UK National Insurance Calculation — methodology

Plain-English summary

National Insurance (NI) is a separate deduction from PAYE income tax. It is taken from your wages by your employer and is split into two halves — what you pay (employee) and what your employer pays (employer). The amount depends on your NI category letter, which reflects things like your age, your contract type, and any reliefs your employer claims.

NI is calculated on a per-pay-period basis (not cumulative like PAYE). That means each payslip is checked in isolation against the relevant thresholds and rates for that period. The three thresholds you need to know are the Primary Threshold (PT) below which no employee NI is due, the Secondary Threshold (ST) above which employer NI starts, and the Upper Earnings Limit (UEL) above which the employee rate drops.

PayslipIQ recomputes NI for the period and category letter shown on the slip and tells you whether the deducted figure is consistent. We also flag categories that may be wrong for the worker's profile — for example a Category C on someone clearly under State Pension age.

NI category letters

Thresholds we use

The exact figures change at the start of each tax year. Our engine pulls the values effective on the pay-date of the slip.

• Lower Earnings Limit (LEL) — earnings count for benefit entitlement
• Primary Threshold (PT) — employee NI starts above this
• Secondary Threshold (ST) — employer NI starts above this
• Upper Earnings Limit (UEL) — employee main rate stops above this
• Upper Secondary Threshold (UST) — relief threshold for under-21 / apprentices
• Veterans Upper Secondary Threshold (VUST) — relief threshold for category V

Step-by-step calculation logic

1. Identify pay frequency. Choose the matching threshold table (weekly, four-weekly, monthly).
2. Determine NIable pay. Start from gross taxable pay; subtract pension salary-sacrifice contributions if visible on the slip; ignore deductions taken net (e.g. payroll giving via net adjustment).
3. Slice the earnings. Split NIable pay into bands: 0 to PT, PT to UEL, above UEL.
4. Apply employee rates by category.
   • A, H, M, V, B (modern) → main rate between PT and UEL, lower rate above UEL
   • C, X → 0% employee NI
   • J, Z → deferred lower rate between PT and UEL, lower rate above UEL
5. Apply employer rates. Standard secondary rate above ST. For categories H, M, V, Z, the secondary rate is 0% up to UST/VUST.
6. Round. NI is rounded to the nearest penny per HMRC convention; some payroll engines truncate, which produces 1p discrepancies we tolerate.
7. Compare. Reconcile against the slip and produce a finding for any difference larger than tolerance (default £1.00).

Worked example

Priya is monthly-paid, NI category A, gross pay £5,000 in May 2026. Assume monthly PT equivalent £1,048 and UEL equivalent £4,189; main employee rate 8%, upper rate 2%.

• Slice 1: £0 to £1,048 → 0% → £0
• Slice 2: £1,048 to £4,189 (£3,141) → 8% → £251.28
• Slice 3: £4,189 to £5,000 (£811) → 2% → £16.22
• Total expected employee NI: £267.50

If the slip shows £255.00, that is a £12.50 gap, which we surface as "needs confirmation". A common explanation is a pension salary-sacrifice contribution we cannot see — we ask the user to check whether their slip shows a sacrifice line.

Edge cases handled

• Directors — annual earnings basis applies; the same thresholds are used cumulatively across the year.
• Category drift — turning 21 or reaching State Pension age mid-year requires a category change. We flag inconsistencies.
• Rounding — HMRC rounds; some engines truncate. We tolerate 1p per slice.
• Bonus periods — a one-off bonus pushes the period above UEL but does not annualise. We treat it correctly per period.
• Statutory pay — SSP, SMP, SAP, SPP, SShPP and SPBP are NIable like normal pay.
• Termination payments — first £30,000 of a redundancy payment is NIC-free; we exclude where flagged.

Limitations and known gaps

• We approximate salary sacrifice from line items — bespoke schemes may yield small variances.
• Multi-employment NI deferments (Cat J/Z) require a confirmed deferment letter, which we cannot see.
• Class 1A/1B benefit charges sit outside the payslip and are not modelled.
• Some employers post NI in arrears for one period; YTD comparison is the correct test in that case.

Sources used

Disclaimer

PayslipIQ provides informational analysis only. We do not provide tax, legal, or employment advice. National Insurance categories are set by your employer based on information you supplied — discuss any apparent mismatch with payroll first, and contact HMRC if it cannot be resolved internally.