Every Self Assessment calculator is doing the same sequence, and the sequence is worth knowing because it explains results that otherwise look wrong. Income is totalled by source, the personal allowance is applied and possibly tapered, the rate bands are worked through in order, National Insurance is added on self-employed profit, charges that sit on top are added, and everything already paid is credited. Then, separately, the payments on account for next year are set. This page walks that order.
Total the income, by source, for one tax year
Employment, self-employment profit, property profit, savings interest, dividends and anything else are brought together for the year ending 5 April. They are kept separate at this stage because they are not all taxed the same way: savings and dividends have their own allowances and their own rates, and self-employed profit alone attracts National Insurance. A calculator with a single income box is quietly assuming everything you earned is the same kind of income, which is the most common reason its answer differs from the return.
Apply the allowance, then the bands in order
The personal allowance comes off first, and it is withdrawn gradually once income passes the taper threshold, which means an extra pound of income in that zone costs more than the headline rate suggests. What remains is charged through the bands in order: basic, then higher, then additional. The band is decided by your total taxable income, not by any one source, which is why a modest rental profit can be taxed at 40% if your salary has already filled the basic-rate band.
Add National Insurance and anything that sits on top
Class 4 National Insurance is charged on self-employed profit between thresholds, and Class 2 sits alongside it with its own rules about voluntary payment and the state pension record. Then come the charges that are collected through the return rather than through the rate bands: the High Income Child Benefit Charge is the one most people meet, and it is why a pay rise can produce a bill that seems out of proportion to it. Student loan repayments are collected here too.
Credit what has been paid, then set next year's instalments
PAYE already deducted, CIS deducted, and payments on account already made are all set against the figure to give the balancing payment. Only then are the following year's payments on account set, normally at half the year's bill each. This last step is separate from your tax for the year, and forgetting it is why a January demand so often exceeds the tax anyone expected. A calculator that stops at the bill has answered half the question.
Questions people ask about self assessment tax calculator uk
Why is the calculator's figure different from HMRC's?
Usually because of income mix, allowances that depend on your circumstances, or payments on account. A calculator applies rates to what you typed; the return applies them to your whole position.
Does the calculator include National Insurance?
The estimator on this site adds Class 4 on self-employed profit and shows it as a separate line, because it is charged on profit rather than on total income.
Is the figure my payment due in January?
Not necessarily. January usually carries the balancing payment for the year just ended plus the first payment on account for the year you are in.
Can I rely on it for the return?
No. It shows the arithmetic so you can check it. Only your filed return, and HMRC, determine what you owe.