Last Updated: 30 July 2026
Replacing a shower door seal is easier once you know which measurements affect the fit. The overall length matters, but it is rarely the best place to start.
First check the thickness of the glass, the space the seal needs to cover and the way the old profile was fitted. A seal may fit the glass but still be wrong for the gap. Another may have the right fin length but use a different fitting channel.
Use millimetres throughout, as the measurements on our product pages are shown in millimetres. For a clip-on seal, measure the exposed glass rather than the outside of the old plastic channel. Keep the old seal nearby, though, because its end shape can help you identify the type of replacement you need.
If the fitting position or profile is still unclear, our wider guide to choosing the right shower door seal explains how the door type, gap and old seal shape narrow the choice.
What You May Need
A rigid metric ruler or steel tape measure is enough for most gaps and lengths. Digital callipers give a clearer reading for glass thickness, although a ruler can still provide a useful first estimate.
Keep your phone nearby to save the readings and take reference photos. A fabric tape is less useful for small gaps because it can bend or sit at an angle.
How to Measure Glass Thickness
Most modern UK shower screens use toughened safety glass. Common thicknesses include 4mm, 5mm, 6mm, 8mm and 10mm.
Measure across an exposed edge of the glass. Choose a clean, undamaged section and avoid any area that is chipped or heavily polished.
Using Digital Callipers
Hold the callipers square to the glass and close them gently. Do not press hard, and do not measure over the plastic channel of the old seal.
Take more than one reading where possible. If one result is noticeably different, move the callipers and check again. Use the most consistent reliable measurement, then compare it with the fitting range shown on the product page.
Small differences often come from the position of the tool or the polished glass edge rather than a real change in thickness.
Measuring Without Callipers
A clear metric ruler can give you a useful first estimate.
If the glass edge is difficult to read, place two flat objects of the same thickness on opposite sides of the glass. Measure the full distance across the objects and the glass, then subtract the combined thickness of the two objects.
Matching coins or Lego bricks can be used for this first check, but the result will be less precise than a direct reading with digital callipers.
What if the Glass Measures Around 5mm?
Some UK bath screens and lighter shower doors measure close to 5mm, especially when checked with a ruler.
Glass around 5mm will often fall within a 4–6mm fitting range. This does not mean every seal marked 4–6mm will fit, as the profile shape, fitting position and gap must also suit the door.
When comparing a 5mm shower screen seal, check the full fitting information shown on the product page.
How to Measure the Bottom Gap
For a shower door bottom seal, measure from the lower edge of the glass to the shower tray, threshold or surface below. Do not use the length of the old fin as the gap measurement.
Let the door rest in its normal closed position. Do not press it down or push it further shut, as this may make the gap appear smaller than it is during everyday use.
Make sure the tray and the bottom of the door are clean and dry. Measure near the left side, in the centre and near the right side, then record all three readings.
If the readings are close, compare them with the fin length shown on the product page. The flexible section needs to reach the surface below without pressing against it so heavily that the door becomes difficult to move.
When the Bottom Gap Is Uneven
An older shower door may sit slightly lower at one end. This can leave a small gap on one side and a larger one on the other.
When the three readings differ noticeably, consider both the smallest and largest gap. Choosing a seal from the largest figure alone may cause the fin to drag where the space becomes narrower.
Our guide for an uneven gap under a shower door explains how changing gap sizes can affect the type of seal that will work.
The fin needs to reach the surface below, but a longer fin is not always the better fit, particularly where the gap becomes narrower.
Once the gap is clear, compare it with the glass thickness and fitting style shown for the product.
How to Measure the Seal Length
Many bottom seals are supplied in lengths of around 900–1000mm, while vertical profiles are often around 2000mm. Check the stated product length and allow for trimming.
For unusually tall doors or bespoke screens, contact us before ordering.
How to Measure a Curved Shower Screen
For a curved, quadrant or P-shaped screen, check the measuring image at the top of our curved shower screen seal collection. Follow the points shown, compare your glass thickness and measurements, then choose a suitable profile.
How to Measure Side and Magnetic Seals
A seal fitted along the side or closing edge of a shower door may cover a wall gap, sit between two overlapping glass panels or slot into an aluminium frame. The correct measurement depends on which arrangement you have.
Measuring a Side Gap
For a standard vertical shower door seal, measure between the edge of the glass and the wall, frame or fixed panel beside it.
Use the widest space the flexible section needs to cover. If the door and the adjoining surface do not look parallel, check the gap at the upper and lower parts of the door.
Measuring Between Sliding Glass Panels
On a sliding shower door, measure the clearance between the two glass panels where they overlap.
Do not measure the full width of the metal track. If the seal clips onto one sliding panel and its fin reaches towards the other, compare F-shape seals for overlapping shower doors by glass thickness and gap coverage.
Checking a Framed Magnetic Strip
Some magnetic strips fit into a channel inside an aluminium frame rather than clipping directly onto the glass.
For these profiles, glass thickness is not the main fitting measurement. The T-shaped fixing section must suit the existing channel, and the magnetic edge needs to reach the correct closing position.
The T-shaped fixing head, track opening and magnetic reach are explained in our framed shower door magnetic strip guide .
What to Send Us if You Are Unsure
If the profile is difficult to identify, send a close photo of the old seal from the end, together with a wider view showing where it fits.
Include the glass thickness and place a ruler beside the gap the seal needs to cover. A photo of the door in its normal closed position can also help show how the two surfaces meet.
If the old seal has already been removed, photograph the bare glass edge, the shower tray gap and the area where the door closes. These details are usually enough for us to rule out profiles that will not fit.
Choosing the Right Seal
Once the measurements are clear, begin with the part of the door where the old seal was fitted. A gap beneath the glass points towards a bottom seal, while a gap along the edge is more likely to need a side or vertical profile.
Magnetic and framed seals need one further check because the closing position or fixing channel must also match. The final choice should suit the glass thickness, fitting method and the space the seal needs to cover.
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