be properly flashed to avoid leaks; many homeowners combine skylight work with a roof replacement.