Data Source and Methodology
- HM Revenue & Customs (HMRC) – Estimate your Income Tax: gov.uk/estimate-income-tax (accessed current tax year).
- HMRC – Income Tax rates and allowances (2024 to 2025): gov.uk/income-tax-rates.
- HMRC – National Insurance rates and thresholds (Class 1, 2024 to 2025): gov.uk/national-insurance-rates-letters.
- Student Loans Company/HMRC – Repayment thresholds and rates (2024 to 2025): gov.uk/repaying-your-student-loan.
Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.
The Formula Explained
Personal Allowance from a standard numeric tax code: $$PA = 10 \times \text{codeDigits} \quad \text{(e.g., 1257L → } PA=12{,}570\text{)}$$
Tapering when Adjusted Net Income exceeds £100,000: $$PA^* = \max\left(0,\; PA - \frac{\max(0,\; ANI - 100{,}000)}{2}\right)$$
Taxable income and banded tax: $$TI = \max(0,\; G_t - PA^*)$$ $$Tax = \sum_{b \in \text{bands}} \left( \min(\max(0, TI - b_{low}),\; b_{width}) \times b_{rate} \right)$$
Employee Class 1 NI per pay period (annualised): $$NI_{period} = 0.08 \cdot \max(0,\min(E, UEL) - PT) + 0.02 \cdot \max(0,\; E - UEL)$$ $$NI_{annual} = NI_{period} \times N_{periods}$$
Student Loan and Postgraduate Loan: $$SL = r_{sl} \cdot \max(0,\; E_{SL} - T_{sl}) \quad,\quad PGL = 0.06 \cdot \max(0,\; E_{SL} - 21{,}000)$$
Pension impact (p = contribution rate): $$G_t = \begin{cases} G \cdot (1 - p) & \text{Salary Sacrifice} \\ G - G\cdot p & \text{Net Pay Arrangement} \\ G & \text{Relief at Source} \end{cases} \quad,\quad E_{NI} = \begin{cases} G \cdot (1 - p) & \text{Salary Sacrifice} \\ G & \text{otherwise} \end{cases}$$
Glossary of Variables
- G: Gross pay before deductions.
- G_t: Taxable gross after pension method adjustments.
- ANI: Adjusted Net Income used for Personal Allowance taper (after salary sacrifice or net pay contributions).
- PA: Personal Allowance; PA*: tapered Personal Allowance.
- TI: Taxable income after allowance.
- E: Earnings in a pay period used for NI; PT: Primary Threshold; UEL: Upper Earnings Limit.
- SL, PGL: Student Loan and Postgraduate Loan deductions; r_sl is 9% for Plans 1/2/4/5; PGL is 6%.
- p: Employee pension rate (percentage of gross).
How It Works: A Step-By-Step Example
Inputs: Salary £45,000/year, Region rUK, Tax code 1257L, No pension, No loans, Tax year 2024/25.
- Personal Allowance: 1257L → PA = £12,570. No taper since ANI ≤ £100,000.
- Taxable income TI = £45,000 − £12,570 = £32,430.
- rUK bands: Entire TI in basic band at 20% → Tax = 0.20 × £32,430 = £6,486.
- NI (annual): PT £12,570, UEL £50,270. NI = 8% × (£45,000 − £12,570) = £2,596.
- Net pay = £45,000 − £6,486 − £2,596 = £35,918 (~£2,993/month).
Frequently Asked Questions (FAQ)
What tax years are supported?
2024/25 (default) and 2023/24. Rates update when HMRC publishes new thresholds.
How accurate is the NI calculation?
NI is calculated per your chosen pay frequency using period thresholds (weekly, monthly or annual) and then annualised, matching real PAYE behaviour closely.
Do you support Scottish Income Tax?
Yes. Selecting Scotland applies the 2024/25 six-band structure (Starter, Basic, Intermediate, Higher, Advanced, Top).
Can I simulate salary sacrifice vs net pay pension?
Yes. Choose a pension rate and method. Salary Sacrifice reduces both tax and NI; Net Pay reduces tax only; Relief at Source is taken from net pay and your provider claims basic-rate relief.
Which student loan plans are included?
Plans 1, 2, 4 and 5 at 9% above their thresholds, plus Postgraduate Loan at 6% above £21,000 (2024/25).
Are the results advice?
No. Results are estimates for guidance only. For personal advice, consult a qualified tax professional.