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What The Second Job Tax Calculator Works Out
- Tax on the second job: income tax on your combined income minus the tax on the main job alone. There is no second job tax rate: the second job simply arrives with no personal allowance attached, so PAYE puts it on a BR code (20% on everything), or D0 (40%) once the main job already fills the basic rate band. GBP 8,000 on top of GBP 25,000 costs GBP 1,600, exactly what the same money would cost as overtime.
- NI on the second job: the genuinely favourable quirk. National Insurance thresholds apply to each employment separately, so a second job paying under GBP 12,570 attracts no NI at all, and one paying more is charged 8% only on its own pay above the threshold.
- Second job take-home: the second income minus its tax and NI, per year and per month.
- Self-employment / side hustle (advanced): switches the second income to self assessment. The GBP 1,000 trading allowance (or your actual expenses if higher) comes off first, income tax applies to the profit at your marginal rate, and Class 4 NI at 6% only starts on profits above GBP 12,570, so most small side incomes pay no NI. Register for self assessment once gross trading income passes GBP 1,000.
- Student loan on the second job (advanced): through PAYE each employer applies the plan threshold to its own pay, so a second job below the threshold has nothing deducted even if your combined income is above it. Self-employed income is assessed on combined income through the tax return instead.
- Take-home per hour (advanced): the net second income after tax, NI and student loan divided by 52 weeks and your hours, which is the number to weigh against your free time.
- If earned as overtime in the main job (advanced): the same gross through one payslip pays identical income tax but NI on the combined pay, so a separate job usually leaves you better off by the NI saved.
- Total deductions across both incomes (advanced): tax, NI and student loan on both jobs together, as a percentage of combined income. Check both tax codes in your personal tax account after starting: codes split badly across jobs cause over-collection that is refunded only after HMRC reconciles.
- Housekeeping: tick "this is not my only job" on the starter checklist, read your main contract for moonlighting clauses, and remember the 48-hour working time average applies across all jobs. Not tax advice.
Source and further reading: GOV.UK (HMRC): tax on a second job.