How to Measure a Shower Door Seal Before Buying a Replacement

What You'll Need

A metric ruler or tape measure will do for most gaps and lengths. For glass thickness, digital callipers will give you a more accurate reading.

It's also worth having your phone handy to record measurements and take a few photos.

How to Measure Glass Thickness

Shower screens in the UK commonly use glass that's 4mm, 5mm, 6mm, 8mm or 10mm thick.

Find an exposed edge of the glass and measure across it, avoiding any chipped or damaged areas. Make sure you're measuring the glass itself, not the plastic channel of the old seal.

Using Digital Callipers

Hold the callipers square to the glass and close them gently until both jaws touch the surface.

Take a couple of readings to make sure you get a consistent result. You can then compare the measurement with the fitting range on the product page.

No Callipers? Try This

A metric ruler can give you a rough idea of the thickness.

If the glass edge is difficult to read, place two matching flat objects on either side of the glass. Measure the total width, then subtract the combined thickness of the objects.

Matching coins or Lego bricks can work for a first check, although they won't be as accurate as callipers.

What if Your Glass Is About 5mm Thick?

Glass measuring around 5mm will often fit a seal designed for 4–6mm glass.

You'll still need to check the shape of the seal and the gap it needs to cover. Two seals can fit the same glass thickness but work quite differently once fitted.

You can compare the options in our 5mm shower screen seal collection.

How to Measure the Gap Under Your Shower Door

Before choosing a shower door bottom seal, close the door as you normally would and measure from the bottom edge of the glass to the shower tray or bath rim.

Take a reading near each end of the door and another in the middle. The gap isn't always the same all the way across.

Watch Out for Raised Threshold Strips

If there's a raised metal threshold beneath your shower door, check how close its highest point comes to the glass. Some threshold strips are higher on one side than the other, which makes this easy to miss.

We had a customer who measured a gap of around 12mm and bought a seal with a 12mm fin. The measurement seemed right, but once the seal was fitted, the door became difficult to open and close.

It turned out there was only 3–5mm of clearance between the glass and the highest part of the threshold. Every time the door moved, the 12mm fin had to squeeze through that narrow space. It caused extra resistance and could lead to the fin bending or wearing out sooner.

With a wider clearance of around 7–12mm, some softer fins may have enough room to flex and move past the threshold. A gap of just 3–5mm needs more care, especially with a longer or stiffer fin. The shape and flexibility of the seal make a difference.

Before buying, check both the gap you need to seal when the door is closed and the narrowest space the fin has to pass through as the door moves.

What if the Gap Is Wider at One End?

Older shower doors sometimes sit slightly lower on one side, leaving an uneven gap underneath.

Measure both the smallest and largest gaps. If you choose a fin based only on the wider end, it may drag or fold where the space becomes narrower.

Our guides to uneven gaps beneath shower doors and why longer fins aren't always better explain what to look out for.

How to Measure the Seal Length

Measure along the glass edge where the replacement seal will sit.

Many bottom seals are supplied in lengths of around 500–900mm, while vertical seals are often around 1700–1900mm. These are only general examples, so check the actual length available on the product page.

Allow for any trimming needed around hinges or fittings. If you have an unusually tall door or a bespoke shower screen, it's worth checking with us before ordering.

How to Measure a Curved Shower Screen

For a curved, quadrant or P-shaped shower screen, follow the measuring diagram below. Check the measurements shown, along with your glass thickness, to help you find a suitable curved bottom seal profile.

How to Measure Side and Magnetic Seals

Not every shower door seal clips onto the glass in the same way. Some cover a gap beside the door, others sit between overlapping glass panels, and certain magnetic seals fit into a channel in the frame.

Here's what to measure for each type.

Vertical Side Seals

For a vertical shower door seal, measure the space between the edge of the glass and the wall, frame or fixed panel beside it.

Check near the top and bottom if the gap looks uneven. The flexible part of the seal needs to reach across the wider section without interfering with the door.

Sliding Shower Doors

On a sliding shower door, measure the clearance between the glass panels where they overlap, rather than the width of the metal track.

If the seal clips onto one panel and its fin reaches towards the other, compare your glass thickness and the overlap gap with the available F-shaped shower door seals.

Framed Magnetic Seals

Some magnetic strips fit into an aluminium frame instead of clipping directly onto the glass.

For these, it's the fixing channel that needs measuring. Check the size of the T-shaped fitting section, the opening in the frame and how far the magnetic strip needs to reach when the door closes.

Our framed shower door magnetic strip guide explains these measurements in more detail.