Most people know their gross salary and their bank balance, and have never been shown what happens between the two. Payslips do not help much: they list deductions without explaining why each one is the size it is, and the numbers rarely divide into anything obvious.
This is the whole mechanism, in the order it happens. Deliberately without current figures — those change every April, and they belong in the calculator, which states which tax year it is using and when a person last checked it. What follows does not change in April.
The single most useful thing to understand
Income tax and National Insurance are two entirely separate calculations. They are not two slices of one deduction. They use different thresholds, different bands, and different rules about what counts as pay. Someone can be a higher-rate taxpayer for income tax and be paying the lower National Insurance rate at the same time, because the point at which NI drops is not the point at which income tax rises.
Almost every confusion about payslips comes from assuming there is one combined rate. There is not, and no single percentage describes what leaves your pay.
Step one: the personal allowance
Everyone starts with an amount they can earn before paying income tax at all. Tax is charged only on what is left after it — your taxable income, which is a smaller number than your salary and the one every income tax band is measured against.
This is the first place people go wrong reading a rates table. When a table says the basic rate applies up to a certain amount, that limit is usually stated in taxable income, not salary. The salary at which you become a higher-rate taxpayer is that limit plus your allowance.
The 60% band nobody advertises
Above a certain income the personal allowance is withdrawn, at a rate of £1 of allowance for every £2 earned. That produces something genuinely strange: within that stretch, each extra pound is taxed at the higher rate and costs you 50p of allowance, which is itself then taxed. The effective rate on income in that range is substantially above the headline higher rate — around 60% for most people, and higher again for anyone also repaying a student loan.
It is not a band, it is not on any rates table, and it appears nowhere on a payslip. But it is real, it is the reason pension contributions are unusually effective in that range, and it ends once the allowance is gone.
The same mechanism, in a different place, produces the Child Benefit charge — an income range where each extra pound claws back part of a benefit. That one is calculated here, and its effective rate depends on how many children you have.
Step two: National Insurance, which works differently
National Insurance is charged on your earnings, not on your taxable income, and it has no personal allowance. It has its own threshold, and — unlike income tax — the rate goes down above the upper limit rather than up.
That falling rate is why National Insurance is a smaller part of the picture at high salaries and a large part at modest ones, and why comparing “tax rates” between countries using income tax alone is misleading for the UK.
It is also charged per pay period, not per year. Someone paid an unusual bonus in a single month can pay more National Insurance across the year than someone earning the same annual amount evenly, because the bonus month breaches the upper limit while the other months sit below the threshold. Income tax, being cumulative through PAYE, corrects itself. National Insurance does not.
Step three: student loan, which is not a tax
Student loan repayments are a percentage of everything above a threshold that depends on your plan. Several plans exist, they have different thresholds, and postgraduate loans are repaid in addition to an undergraduate plan rather than instead of it.
Two things surprise people. First, the repayment is calculated on earnings above the threshold, not on the whole salary. Second, it is unrelated to the balance outstanding — paying more does not shorten the term in the way a mortgage overpayment does, because the debt is written off after a set period regardless. Whether it is worth overpaying is a question about your expected lifetime earnings, and not one arithmetic alone can settle.
Where pensions sit, and why the arrangement matters more than the amount
This is the part with the most money in it and the least clear explanation anywhere.
Salary sacrifice means you formally give up part of your salary before it is ever earned. Your gross pay is genuinely lower, so income tax and National Insurance are both reduced, and so is the personal allowance taper. It is the most efficient arrangement available to most employees.
Net pay arrangement takes the contribution from pay before income tax but after National Insurance. You get income tax relief immediately; you do not save National Insurance.
Relief at source takes the contribution from pay after tax. The provider reclaims basic-rate relief and adds it to your pot. If you are a higher-rate taxpayer, the rest is not automatic — you claim it through Self Assessment, and a great many people never do.
Three arrangements, the same contribution, three different costs to take-home pay. The salary sacrifice calculator works out the first one exactly, and explains why a single marginal rate overstates the cost of a contribution that spans two bands.
Why the order matters
Deductions are not independent percentages applied to the same number. Salary sacrifice reduces the figure that income tax, National Insurance and the personal allowance taper are all measured against — so a contribution can reduce three things at once. A net pay contribution reduces one. Applying a flat “tax rate” to a salary, however carefully chosen, cannot represent this.
That is the argument for showing the working rather than a single figure, and it is why the take-home pay calculator itemises every line rather than printing one number.
What this means for a pay rise
A rise is worth less than its headline, always — but by a proportion that changes with where you are. Crossing into a higher band, into the allowance taper, past a student loan threshold, or over the Child Benefit threshold each changes the fraction you keep. Two people receiving identical rises can keep noticeably different amounts.
What a rise is actually worth is its own calculation for that reason.
Where the figures live
Every rate and threshold mentioned as a mechanism here is published by HMRC and linked below. The current values are on the calculators, each of which states its tax year and the date a person last checked it against the source — because the mechanism above is stable and the numbers are not, and mixing the two is how an article that reads as current quietly stops being true.