Presented by VELUX


What size skylight do I need?


The optimal skylight size for your space depends on a few factors: the size of your room, the number of windows in it, and the angle of the roof above it. A skylight that is too big can cause a room to overheat whereas one that is too small will not reap the full benefits skylights have to offer.

When trying to figure out the best size skylight, it’s best to reach out to a skylight installer for expert advice, but here are a few guidelines to determine the right size skylight for your space if you’re DIY-ing your installation.

Measuring For a New Skylight

The industry standard for a new in-home skylight is typically 5% of your floor space for rooms that already have a lot of windows and 15% of your floor space for rooms that do not have many windows. Abiding by these measurements helps keep your space as energy efficient as possible while enabling you to enjoy the sunshine and fresh air that skylights offer.

  • Using a tape measurer, calculate the square footage of your floor by multiplying the length and width.
  • If your room is dark with few windows, multiply the square footage of your floor by .15 (15%) and if it has plenty of windows multiply by .05 (5%).
  • The final number is the approximate square footage your skylight should be for that space.

Measuring for a Replacement Skylight

If you need to measure an existing skylight for a replacement, you’ll need accurate measurements for that perfect fit. Your contractor or roofer will likely take measurements of their own, but if you’re more the DIY type or you just want to know more about your skylight, here’s how to measure your skylight for replacements.

  • On the roof, measure the length and width of your existing skylight starting along the side where the shingles meet the flashing.
  • On the inside of your skylight, you’re measuring the width and length of the drywall around the skylight’s opening.

Pro-Tip: When you’re taking your measurements, snap a picture of your tape measurer showing the lengths, widths, and heights. Pass these along to your contractor or roofer to support the accuracy of your measurements.

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