Self Assessment Penalty Calculator

Late filing penalty + late payment surcharge + HMRC interest — tier by tier, in plain numbers.

Estimates only – not tax advice. For planning purposes only; does not replace professional advice or official HMRC calculations. Full disclaimer

Guidance, not advice. This calculator runs the rules as published, it doesn't assess your circumstances. Your actual tax may be affected by factors it doesn't cover (allowances used elsewhere, reliefs, marriage allowance, scheme-specific adjustments). Always seek financial or tax advice from your accountant, or contact HMRC. Read our editorial scope →

Penalty escalation timeline

1. Day 1Triggered

   £100 fixed filing penalty.

2. 30 daysTriggered

   5% surcharge on any unpaid tax.

3. 3 months

   £10/day filing penalty starts (max £900 over 90 days).

4. 6 months

   Additional 5% filing penalty (or £300) + 5% payment surcharge.

5. 12 months

   Another 5% filing penalty (or £300) + 5% payment surcharge. Behaviour uplifts possible.

How SA penalties stack

Filing penalties — charged on the return being late, regardless of tax owed.

• 1 day late: £100 fixed
• 3 months late: +£10/day for up to 90 days (max £900)
• 6 months late: +5% of tax owed (or £300 if higher)
• 12 months late: another +5% (or £300 if higher); higher if deliberate

Payment penalties — charged on tax paid late.

• 30 days late: 5% of unpaid tax
• 6 months late: another 5%
• 12 months late: another 5%

Interest runs daily from the original due date until payment, at HMRC's published rate (2025/26: ~7.75%).

See our HMRC letter guide and common tax mistakes guide for context on responding once a penalty notice arrives.