The Most Common P800 Tax Calculation Mistakes

Introduction

Every year, HMRC issues millions of P800 tax calculations to employees and pensioners across the UK. These letters show whether you’ve overpaid or underpaid tax during the previous tax year.

For many people, a P800 is good news — it means you’re due a tax refund. But mistakes on P800s are more common than you might think, and they can lead to missed refunds or unexpected tax bills.

This guide explains what a P800 is, how to check it for errors, and the most common mistakes to avoid when claiming your refund — with expert help from Rebate My Tax Ltd, The UK’s Number 1 Tax App & Tax Services.

What Is a P800 Tax Calculation?

A P800 is a document sent by HMRC to tell you whether you have paid the right amount of tax under the PAYE (Pay As You Earn) system. It summarises your income, tax paid, and any benefits or allowances during the tax year (6 April to 5 April).

You’ll receive a P800 if HMRC believes you’ve either:

• Overpaid tax (and may be due a refund), or

• Underpaid tax (and owe money).

P800s are usually sent out between June and November after the end of the tax year, once HMRC has processed all the data from employers and pension providers.

How to Check If You’re Due a P800 Refund

You can view your P800 online by logging into your Personal Tax Account on GOV.UK.
If you’re owed money, the letter will tell you how to claim it — usually by direct bank transfer through your online account.

But don’t assume it’s always correct. HMRC uses data provided by employers and pension providers, and sometimes that information is incomplete, delayed, or duplicated.

Common P800 Mistakes to Watch Out For

Here are the most frequent issues that cause incorrect P800s — and how to fix them.

1. Missing Income Details

If you’ve had more than one job during the year, or a mix of jobs and pensions, HMRC might not have all your income information.

A missing P45 or incorrect PAYE report can lead to an incomplete picture, meaning your refund (or bill) is wrong.

Fix:
Always keep your P45s and P60s. Compare them with the income shown on your P800. If something’s missing, contact HMRC to update your record.

2. Duplicated Income Records

Sometimes HMRC accidentally counts the same income twice — often when you change employers or payroll systems mid-year. This can make it look like you’ve earned more than you actually did, resulting in an underpayment notice instead of a refund.

Fix:
Check that each job listed on your P800 appears once only. If you see duplication, call HMRC and provide your correct employer details.

3. Incorrect Tax Codes

Wrong tax codes are a major source of overpayments and incorrect P800s.
If you’ve had a BR, D0, or emergency tax code, it’s likely that too much tax was taken from your pay.

Fix:
Review your tax code history using your Personal Tax Account. If your P800 doesn’t reflect corrections or code changes, you may be entitled to an additional refund.

Rebate My Tax Ltd can check past codes for multiple years to ensure you haven’t missed any hidden overpayments.

4. Unreported Benefits or Allowances

If you receive benefits in kind (like a company car, fuel card, or private medical cover), they must be included in your tax calculation. If HMRC misses or miscalculates these, your P800 will be wrong.

Similarly, missing allowances (like the Marriage Allowance or Blind Person’s Allowance) can lead to underclaims.

Fix:

Verify that all your benefits and allowances are listed accurately. If not, contact HMRC or let Rebate My Tax Ltd amend your record and reclaim the difference.

5. Old or Outdated Information

HMRC sometimes uses data from the previous tax year or fails to remove old employment records. This can trigger errors where income from past jobs appears again.

Fix:
Check the dates of each employment listed. If you stopped working for an employer earlier than shown, ask HMRC to remove or correct that record.

6. Overlooking Earlier Tax Years

Even if your latest P800 looks fine, you might still be owed money from previous years. Many taxpayers assume a refund for one year means everything is now correct — but tax codes and PAYE errors often go unnoticed for multiple years.

Fix:
You can claim tax refunds for up to four previous tax years. Rebate My Tax Ltd can review all years together to ensure you claim the maximum amount owed.

7. Not Claiming Your Refund in Time

If you don’t claim your P800 refund within the time HMRC allows, you could lose it completely. Typically, refunds must be claimed within four years of the end of the tax year.

Fix:
Act quickly once you receive your P800 — especially if it’s for an older tax year. If you’re unsure, Rebate My Tax Ltd can handle the claim and ensure you don’t miss the deadline.

How to Correct a P800 Mistake

If you notice a problem, you can:

1. Log in to your Personal Tax Account on GOV.UK.

2. Review the income, tax codes, and employment details HMRC holds.

3. Contact HMRC via phone (0300 200 3300) or secure message to explain the issue.

4. Provide evidence — like payslips, P45s, or P60s — to back up your claim.

HMRC will issue an updated P800 or P810 calculation once they’ve reviewed your case. If they confirm you’ve overpaid, you’ll receive a refund directly to your bank account.

Can You Still Claim If You Haven’t Received a P800?

Yes. Not everyone receives a P800 automatically — for example, if HMRC lacks full employer data or if you file a Self Assessment return.

If you think you’ve overpaid but haven’t been contacted, you can still proactively claim a refund. Rebate My Tax Ltd can do this on your behalf by reviewing your pay and tax history to uncover any missed refunds.

Why It Pays to Double-Check

Even small mistakes can add up. Many UK taxpayers are owed hundreds or even thousands of pounds in overpaid tax simply because of:

• Misreported income,

• Incorrect tax codes, or

• Missed allowances.

A quick review can ensure you’re not leaving money behind.

How Rebate My Tax Ltd Can Help

At Rebate My Tax Ltd, we specialise in spotting the errors HMRC and employers often overlook.

Using The UK’s Number 1 Tax App & Tax Services, our experts:

• Review your P800s from current and past tax years.

• Check for code errors, duplicated records, and missed allowances.

• Contact HMRC directly to correct issues and claim refunds fast.

• Keep you updated every step of the way through a simple, secure process.

We’ve helped thousands of people across the UK recover overpaid tax caused by P800 errors — often within just a few weeks.

Avoiding Future Mistakes

To prevent further P800 issues, follow these simple tips:

• Check your payslips regularly. Make sure your tax code is consistent.

• Keep your P45s and P60s. They’re vital for checking HMRC data accuracy.

• Update HMRC when you change jobs or circumstances.

• Review your online tax account annually.

Taking five minutes now could save you hundreds later.

Conclusion

Your P800 might look official, but that doesn’t mean it’s always right. Double-checking your tax calculation ensures you don’t lose money to simple errors or missing information.

If you’ve received a P800 — or think you might have overpaid — let Rebate My Tax Ltd handle the hard work. We’ll review your records, identify any mistakes, and help you claim what’s rightfully yours.