Overpaid Tax Calculator UK — Check If HMRC Owes You a Refund
Find out if you've overpaid tax and how to claim it back from HMRC. Check P800 refunds, emergency tax, pension tax relief, and other common overpayments.
·6 min read
HMRC holds over £300 million in unclaimed tax refunds. Here’s how to check if you’ve overpaid and claim your money back.
Common Reasons for Tax Overpayment
Reason
Typical Refund
Wrong tax code
£100 - £1,000+
Emergency tax on new job
£200 - £2,000+
Working only part of the year
£100 - £500
Pension tax relief (higher-rate taxpayers)
£100 - £2,000+/year
Marriage Allowance not claimed
£252/year (up to 5 years = £1,260)
Working from home allowance
£62/year (up to £310 for 5 years)
Professional subscriptions
£20 - £200/year
Uniform/tools allowance
£60 - £140/year
Multiple jobs with incorrect codes
£200 - £1,500+
Quick Check: Are You Owed a Refund?
Answer these questions:
Question
If Yes, Possible Refund
Did you start a new job this year without giving your P45?
Emergency tax — likely owed refund
Did you work for only part of the tax year?
May not have used full Personal Allowance
Are you a higher-rate taxpayer with a pension?
Pension tax relief likely unclaimed
Did you work from home during COVID or since?
Working from home allowance
Are you married with one low earner?
Marriage Allowance
Do you pay for work-related subscriptions?
Professional subscriptions relief
Do you wash or buy your own uniform?
Uniform/flat rate expenses
Scenario 1: Emergency Tax (Wrong Tax Code)
How Emergency Tax Works
When an employer doesn’t have your tax information (no P45), they use an emergency tax code. This often results in overpayment.
Emergency Code
What It Means
Likely Outcome
1257L W1/M1
Non-cumulative — each month treated separately
May overpay if early in year
BR
All earnings taxed at 20%
Overpay if below £12,570 annual earnings
0T
No Personal Allowance given
Significant overpayment likely
D0
All earnings taxed at 40%
Large overpayment likely
Calculate Your Emergency Tax Overpayment
If you were on BR (all at 20%) when you should have had 1257L:
Monthly Gross Pay
Tax Paid (BR)
Tax Owed (1257L)
Monthly Overpayment
£2,000
£400
£190
£210
£2,500
£500
£290
£210
£3,000
£600
£390
£210
£3,500
£700
£490
£210
The overpayment is roughly £210 per month for average earners on BR instead of 1257L.
How to Fix
Give your employer your P45
If no P45, complete a starter checklist (form available from HMRC)
Call HMRC on 0300 200 3300 to update your code
Overpayment should be refunded through your wages once corrected
Scenario 2: Pension Tax Relief
Higher and additional rate taxpayers contribute to pensions from taxed income but only get basic rate relief automatically. The extra relief must be claimed.
Unclaimed Pension Relief Calculator
Your Annual Pension Contribution
Basic Rate Relief (automatic)
Higher Rate Relief (must claim)
Additional Rate Relief (must claim)
£5,000
£1,250
£1,000
£1,250
£10,000
£2,500
£2,000
£2,500
£20,000
£5,000
£4,000
£5,000
£40,000
£10,000
£8,000
£10,000
How it works:
You contribute £5,000 from post-tax income
Basic rate relief of £1,250 is added automatically (£6,250 goes into pension)
If you’re a 40% taxpayer, you can claim an additional £1,000 back
If 45% taxpayer, claim £1,250 back
How to Claim
Tax Situation
How to Claim
Self-Assessment filer
Include on your tax return
Not filing Self-Assessment
Call HMRC or complete P87 form
Multiple years unclaimed
Contact HMRC — can go back 4 years
Scenario 3: Working From Home
Tax Relief for Home Workers
Eligibility
Relief Available
Employer required you to work from home
£6/week flat rate (no receipts)
COVID home working (2020-2022)
Could claim for whole year
Regular home workers
Claim for days worked at home
Calculate Your Refund
Tax Rate
£6/week × 52 weeks
Tax Relief
Basic (20%)
£312
£62.40
Higher (40%)
£312
£124.80
Additional (45%)
£312
£140.40
For COVID years (2020/21 and 2021/22), you could claim the full year even if you only worked from home for part of it.