You open an online salary calculator, type in your salary, and see a take-home figure. Then you look at your payslip and it's different — sometimes by hundreds of pounds. This is extremely common, and it almost always has a logical explanation. Here are the 8 most frequent causes.

Your Tax Code Isn't the Default 1257L

Most calculators default to the standard 1257L tax code (Personal Allowance of £12,570). But HMRC assigns personal codes to millions of employees for various reasons — a higher allowance for professional subscriptions, a lower one for unpaid tax from a previous year, or an emergency BR code after a job change. Check your payslip for your actual code, then enter it into our calculator for an accurate match.

Your Pension Comes Out Differently

There are three pension contribution types — Net Pay (before tax), Relief at Source (after tax), and Salary Sacrifice — and they affect your take-home differently. A Relief at Source contribution shows on your payslip as a post-tax deduction but you get basic rate relief added back by your provider. If the calculator uses a different pension type than yours, the results will differ. Our calculator lets you select the exact type.

You Have Benefits in Kind

Company benefits such as a company car, private medical insurance, or a fuel card are considered taxable 'benefits in kind'. HMRC adjusts your tax code to collect tax on these — reducing your effective allowance. A basic web calculator won't know about these. Enter the monthly value of your benefits to correct the estimate.

Student Loan Deductions

Calculators must know your exact plan (1, 2, 4, 5, or Postgraduate). Using the wrong plan or leaving it blank produces the wrong result. If you're on Plan 2 but the calculator assumes no student loan, the difference can be £50–£200/month on a typical graduate salary. Not sure which plan? Check your original loan documents or log into the Student Loans Company portal.

It's Not the First Month of the Tax Year

PAYE operates cumulatively — your employer calculates your total tax due from 6 April to date and adjusts each month based on what's already been deducted. So if your tax liability was slightly over in a previous month, this month might show a lower deduction. Online calculators often show a static monthly average, not the exact amount for a specific payroll date.

Your Employer Has Enhanced NI or Benefits

Some employers have salary sacrifice arrangements for cycle-to-work, electric vehicles, or childcare vouchers that reduce your gross pay before NI is calculated. These won't show in a basic calculator unless you input the sacrifice amount. Similarly, some employers use a different NI category letter (e.g., category M for under-21s, J for those with multiple jobs).

You're on an Emergency Tax Code

Emergency codes (often shown as 1257L W1/M1, BR, or 0T) tax you without accounting for the rest of the year's income or allowances. If you just started a new job without providing a P45, HMRC may have applied an emergency code that deducts more tax than necessary. This will be corrected automatically once HMRC receives payroll data — or you can contact HMRC immediately to resolve it.

The Calculator Is Out of Date

Tax bands, NI thresholds, and student loan rates change every April. If the calculator you're using hasn't been updated for 2026/27, the numbers will be slightly off. Our calculator is fully updated for every new tax year the moment rates are confirmed by HMRC.

How to Get an Exact Match

Use our UK Salary Calculator with these specific inputs from your payslip:

Your exact tax code (bottom of payslip or on your P60)
Your pension contribution type (check your pension scheme documents)
Any salary sacrifice amounts (cycle to work, EV scheme)
Your student loan plan
Your pay period (monthly, weekly, 4-weekly)

With all those in, the calculator will match your payslip to within a few pence for most scenarios.

Why Your Payslip Differs from Online Calculators