Income tax and National Insurance are two separate charges with different rules, and only their SUM explains a payslip. At £45,000 in 2026/27, income tax of £6,486.00 and NI of £2,594.40 come together to £9,080.40, leaving £35,919.60. The calculator this page feeds always shows the two lines separately, with the band each pound fell in.
Two charges, two shapes
Income tax is progressive with a taper: allowance first, then 20%, 40% and 45% slices, and above £100,000 the allowance itself erodes. NI is nearly flat: 8% in a single wide band from £12,570 to £50,270, then 2% without limit. The crossover matters: above £50,270 NI nearly stops rising while tax accelerates, which is why the combined marginal rate goes 28% → 42% at that line rather than 20% → 40%.
The combined picture at three salaries
£30,000: tax £3,486.00 and NI £1,394.40 take 16.27% of gross, leaving £25,119.60. £45,000: tax £6,486.00 and NI £2,594.40 take 20.18%. £60,000: tax £11,432.00 and NI £3,210.60 take 24.4%, leaving £45,357.40. Notice NI barely moves between the last two while tax jumps: past the upper earnings limit, income tax is nearly the whole story of a rise.
What NI buys, and why it is not just another tax
NI contributions build your entitlement record for the State Pension and some benefits: a year counts when you earn above the lower earnings limit, even in weeks where little NI is actually paid. That is also why NI is charged per pay period rather than annually: the record is weekly. For take-home arithmetic, though, treat it exactly as the second deduction line; the calculator's breakdown table names the threshold each slice of your pay crossed.
Questions people ask about ni and tax calculator
Why did my NI go down but my tax go up after a pay rise?
It cannot: both only rise with pay. What changes is the SHARE: past £50,270 NI adds just 2p per pound while tax adds 40p, so tax dominates the increase.
Do I pay NI on pension contributions?
Under salary sacrifice, no: the pay given up escapes both tax and NI. Under a net-pay or relief-at-source scheme, NI is still charged on the full gross.
Does NI apply to income from savings or rent?
No. Class 1 NI is on earnings from employment only. Income tax has the wider base; that is one more way the two charges differ.