Quick Answer: What Should You Measure First?
To buy the correct seal for shower door, measure three things first: the glass thickness, the gap the seal needs to cover, and the shape of your old seal profile.
Always measure in millimetres, measure the glass itself rather than the old seal, and take readings from more than one point.
Why Precise Measurement is Critical
Incorrect seal sizing is one of the most common reasons customers experience shower leaks, poor door closure and repeat purchases. Even a small measuring error can lead to the wrong profile, poor water control or a seal that is too tight or too loose to fit properly.
Because shower seal strips are long and easily damaged in transit, returns can also be inconvenient and costly. Measuring carefully before ordering helps avoid wasted time, extra postage and unnecessary replacement orders.
When purchasing shower door seals, millimetre-level inaccuracies can result in:
- ❌ Inadequate sealing, allowing water to escape through gaps
- ❌ Door malfunction, causing difficulty in closing or opening properly
- ❌ Magnetic misalignment, resulting in loss of adhesive force
- ❌ Improper fit, causing excessive fin compression and reduced product lifespan
- ❌ Returns and exchanges, wasting both time and money
The good news is that most sizing problems can be avoided by taking three simple measurements before ordering.
Part 1: Preparation Before Measurement
Required Tools Checklist
| Tool | Purpose | Alternative |
|---|---|---|
| Steel tape measure (metric/mm) | Precise measurement of length and gaps | Laser distance measurer |
| Digital vernier caliper | Measuring glass thickness | UK Coin Thickness Reference |
| Alcohol wipes | Cleaning the measurement area | Lint-free cloth + white vinegar |
| Paper and pen | Recording measurements from three positions | Smartphone notes app |
| Spirit level (optional) | Checking if the door is vertically aligned | Smartphone level app |
Part 2: Five Critical Measurements Explained
Step-by-Step Glass Thickness Measurement (Most Critical)
Shower enclosure glass in the UK typically complies with BS EN 12150-1 (Thermally Toughened Safety Glass for Building Use). Understanding your glass thickness is essential for selecting the correct seal profile.
| Glass Type | Standard Thickness | Tolerance Range | Suitable Door Types |
|---|---|---|---|
| Light Shower Screen | 4, 5, 6mm | ±0.3mm | Fixed panels, lightweight doors |
| Standard Shower Door | 8mm | ±0.3mm | Hinged doors, pivot doors |
| Heavy Frameless Door | 10mm | ±0.4mm | Large frameless enclosures |
| Luxury Custom Door | 12mm | ±0.4mm | Bespoke high-end enclosures |
Step 1: Select Measurement Positions
- Measure at three positions: top, middle, and bottom
- Position each measurement point at least 50mm from the glass edge
- Avoid areas near hinges, handles, or visible damage
What if My Shower Glass Measures Around 5mm?
Some UK bath screens and lightweight shower doors may appear to measure around 5mm, especially when measured with a ruler rather than digital calipers.
In most cases, glass around 5mm sits within the 4–6mm shower seal fitting range. Many replacement shower screen seals are designed to fit 4mm, 5mm and 6mm glass when the profile shape, fitting position and gap size are correct.
The most important rule is this: measure the exposed glass itself, not the old seal. Old PVC seals can stretch, harden, flatten or change shape over time, which may make the size look different from the actual glass thickness.
If your glass measures around 5mm, browse our 5mm shower screen seal range or choose products marked for 4–6mm glass.
Step 2: Using Digital Calipers (Recommended Method)
| Action | Detail |
|---|---|
| Tool | Digital vernier caliper (±0.1mm accuracy) |
| Technique | Gently clamp the glass—do not apply excessive pressure |
| Reading | Record to nearest 0.1mm |
| Final Value | Use the maximum of three readings |
Why choose the maximum thickness? Selecting the largest measurement ensures the seal fits securely without falling off or being too tight to install.
If your readings vary slightly, do not worry. Small differences are normal on shower glass, especially near polished edges or older installations. Use the largest reliable reading as your ordering reference, then check the product description to confirm the correct glass thickness range.
If you do not have a digital caliper, you can still take a useful first measurement with a steel tape measure or clear ruler, or send us a photo for confirmation.
Step 3: Alternative Method (No Calipers Available)
Don't have digital calipers? We've published a detailed guide on alternative measurement techniques, including the British Coin Comparison Method:
📖 Read: How to Precisely Measure Bottom Shower Door Seal Dimensions
This companion guide covers:
- Using UK coins (1p, 5p, £1) as thickness references
- Photographic comparison techniques
- When to seek professional measurement assistance
Measurement 2️⃣: Bottom Gap Measurement (Critical for Waterproofing)
The bottom seal is the first line of defence against water leakage.
As a practical guide, many shower door bottom gaps sit around 8–12mm, but older or uneven installations may vary.
Step-by-Step Details
| Step | Action | Notes |
|---|---|---|
| Step 1 | Close door completely to normal position | Do not force the door; let it close naturally |
| Step 2 | Measure at three positions: left, centre, right | Use steel tape measure, accurate to 1mm |
| Step 3 | Record maximum and minimum values | Door may tilt due to uneven flooring |
| Step 4 | If gap >15mm, select extended-height seal | Refer to product selection chart |
- ✅ Ensure the shower tray is dry and clean during measurement
- ✅ Water stains or debris can cause 2-3mm measurement errors
- ✅ Measure in daylight or use a torch for better visibility
- ✅ Take a photo of your measurements for customer service reference
Quick Reference Table (Mobile Optimised)
| Measurement Result | Possible Cause | Solution | Recommended Product |
|---|---|---|---|
| <6mm | Door sagging or tray too high | Select small-gap seal | VB Series |
| 8-12mm | Standard range | Select standard single-fin bottom seal | V Series |
| >15mm | Uneven floor or door installation issue | Select extended-height seal | V Series |
| Three-point variation >5mm | Door tilting | Select seal with side-extension fins | S Series |
🆘 Uneven Gap? We Have Solutions
Not sure which seal to choose for uneven gaps? Our engineering team has analysed gap variations from 3mm to 10mm+ and created a comprehensive solution guide.
Read: How to Choose Seals for Uneven Gaps (3mm-10mm+ Solutions)
Ready to Shop?
Measured your bottom gap? Use the largest gap reading to choose the closest fin length. If water is leaking under the door, start with our bottom shower door seals.
- ✅ Free UK delivery on all orders
- ✅ Expert support via live chat
- ✅ 30-day returns policy
Do You Need to Measure Total Length?
| Product Type | Standard Length | Best For | UK Stock Status |
|---|---|---|---|
| Bottom Seal | 1000mm (1 metre) | Shower tray edges | ✅ In Stock |
| Side Seal | 2000mm (2 metres) | Door vertical edges | ✅ In Stock |
Typical UK Shower Door Heights
Standard: 1800-2000mm → 2000mm seal
Walk-in Screen: 1000mm → 1000mm seal
Wet Room: 2200-3500mm → Custom order
What About Custom Sizes?
| Situation | Solution | Lead Time |
|---|---|---|
| Standard doors (<2m height) | Order online directly | Next day delivery for eligible orders |
| Extra-tall doors (>2m height) | Contact customer service | 15-35 working days |
| Bespoke enclosures (2.5m+) | Factory direct order | 35-70 working days |
Curved Edge vs. Straight-Line Measurement
Measuring a Curved Shower Seal is simple once you know these two points:
1. Thickness & Gap Size
The method for measuring glass thickness and gap size for a curved screen is exactly the same as for a straight seal. See Step 1 and Step 2 above.
2. Understanding the Curve: The Arc vs. The Chord
Outer Arc: Measure along the outside curved edge of the glass.
The Chord: Measure the straight-line distance between the two ends.
If your shower screen is curved, quadrant-shaped or P-shaped, use a curved shower screen seal designed to follow the glass shape rather than forcing a straight seal around the curve.
Measurement 3️⃣: How to Measure Vertical & Magnetic Shower Seals
Vertical seals stop water escaping from the sides of your door. Follow these two checks:
The Side Gap (Clearance)
Standard Doors: Measure the gap between the glass and the wall at its widest point.
Sliding Doors: Measure the overlap gap, which is the clearance between the two glass panels.
Side Leak or Magnetic Door?
If your leak is coming from the side of the door, measure the side gap first. For doors that close against another glass panel, also check whether you need a magnetic shower seal pair.
Part 3: Measurement Data Recording Sheet
No need to screenshot. Fill in your measurements directly below.
Use the table below to record your measurements. Use the largest reliable reading as your final ordering value.
| Measurement Position | Top (mm) | Middle (mm) | Bottom (mm) | Final Value (Max) |
|---|---|---|---|---|
| Glass Thickness | - | |||
| Bottom Gap | - | |||
| Side Gap (Left) | - | |||
| Side Gap (Right) | - | |||
| Double Door Overlap | - |
How to Use This Table
| 1 | Fill in all three measurements for each row. |
| 2 | The Final Value column automatically updates to show the maximum value, which is the safest measurement for ordering. |
| 3 | Use the Final Value for product selection on our shop pages. |
| 4 | Take a screenshot of the completed table and attach it when contacting customer service for faster assistance. |
Part 4: Common Measurement Mistakes & How to Avoid Them
Based on analysis of UK customer support questions and common ordering issues.
❌ Mistake 1: Measuring Only One Point
Fix: Always use the Three-Point Method: top, middle and bottom. Record the maximum value.
❌ Mistake 2: Door Not in Natural Closed Position
Fix: Let the door close naturally until magnets just engage. Then measure.
❌ Mistake 3: Ignoring Glass Edge Polishing
Fix: Measure at least 50mm from the edge. Avoid polished or damaged areas.
❌ Mistake 4: Using Inches Instead of Millimetres
Fix: Confirm your tape measure uses mm markings. 1 inch = 25.4mm.
❌ Mistake 5: Measuring the Old Seal Instead of the Glass
Fix: Measure the exposed glass edge directly wherever possible. Use the old seal only as a profile reference, not as the main size measurement.
Quick Conversion Reference
| Inches | Millimetres | Common Use |
|---|---|---|
| 1/4" | 6.35mm | Small gaps |
| 3/8" | 9.5mm | Standard bottom gap |
| 1/2" | 12.7mm | Larger gaps |
| 3/4" | 19mm | Extended seals |
| 1" | 25.4mm | Reference only |
Still Not Sure? Let Our Experts Help You
Don't let measurement stress stop you. Use our Expert Verification Service to get it right the first time.
Ready to Find Your Replacement Seal?
Browse Bottom Seals if water leaks under the door.
Browse Vertical Seals if water escapes from the side.
Browse Curved Seals if your screen is quadrant-shaped or P-shaped.
For magnetic seals or framed door seals, compare the old profile carefully before ordering.
Before ordering, do not choose by length alone. Match the glass thickness, gap size and old profile shape first.
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