UK payroll · bonus & commission FY 2026/27 DUAL-PASS
Most UK payroll software annualises a bonus month and slams it through the higher-rate bands before dividing back by 12, which is why a £3,000 lump sum feels like it loses half its weight on payslip day. This dual-pass calculator runs the exact same arithmetic against the locked 2026/27 monthly thresholds — then subtracts a baseline month to isolate the deductions levied only against the bonus.
Engine note · Dual-pass PAYE engine in integer pence — spike-month annualisation reconciled to the penny against baseline, no telemetry.
Two-pass engine: a baseline month at your contracted salary, then a spike month that inflates the gross by your bonus, re-runs PAYE under 2026/27-locked monthly thresholds (£1,047.50 PA · £4,189.17 BR · £10,428.33 AR), and subtracts to surface the brutal effective marginal rate that hits the lump sum on payslip day.
The engine reads monthly PA / BR / AR thresholds from the selected year — change to model a historical payslip.
SL applies a flat 9% (Plan 1/2/5) or 6% (PGL) on the monthly gross above the plan's threshold — non-cumulative.
Receiving a workplace bonus or corporate performance allocation represents a major financial milestone for any working professional. The initial feeling of success is frequently replaced by confusion the moment the corresponding payslip arrives. Employees are routinely blindsided by the intense level of tax deductions levied against their one-off bonus cash.
This over-taxation is not a processing error by your company's HR department. It is a direct mathematical result of how statutory PAYE clearing codes and National Insurance tracking systems handle irregular lump-sum injections within isolated monthly payroll runs.
The root cause of the bonus tax shock lies in the structural design of automated PAYE calculation engines. Standard UK payroll systems are engineered to operate in the present moment, running calculations using isolated monthly segments.
When a bonus lump sum of £5,000 is injected on top of a standard £3,500 monthly salary line, the software sees a total gross intake of £8,500 for that single month. Rather than recognising this as a unique, one-off event, the algorithm multiplies this monthly spike across the entire 12-month fiscal calendar, assuming your annualised earning capacity has suddenly expanded to £102,000. As a result, the engine dynamically applies higher-rate 40% income tax bands and personal-allowance clawback rules to your bonus cash from the very first pound — producing the heavily distorted deduction profile you see on the payslip.
While Income Tax (PAYE) can self-correct over time under a cumulative tax code, Employee National Insurance Contributions (NICs) operate on a strictly non-cumulative basis. Each weekly or monthly pay interval is treated as a completely closed box.
For the active 2026/27 fiscal cycle, employee Class 1 National Insurance charges a main rate of 8% on monthly earnings between the Primary Threshold (£1,047.50) and the Upper Earnings Limit (£4,189.17) and drops to a 2% upper rate on everything above that line. When a bonus pushes your monthly pay packet past the UEL, the portion of income exceeding the limit benefits from the lower 2% rate. While this non-cumulative structure provides a minor tax advantage for high earners compared to spreading the bonus evenly across the year, the intense combination of higher-rate PAYE brackets still results in a massive immediate deduction hit.
If your paycheck operates under a standard cumulative tax code (such as 1257L), your payroll software will systematically correct any initial over-taxation across subsequent months. In the months following your bonus spike, your actual monthly earnings will drop back down to your baseline salary, while your cumulative year-to-date (YTD) allowance pool continues to grow by an extra £1,047.50 each month.
As the system reviews your YTD figures over the remainder of the fiscal year, it recognises that your total annualised income will fall below the inflated estimate it calculated during the spike month. The software then applies an automatic downward adjustment to your income tax deductions, effectively returning your overpaid tax through slightly higher net take-home pay packets until your tax affairs are fully balanced at year-end. NI and student loan deductions, by contrast, are permanently lost — they don't smooth.
The summary card reports a single “effective marginal deduction
rate” computed by the dual-pass engine:
rate = bonus deductions delta ÷ gross bonus. For a basic-rate earner pushed across the higher-rate cliff by a
bonus, this typically lands at 42% (40% PAYE + 2% NI). Add an
undergraduate Student Loan Plan and it climbs to 51%; bring a Plan 5 + PGL stack to bear and it can crest
57%. The colour of the chip tracks the band — green inside basic, amber for
higher, red for additional or PA-taper territory — so a single glance tells
you which tax cliff your bonus has crossed. The cleanest way to avoid the
higher-rate cliff is sacrificing the bonus into your pension — explore that split in the Sacrifice Optimiser.
The per-period bands the engine reads to compute the spike-month deduction stack.
Why a £3,000 bonus feels like it loses half its weight on payslip day. The 4-step PAYE annualisation algorithm, the 40% cliff at £50,270, and how cumulative 1257L codes smooth the over-deduction back over later months.
Non-cumulative NI caps, non-cumulative student-loan deductions, and the higher-rate PAYE band stack on top of each other to strip 51% off every commission pound that crosses the monthly UEL. The exact math, line by line.
Every £1 between £100k and £125,140 costs 62p in tax + NI because Personal Allowance tapers away. The escape is salary sacrifice — and the math is generous.
Three pipes that all deposit into the same pension. Only one saves you NI; only one gives higher-rate relief automatically.