Why salary calculators disagree

Run the same salary through three calculators and you can easily get three answers. That is not because two of them are broken. Almost all of them apply the published rates correctly; what differs is the assumptions they make about you in the places you have not told them anything.

Here is what actually causes it, most common first.

1. Your tax code, which is the big one

Nearly every calculator assumes the standard code and most do not say so. If yours is different, every figure below it moves - and a code is not a detail, it is the whole Personal Allowance. The salary calculator on this site takes your code under More options and works Income Tax out the way payroll does under it, then says whether that is more or less than the year's rules would charge.

Three cases cause most of the confusion:

• A different allowance. Underpaid tax from an earlier year, a company car or medical insurance all reduce the code. The calculator assumes £12,570 is free of tax; your code may say otherwise.
• An emergency or non-cumulative code - one ending W1, M1 or X. These treat every pay packet as if it were the first of the year, so the allowance is not carried forward and the tax is usually too high until it settles. No annual calculator models this, because it is not an annual effect.
• BR, D0 or D1, which tax the whole of that job at a single rate - normal for a second job, wrong-looking on a payslip if you do not know why.

The tax code explainer covers what each one means.

2. Which kind of pension you are in

This is the one that produces a small, constant gap that nobody can explain, and it is the most common reason two honest calculators disagree. There are three arrangements and they do not produce the same answer:

Arrangement	Income Tax relief	National Insurance relief
Salary sacrifice	Yes	Yes
Net pay arrangement	Yes	No
Relief at source	Yes, 20% added by the provider; the rest claimed back	No

A calculator that models your contribution as salary sacrifice when you are actually in a net pay scheme will overstate your take-home, because it has given you National Insurance relief you do not get. This matters most in the public sector: the NHS Pension Scheme is a net pay arrangement, which is why the NHS calculator on this site runs the engine twice - once for the Income Tax relief and once for National Insurance on the full gross - rather than treating the contribution as sacrifice.

Relief at source adds a further wrinkle for Scottish taxpayers: gov.uk confirms that a saver paying the 19% starter rate still gets relief at 20% and does not have to repay the difference.

3. Whether it works per year or per pay packet

Income Tax under PAYE is cumulative across the year. National Insurance is not: it is charged on the earnings in each pay period against that period's own thresholds. HMRC publishes a separate figure for each period, which is the clue.

Threshold	Weekly	Monthly	Annual
Primary Threshold	£242	£1,048	£12,570
Upper Earnings Limit	£967	£4,189	£50,270

Twelve monthly thresholds come to £12,576; the annual one is £12,570. The £6 difference is small on an ordinary salary and not small at all on lumpy pay, where a big month can run past that month's Upper Earnings Limit into the 2% band while quieter months waste their thresholds. Nothing carries forward.

Company directors are assessed annually however often they are paid, under Regulation 8 of the Social Security (Contributions) Regulations 2001 - so a director and an employee on identical pay genuinely owe different amounts. The National Insurance calculator models all three bases.

4. The smaller ones, which still move the pennies

• Student loan reporting. HMRC requires the deduction to be reported on the FPS and P60 in "whole pounds only", so a calculator showing pence will not match a payslip exactly even when the rule is applied identically.
• Scotland. Different rates and bands on earned income, but not on dividends, savings or the Personal Allowance, which are reserved. A calculator that applies Scottish rates to dividend income is wrong; see the England vs Scotland comparison.
• Starting or leaving mid-year. PAYE is cumulative, so a part-year employment does not pay a pro-rated share of the annual tax - it front-loads the allowance. An annual calculator cannot see this.
• Payrolled benefits in kind. If your employer payrolls your medical insurance or car, it is taxed through the payslip rather than the code, and the gross on your payslip is not the gross you think it is.
• Weeks in the year. Some calculators divide by 52, some by 52.14. Over a year that is nearly a day's pay of difference in the weekly figure.

5. How to spot one that has not been updated

Test a figure that moved, not one that is frozen. The Personal Allowance is a bad test: £12,570 has been frozen for years and runs to 5 April 2031, so a badly stale calculator still shows it correctly.

Employer National Insurance is the good test. For 2026/27 it is 15% above a £5,000 secondary threshold. A calculator still showing 13.8% above £9,100 is running on pre-April-2025 rates, and whatever else it tells you is from the same vintage. Class 2 National Insurance is a second good test: it has not been a charge above the Small Profits Threshold since April 2024, so a self-employed calculator that still adds it to your bill is two years behind.

What this site assumes

Stated because a calculator that will not say this is asking to be believed rather than checked. Unless you tell it otherwise, the salary calculator here assumes:

• The standard tax code and a full Personal Allowance, cumulative, for a whole tax year - unless you enter a code, in which case Income Tax is what payroll deducts under it.
• One employment, paid evenly, with no bonus and no overtime.
• A pension contribution is salary sacrifice unless you pick net pay or relief at source in the pension type box; the NHS and teacher calculators are fixed to net pay because those schemes are.
• No taxable benefits in kind and nothing payrolled.
• Under State Pension age and on NI category letter A, unless you pick another letter.
• No Child Benefit claimed, so no High Income Child Benefit Charge unless you enter one.

What it does not model at all: the pay-period effect of an emergency or non-cumulative code outside the bonus-month panel (the annual figure is shown as if pay were level), mid-year starts and leavers, a change of National Insurance category letter part-way through the year, and the annual maximum for someone with several jobs. Where a figure cannot be sourced it is left out rather than estimated - the methodology sets out how that is decided, and every rate traces to a primary source in the data sources audit.

When to believe your payslip instead

Almost always. A calculator knows the rules; your payroll knows your code, your start date, your benefits and your scheme. If the two disagree, the calculator is usually right about the law and wrong about you.

The exception is worth knowing: payroll errors do happen, and the way to settle it is not another calculator. Your personal tax account shows what HMRC actually holds - your code, your employments, your estimated pay. If that disagrees with your payslip, one of them needs correcting, and HMRC's record is the one that drives your tax.

If you think something on this site is wrong, say so - the corrections page logs what we have already fixed, and about has the contact address.