How to measure your skylight for a perfect fit | Skylight size guide

Getting the right skylight size is essential to ensure your new dome fits perfectly on your flat roof. But don’t worry — it’s much simpler than it sounds. This step-by-step guide will show you exactly how to measure your skylight. You’ll learn the difference between key terms such as daylight size, outer size, and roof opening, and how to measure each of them accurately. Follow the simple instructions below and you’ll be confident you’ve chosen the ideal skylight for your flat roof. Let’s get started.

Determine the daylight size

The daylight size is always measured from the inside, at the top of the inner edges of your existing upstand (see photo). To make your order as simple and accurate as possible, we work only with two measurements: daylight size and roof opening. All dimensions shown on this webshop refer to the daylight size. Take a moment to measure carefully — it’s the best way to ensure your new dome fits perfectly the first time.

If you are measuring from the outside

When replacing or renovating a skylight, you’ll often start from the dimensions of your existing dome. Keep in mind that the outer measured size is always 16 cm larger than the daylight size (B + 16 cm = measured size). This difference exists because the dome includes an 8 cm flange on each side. For example, an outer measured size of 116 x 116 cm corresponds to a daylight size of 100 x 100 cm. Knowing this simple rule helps you avoid mistakes and ensures you order the perfect fit for your flat roof.

View All

Skylight 100x100 cm

From €226,66

Budget Skylight

From €53,65

View All

Flat roof window Vitria 100x100 cm

From €570,65

iWindow - Flat roof window 100x100 cm

From €756,00

View All

iWindow - Opening Flat Roof Window - Complete Set 100x100 cm

From €1.635,32

Flat roof window Vitria 100x100 cm

From €570,65

View All

Budget Curb 16/20X

From €130,12

PVC-Curb 100x100 cm

From €379,34

In this short video, we guide you through the entire measuring process step by step. You’ll learn how to determine the daylight size, overall size, and roof opening, along with practical tips to avoid common mistakes. With this video, measuring your skylight becomes quick, easy, and error-free — the perfect start to choosing your new dome with confidence.

Start from the roof opening

The roof opening (C) — the hole in your roof — depends on the type of curb (or upstand) your skylight rests on. There are two types: straight and splayed. If the curb is straight (its code ends with /00), the roof opening must be exactly the same as the daylight size (B).

If the curb is splayed, the opening needs to be larger. The number at the end of the code shows how much: /20 means +20 cm, /30 means +30 cm, and so on.

Example: with a splayed curb /20 and a daylight size of 100 x 100 cm (B), your roof opening (C) should be 120 x 120 cm — 20 cm extra on each side.

Tip: always check the type of curb before ordering to make sure your skylight fits perfectly.

Check again

Before ordering, take a moment to double-check all your measurements:

B = daylight size → the inner dimensions of the curb
C = roof opening → the hole in your roof

It’s always worth verifying your measurements to make sure you order the correct skylight size. If you’re unsure, it’s best to ask a professional for help.

Tip: keep your tape measure flat and straight while measuring — even a small error can make a big difference when fitting your skylight.

With these simple steps, you can measure with confidence and choose the perfect skylight for your flat roof. Remember: measuring is knowing.