MOT Windscreen Failure: Chip & Crack Size Limits 2025

Windscreen and glass defects account for 8.53% of all MOT failures in the UK, making it the sixth most common reason vehicles fail their test. In 2024, over 3 million vehicles had windscreen or glass issues identified during MOT testing, with approximately 1.6 million failing due to chips, cracks, or damaged glass.

The frustrating part? Many windscreen failures are preventable with timely repairs, which often cost nothing due to insurance cover. This comprehensive guide explains the exact MOT windscreen rules, size limits for chips and cracks, repair vs replacement costs, insurance considerations, and when damage will definitely fail your MOT.

Quick Summary: Windscreen MOT Limits 2025

Repair cost: £40-£80 (usually £0 with insurance, no excess on many policies) Replacement cost: £150-£500+ (insurance excess typically £75-£100)

A-Zone = Area swept by driver's side wiper blade (critical visibility area)

Understanding MOT Windscreen Testing

MOT testers assess all glass for damage that could affect driver vision or structural integrity:

What Gets Tested

1. Front Windscreen Primary focus area - most critical for safety:

• A-zone (wiper swept area): Strict 10mm chip limit
• B-zone (rest of windscreen): 40mm chip limit
• Cracks: Any crack in driver's line of sight fails
• Obscuration: Any damage blocking view fails

2. Front Side Windows Driver's windows (both sides):

• Must provide clear view of road
• No damage obscuring vision
• Working properly (must open/close for emergency exit)

3. Rear Window Less critical but still tested:

• Must not be shattered/broken
• No sharp edges (safety issue)
• Rear view must be adequate

4. Door Mirrors Essential if rear window view obscured:

• Minimum one working mirror (driver's side)
• Preferably two working mirrors
• Glass not cracked or missing
• Secure mounting

MOT Windscreen Damage Zones

A-Zone (Most Strict):

• Area swept by driver's wiper blade
• Approximately 290mm from steering wheel center
• Critical for driver visibility
• Maximum chip size: 10mm
• Any crack = FAIL

B-Zone (Less Strict):

• Rest of windscreen outside A-zone
• Less critical but still important
• Maximum chip size: 40mm
• Crack outside driver's view = May pass with advisory

Measurement Method:

• Chip size = diameter of damage including any 'legs' or spreading
• Not just the impact point - the entire damaged area
• Measured with calibrated gauge or ruler

Pass/Fail Criteria

Automatic FAIL if:

• ❌ Chip larger than 10mm in A-zone (wiper area)
• ❌ Chip larger than 40mm anywhere on windscreen
• ❌ Any crack in driver's direct line of sight
• ❌ Multiple chips creating obscured vision
• ❌ Damage obscuring VIN plate view
• ❌ Damaged area shows signs of delamination
• ❌ Windscreen not securely fixed
• ❌ Dark tinted windscreen (legal limit: 75% light transmission minimum)

Will PASS if:

• ✅ Chip 10mm or less in A-zone
• ✅ Chip 40mm or less in B-zone
• ✅ Small stone chips outside driver's view
• ✅ Minor scratches not obscuring view

Advisory (passes but noted) if:

• ⚠️ Chip approaching 10mm in A-zone
• ⚠️ Small crack not in driver's line of sight
• ⚠️ Minor damage that could worsen
• ⚠️ Multiple small chips (each under limit)

Common Windscreen Failures: Causes & Costs

1. Stone Chip in A-Zone Over 10mm

Failure Rate: ~50% of windscreen-related MOT failures

Why It Happens

• Flying stones: Most common cause (gravel trucks, motorways)
• Chip expansion: Small chip spreads due to:
  • Temperature changes (heat/cold cycles)
  • Vibration from driving
  • Pressure from wind
  • Structural stress
• Delayed repair: 24-48 hours to repair before spreading
• Seasonal: More common in winter (temperature changes)

How Chips Spread

Timeline of chip damage:

1. Initial impact: Small 2-5mm chip
2. First 24 hours: Most critical period
3. Temperature change: Expansion/contraction stress
4. Vibration: Driving spreads damage
5. Result: 10mm+ chip or crack forming

Speed of spreading:

• Can spread from 5mm to 15mm in 24 hours
• Temperature changes accelerate spreading
• Once spreading starts, difficult to stop
• Becomes irreparable quickly

Symptoms

• Visible chip in windscreen
• Often with 'legs' radiating outward
• May see rainbow effect (delamination)
• Clicking sound when driving (air in damage)
• Spreads over time

Repair vs Replace Decision

Repairable if:

• ✅ Chip smaller than £2 coin (20mm diameter)
• ✅ Not in A-zone, OR in A-zone but under 10mm
• ✅ Not directly in driver's line of sight
• ✅ Not at windscreen edge (within 3cm of frame)
• ✅ No significant spreading/cracking
• ✅ Single layer damage only (not delamination)

Must replace if:

• ❌ Chip larger than 20mm (£2 coin)
• ❌ Multiple chips in close proximity
• ❌ Crack longer than credit card
• ❌ Damage at edge of windscreen
• ❌ Delamination visible (rainbow effect)
• ❌ Driver's vision obscured after repair

Repair Costs

Windscreen Chip Repair:

• Without insurance: £40-£80
• With insurance: Often £0 (no excess)
• Time: 30-60 minutes
• Method: Resin injection under pressure
• Mobile service: Usually included
• Success rate: 85-95% if caught early

Process:

1. Clean damaged area
2. Apply vacuum to remove air
3. Inject clear resin under pressure
4. UV cure the resin (hardens)
5. Polish surface
6. Total time: 30-60 minutes

Limitations:

• Cannot repair large chips (20mm+)
• Cannot repair cracks
• Cannot repair edge damage
• Some distortion may remain
• Success depends on catching it early

Replacement Costs

Full Windscreen Replacement:

Without Insurance:

• Standard car: £150-£300
• Premium/luxury car: £300-£600
• Acoustic/heated screen: £400-£800
• Head-up display screen: £600-£1,200+
• Mobile service: +£30-£50

With Insurance:

• Your excess: Typically £75-£100
• Some policies: £0 excess for windscreens
• No claims bonus: Usually protected (check policy)

Additional costs included:

• Removal of old screen
• Fitting new screen
• Adhesive/sealer
• Calibration (if sensors fitted)
• ADAS recalibration (cameras/sensors): +£100-£300

Breakdown:

• Windscreen glass: £80-£400
• Labour: £50-£150
• Adhesive: £20-£40
• Calibration: £50-£300 (if needed)
• Mobile service: £0-£50

Insurance Considerations

Windscreen Cover: Most comprehensive policies include:

• Chip repair: Free (no excess)
• Windscreen replacement: Pay excess only
• Mobile service: Usually included
• Claims: Often don't affect no claims bonus

Check your policy:

• Windscreen excess amount (£50-£150 typical)
• Chip repair excess (often £0)
• Approved repairers list
• No claims protection status
• Claim limits (some cap at 2-3 claims per year)

Excess Examples:

• Budget policy: £100-£150 excess
• Standard policy: £75-£100 excess
• Premium policy: £50-£75 excess, or £0 for chips
• Some policies: £0 excess for windscreens entirely

Is it worth claiming?

Chip repair (£40-£80 cost): ✅ Always claim if covered - Usually £0 excess

• No impact on premium typically
• Quick mobile service
• Better than replacing later

Replacement (£150-£600 cost): ✅ Claim if screen costs more than excess + potential premium increase

Example calculation:

• Replacement cost: £300
• Your excess: £75
• Claim saves you: £225
• Premium increase: Maybe £30-£50 next year
• Net benefit: £175-£195 ✅ Worth claiming

Don't claim if:

• Replacement cost less than excess
• Close to renewal (may affect premium significantly)
• Already made multiple claims this year

2. Crack in Driver's Line of Sight

Failure Rate: ~30% of windscreen-related MOT failures

Why Cracks Fail MOT

• Visibility: Obscures driver's view of road
• Distraction: Eye drawn to crack
• Refraction: Light distorts through crack
• Structural: Weakens windscreen integrity
• Safety: May shatter in accident

Common Crack Causes

• Chip expansion: Small chip spreads into crack
• Temperature stress: Extreme heat or cold
• Structural stress: Body flex, slamming doors
• Impact damage: Larger stone hits
• Manufacturing defect: Rare but possible
• Age: Old windscreens more susceptible

Crack Locations & MOT

Driver's direct line of sight:

• ❌ Any crack = FAIL
• Definition: Area driver looks through normally
• Approximately 30cm x 30cm area
• Centered on driver's seating position

Outside driver's line of sight:

• ⚠️ May pass with advisory
• Tester's discretion based on:
  • Visibility impact
  • Crack length
  • Position
  • Structural concern

Passenger side:

• ⚠️ Usually advisory only
• Unless crack very long (structural concern)
• Or obscures passenger side mirror view

Crack Repair - Usually NOT Possible

Cannot repair if:

• Crack longer than credit card (~8cm)
• Crack reaches edge of windscreen
• Crack in laminated section
• Multiple cracks
• Crack showing delamination

Rare exceptions:

• Very short crack (under 2cm)
• Not in A-zone
• Caught within hours of formation
• Specialist resin available

Verdict: Cracks almost always require full replacement

Replacement Costs

Same as chip replacement section above (£150-£800 depending on vehicle)

3. Multiple Chips Creating Obscured Vision

Failure Rate: ~10% of windscreen-related failures

When Multiple Chips Fail MOT

Individual assessment:

• Each chip measured separately
• One over limit = FAIL
• Multiple under limit might still fail if:
  • Clustered together (obscure vision)
  • Create distraction
  • Structurally weaken screen

Example scenarios:

Scenario 1 - PASS:

• Three 8mm chips
• All in different areas
• None in A-zone
• Verdict: PASS (each under 40mm limit)

Scenario 2 - FAIL:

• Three 8mm chips
• All within 10cm of each other in A-zone
• Create obscured area
• Verdict: FAIL (combined obscuration)

Scenario 3 - FAIL:

• One 12mm chip in A-zone
• Two 8mm chips elsewhere
• Verdict: FAIL (one exceeds A-zone limit)

Repair Options

If chips are repairable individually:

• Can repair multiple chips same visit
• Cost: £40-£80 per chip (or £0 with insurance)
• Insurance usually covers multiple chips
• Better than replacement (£150-£600)

If too many chips:

• Replacement more economical
• Structural integrity concern
• Visual clarity issues
• Insurance may insist on replacement

4. Damage at Windscreen Edge

Failure Rate: ~5% of windscreen-related failures

Why Edge Damage is Serious

• Structural integrity: Windscreen bonded to frame at edges
• Rapid spreading: Edge cracks spread quickly
• Seal compromise: Water ingress possible
• MOT failure: Always fails if damage within 3cm of edge

Edge Damage Types

• Chips at edge (usually from stone hits during windscreen replacement)
• Cracks starting at edge
• Delamination at bonding area
• Seal separation

Cannot Be Repaired

• Edge damage always requires replacement
• Resin cannot bond effectively at edge
• Structural concern
• Replacement only option: £150-£800

5. Tinted Windscreen Too Dark

Failure Rate: ~2% of windscreen-related failures

Legal Tinting Limits

Front windscreen:

• Minimum light transmission: 75%
• Only very light tinting allowed
• Usually just factory-fitted tinted strip at top (legal)
• Aftermarket tinting almost always illegal

Front side windows (driver + front passenger):

• Minimum light transmission: 70%
• Slight tinting allowed
• Most factory tinting complies
• Aftermarket tinting often too dark

Rear windows:

• No restrictions (can be fully tinted)
• Provided vehicle has two working mirrors

MOT Testing

• Tester uses light meter
• Measures light transmission percentage
• Below legal limit = FAIL
• No tolerance or margin

Remedy

• Cannot repair: Tint must be removed
• Professional removal: £50-£150
• DIY removal: Difficult (heat gun, scraping)
• Replacement if tint won't remove cleanly: £150-£600

Other Glass & Mirror Requirements

Front Side Windows

MOT requirements:

• Must open (emergency exit requirement)
• Clear view for mirrors/junctions
• Not shattered or sharp
• Minimal damage acceptable

Failure causes:

• Cannot open (stuck/broken mechanism)
• Shattered glass
• Excessive damage
• Sharp edges (safety)

Rear Window

Requirements:

• Not shattered/broken
• If broken, no sharp edges
• Rear view mirror operational (or two wing mirrors)

Less critical:

• Small damage usually advisory only
• Unless severely obscured

Door Mirrors

Minimum requirement:

• At least one working mirror (driver's side)
• Preferably two mirrors

MOT failure if:

• No working mirrors
• Glass cracked/missing
• Mirror insecure/damaged
• Inadequate view

Replacement costs:

• Mirror glass only: £15-£40
• Complete mirror unit: £50-£200
• Heated/electric: £80-£300

Pre-MOT Windscreen Checks (5 Minutes)

1. Inspect in Good Light (2 minutes)

Park in bright area or use torch:

• Check entire windscreen surface
• Look from both inside and outside
• Note any chips or cracks
• Measure questionable chips

Focus areas:

• A-zone (wiper swept area)
• Edges (within 3cm of frame)
• Driver's direct line of sight
• Corners (stress areas)

2. Measure Chip Sizes (2 minutes)

Use ruler or coin comparison:

• 10p coin: ~24mm diameter (too big for A-zone, OK for B-zone)
• 20mm (£2 coin): Maximum repairable size
• 10mm: Maximum for A-zone
• 40mm: Maximum for B-zone

Measure full damage extent:

• Include any 'legs' or spreading
• Not just the impact point
• Entire damaged area counts

3. Check Crack Locations (1 minute)

Sit in driver's seat:

• Look through windscreen normally
• Note any cracks in line of sight
• Check crack length
• Look for spreading

Critical:

• Any crack in driver's view = will fail
• Even small cracks can fail
• Spreading cracks get worse quickly

Red Flags - Fix Before MOT

Replace immediately if:

• ❌ Chip over 10mm in A-zone
• ❌ Any crack in driver's line of sight
• ❌ Chip over 40mm anywhere
• ❌ Multiple chips obscuring view
• ❌ Damage at windscreen edge

Repair quickly if:

• ⚠️ Any chip 5-10mm (before it spreads)
• ⚠️ Small crack under 2cm (rare repair option)
• ⚠️ Multiple repairable chips

Monitor if:

• ✓ Tiny chips under 5mm in B-zone
• ✓ Minor scratches not obscuring view
• ✓ Damage outside driver's area

Cost-Saving Strategies

1. Repair Chips Immediately

Prevent replacement costs:

• Stone chip happens: £0 damage initially
• Repair within 24 hours: £0-£80 (free with insurance)
• Wait 1 week, spreads: £150-£600 replacement needed

Savings: £150-£520 by repairing early

2. Use Insurance for Windscreen Claims

Typical insurance benefits:

• Chip repair: £0 (no excess usually)
• Replacement: Pay excess only (£75-£100)
• No claims bonus: Often protected
• Mobile service: Included

Check policy for:

• Windscreen excess amount
• Chip repair excess
• No claims impact
• Approved repairer list

3. Mobile Windscreen Services

Convenience benefits:

• Come to your home/work
• Usually no extra cost
• Insurance approved
• Quick service (30-60 mins)

Providers:

• Autoglass
• National Windscreens
• A1 Windscreens
• Local mobile services

4. Shop Around for Replacement

Price varies significantly:

• Main dealer: £400-£1,200
• Branded specialists (Autoglass): £200-£600
• Independent fitters: £150-£400
• Mobile services: £150-£500

Example (standard car windscreen):

• Main dealer: £450
• Autoglass: £280
• Independent: £190
• Saving: £260 by shopping around

Consider:

• OEM glass quality
• Installation warranty
• Calibration included
• Mobile service option

5. Avoid Preventable Damage

Free prevention:

• Don't follow gravel trucks closely
• Increase distance on motorways
• Replace worn wiper blades (scratch prevention)
• Clear ice properly (don't use hot water)
• Fix chips immediately (prevent spreading)

Potential saving: Avoiding £150-£600 replacement

When to Repair vs Replace

Repair if Possible:

✅ Chip 20mm or smaller

• Quick (30-60 mins)
• Cheap (£0-£80)
• Effective (85-95% success)
• Preserves original screen

✅ Not at windscreen edge

• Repairable in main screen area
• 3cm+ from edge

✅ Caught early

• Within days of damage
• No significant spreading

✅ Single layer damage

• No delamination
• No rainbow effect

Replace if:

❌ Chip over 20mm

• Too large to repair
• Structural concern

❌ Any crack

• Cannot repair cracks effectively
• Safety/structural issue

❌ Edge damage

• Within 3cm of frame
• Rapid spreading risk

❌ Multiple chips clustered

• Too many damages
• Visual obscuration

❌ Failed previous repair

• Repair hasn't held
• Spreading continues

Frequently Asked Questions

Can I pass MOT with a small chip in my windscreen?

Yes, if:

• Chip is 10mm or smaller in A-zone (wiper area)
• Chip is 40mm or smaller elsewhere on windscreen
• Chip is not in driver's direct line of sight

Chips slightly below these limits may pass but get an advisory. Best to repair any chip before MOT to guarantee passing.

Will insurance cover my windscreen repair for MOT?

Usually yes. Most comprehensive insurance policies cover:

• Chip repairs with £0 excess (free)
• Windscreen replacement (you pay your excess, typically £75-£100)
• Often no impact on no claims bonus

Check your policy documents or call your insurer to confirm coverage.

How is windscreen damage measured for MOT?

Chips are measured by total diameter of damage including any 'legs' or spreading - not just the impact point. The measurement includes:

• Main damaged area
• Any cracks radiating outward
• Delaminated area
• Discolored region

Testers use a calibrated gauge or ruler. If in doubt, they measure conservatively (larger measurement).

Can a crack be repaired?

Rarely. Cracks can only sometimes be repaired if:

• Very short (under 2-3cm)
• Caught within hours of forming
• Not in critical viewing area
• Not at windscreen edge

Most cracks require full windscreen replacement (£150-£600). Small cracks spread quickly, especially with temperature changes.

What is the A-zone on a windscreen?

The A-zone is the area swept by the driver's side wiper blade. It's the most critical visibility area and has the strictest MOT limits:

• Maximum chip size: 10mm
• Any crack: FAIL

The A-zone is typically about 290mm wide (measured from the center of the steering wheel). Think of it as your primary viewing area when driving.

Is it cheaper to repair or replace a windscreen?

Repair is much cheaper if the damage is repairable:

• Repair: £40-£80 (often £0 with insurance)
• Replace: £150-£600+ (pay insurance excess if covered)

However, replacement is necessary for:

• Large chips (over 20mm)
• Any cracks
• Edge damage
• Multiple chips

Always try to repair chips quickly before they spread into cracks requiring expensive replacement.

Does a windscreen chip repair affect MOT?

No. A properly repaired chip will pass MOT, provided:

• Repair is professional quality
• No distortion in driver's view
• Screen structurally sound
• Repair hasn't failed/spread

Some minor distortion may remain after repair but shouldn't affect MOT if done correctly.

Can I drive with a cracked windscreen?

Legally depends on location:

• ✅ Small crack outside driver's view: Legal but risky (could spread)
• ❌ Crack in driver's line of sight: Illegal and dangerous
• ❌ Large crack compromising structure: Illegal

Insurance implications:

• Driving with damaged windscreen may invalidate insurance
• If involved in accident with damaged screen, insurer may not pay out
• Police can issue prohibition notice for dangerous windscreen

Recommendation: Get cracked windscreens replaced immediately.

Will my no claims bonus be affected?

Usually not. Most insurers:

• Don't count windscreen claims against no claims bonus
• Treat glass claims separately from other claims
• Allow 1-3 glass claims per year without impact

Check your policy:

• Some budget policies may be affected
• Read glass cover terms
• Call insurer to confirm

How long does windscreen repair take?

Chip repair: 30-60 minutes

• Clean area: 5 minutes
• Apply vacuum: 5 minutes
• Inject resin: 10-15 minutes
• UV cure: 10-15 minutes
• Polish: 5-10 minutes

Windscreen replacement: 1-3 hours

• Remove old screen: 30-45 minutes
• Clean frame: 15-20 minutes
• Fit new screen: 30-45 minutes
• Cure adhesive: 1-2 hours before driving
• ADAS calibration (if needed): 30-60 minutes extra

Mobile services come to you, saving travel time.

Conclusion: Preventing Windscreen MOT Failures

Windscreen damage accounts for 8.53% of MOT failures, but most failures are preventable with prompt action when damage occurs.

Key takeaways:

1. Know the limits:

   • 10mm maximum chip in A-zone
   • 40mm maximum chip elsewhere
   • Any crack in driver's view fails

2. Act fast:

   • Repair chips within 24-48 hours
   • Prevents spreading into cracks
   • Usually free with insurance

3. Use insurance:

   • Chip repairs often £0 excess
   • Replacement requires excess (£75-£100 typical)
   • Usually no impact on no claims

4. Pre-MOT check:

   • Inspect entire windscreen
   • Measure questionable chips
   • Repair/replace before test

5. Prevention:

   • Avoid following gravel trucks
   • Increase motorway distance
   • Fix chips immediately

Cost comparison:

• Repair chip promptly: £0-£80 (free with insurance)
• Wait, spreads to crack, replacement needed: £150-£600+
• Saving by acting fast: £150-£520

Before your next MOT:

1. Inspect windscreen in good light
2. Measure any chips (use 10p coin)
3. Check for cracks in driver's view
4. Repair chips over 5mm immediately
5. Replace if chip over 10mm in A-zone or any crack present

Check your MOT history now to see if previous tests noted windscreen advisories and predict if current damage will fail.