UK National Insurance Calculator
This professional-grade National Insurance (NI) calculator estimates employee (Class 1) and self‑employed (Class 2 and Class 4) contributions for the UK 2024/25 tax year. It’s designed for employees, sole traders, payroll admins, and advisors who need quick, accurate, and accessible NI figures.
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All fields marked with * are required. Values are processed using HMRC thresholds for your selected pay period.
Results
Enter your details to see your estimated National Insurance.
Calculations assume employee Category A or standard self‑employed rules for 2024/25. This tool is for guidance only and is not tax advice.
Data Source and Methodology
- HM Government — Work out an employee’s National Insurance contributions (updated for 2024/25). gov.uk/guidance/work-out-an-employees-national-insurance-contributions
- HMRC — Rates and thresholds for employers 2024 to 2025. gov.uk/guidance/rates-and-thresholds-for-employers-2024-to-2025
- HM Government — Self‑employed National Insurance rates. gov.uk/self-employed-national-insurance-rates
Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.
The Formula Explained
Employee (Class 1, Category A):
For annual pay P, Primary Threshold PT, Upper Earnings Limit UEL, main rate r_m and upper rate r_u:
$$ \mathrm{NI}_{\text{main}} = \max\!\bigl(0, \min(UEL, P) - PT\bigr)\times r_m $$
$$ \mathrm{NI}_{\text{upper}} = \max\!\bigl(0, P - UEL\bigr)\times r_u $$
$$ \mathrm{NI}_{\text{annual}} = \mathrm{NI}_{\text{main}} + \mathrm{NI}_{\text{upper}} $$
Self‑employed (Class 4 and optional Class 2):
For annual profits S, Lower Profits Limit LPL, Upper Profits Limit UPL, Class 4 rates r_{4m}, r_{4u}, weekly Class 2 rate c_2, Small Profits Threshold SPT, and an optionality flag v ∈ {0,1}:
$$ C4_{\text{main}} = \max\!\bigl(0, \min(UPL, S) - LPL\bigr)\times r_{4m} $$
$$ C4_{\text{upper}} = \max\!\bigl(0, S - UPL\bigr)\times r_{4u} $$
$$ C2_{\text{vol}} = \begin{cases} 52 \times c_2 \times v, & \text{if } S < SPT \\ 0, & \text{if } S \ge SPT \end{cases} $$
$$ \mathrm{NI}_{\text{annual}} = C4_{\text{main}} + C4_{\text{upper}} + C2_{\text{vol}} $$
Glossary of Variables
- P: Annual gross pay (employee).
- S: Annual taxable profits (self‑employed).
- PT: Primary Threshold (employee) — below this, employee NI is 0%.
- UEL: Upper Earnings Limit (employee) — earnings above are charged at the upper rate.
- LPL: Lower Profits Limit (self‑employed, Class 4).
- UPL: Upper Profits Limit (self‑employed, Class 4).
- SPT: Small Profits Threshold — below this you may opt to pay Class 2 voluntarily.
- r_m / r_u: Employee Class 1 main and upper rates.
- r_{4m} / r_{4u}: Self‑employed Class 4 main and upper rates.
- c_2: Weekly Class 2 rate (voluntary when profits are below SPT in 2024/25).
- Effective NI rate: Total NI divided by total gross pay or profits.
Worked Example
How It Works: A Step‑by‑Step Example
Scenario: Employee on £3,000 per month in 2024/25, Category A.
- Annualise: £3,000 × 12 = £36,000.
- Apply thresholds: PT £12,570; UEL £50,270.
- Main band: min(£36,000, £50,270) − £12,570 = £23,430.
- Upper band: max(£36,000 − £50,270, 0) = £0.
- NI: £23,430 × 8% = £1,874.40 per year ⇒ about £156.20 per month.
Frequently Asked Questions (FAQ)
Which tax year is supported?
2024/25 (6 April 2024 to 5 April 2025). We update rates promptly when HMRC publishes changes.
Does this calculator include employer NI?
No. It shows employee Class 1 and self‑employed Class 4 (plus optional Class 2). Employer NI is excluded.
How are weekly and monthly thresholds handled?
HMRC defines thresholds per period. We use the compliant PT/UEL weekly and monthly values so your results match HMRC methods.
What about special NI categories (e.g., under 21, apprentices)?
This tool assumes Category A for employees. Other categories can differ and are not included here.
Is Class 2 mandatory for the self‑employed in 2024/25?
No, not if profits are at or above the Small Profits Threshold. If profits are below SPT, you can choose to pay Class 2 voluntarily to protect State Pension entitlement.
Does salary sacrifice affect NI?
Yes. Salary sacrifice can reduce NI. This tool assumes straightforward gross pay without salary sacrifice.
Is this tax advice?
No. It’s an educational tool. Consult a qualified advisor for your specific situation.
Formula (LaTeX) + variables + units
\mathrm{NI}_{\text{main}} = \max\!\bigl(0, \min(UEL, P) - PT\bigr)\times r_m
\mathrm{NI}_{\text{upper}} = \max\!\bigl(0, P - UEL\bigr)\times r_u
\mathrm{NI}_{\text{annual}} = \mathrm{NI}_{\text{main}} + \mathrm{NI}_{\text{upper}}
C4_{\text{main}} = \max\!\bigl(0, \min(UPL, S) - LPL\bigr)\times r_{4m}
C4_{\text{upper}} = \max\!\bigl(0, S - UPL\bigr)\times r_{4u}
C2_{\text{vol}} = \begin{cases} 52 \times c_2 \times v, & \text{if } S < SPT \\ 0, & \text{if } S \ge SPT \end{cases}
Employee (Class 1, Category A): For annual pay P, Primary Threshold PT, Upper Earnings Limit UEL, main rate r_m and upper rate r_u: $ \mathrm{NI}_{\text{main}} = \max\!\bigl(0, \min(UEL, P) - PT\bigr)\times r_m $ $ \mathrm{NI}_{\text{upper}} = \max\!\bigl(0, P - UEL\bigr)\times r_u $ $ \mathrm{NI}_{\text{annual}} = \mathrm{NI}_{\text{main}} + \mathrm{NI}_{\text{upper}} $ Self‑employed (Class 4 and optional Class 2): For annual profits S, Lower Profits Limit LPL, Upper Profits Limit UPL, Class 4 rates r_{4m}, r_{4u}, weekly Class 2 rate c_2, Small Profits Threshold SPT, and an optionality flag v ∈ {0,1}: $ C4_{\text{main}} = \max\!\bigl(0, \min(UPL, S) - LPL\bigr)\times r_{4m} $ $ C4_{\text{upper}} = \max\!\bigl(0, S - UPL\bigr)\times r_{4u} $ $ C2_{\text{vol}} = \begin{cases} 52 \times c_2 \times v, & \text{if } S < SPT \\ 0, & \text{if } S \ge SPT \end{cases} $ $ \mathrm{NI}_{\text{annual}} = C4_{\text{main}} + C4_{\text{upper}} + C2_{\text{vol}} $
- T = property tax (annual or monthly depending on input) (currency)
- I = homeowners insurance (annual or monthly depending on input) (currency)
- gov.uk/guidance/work-out-an-employees-national-insurance-contributions — gov.uk · Accessed 2026-01-19
https://www.gov.uk/guidance/work-out-an-employees-national-insurance-contributions - gov.uk/guidance/rates-and-thresholds-for-employers-2024-to-2025 — gov.uk · Accessed 2026-01-19
https://www.gov.uk/guidance/rates-and-thresholds-for-employers-2024-to-2025 - gov.uk/self-employed-national-insurance-rates — gov.uk · Accessed 2026-01-19
https://www.gov.uk/self-employed-national-insurance-rates
Last code update: 2026-01-19
- Initial audit spec draft generated from HTML extraction (review required).
- Verify formulas match the calculator engine and convert any text-only formulas to LaTeX.
- Confirm sources are authoritative and relevant to the calculator methodology.