Estimate your pension allowance based on your salary and pension contributions.
Your calculated pension allownace and incomes will be shown below. If your remaining allowance is negative you will need to pay tax on that amount.
The amount you can contribute to a pension per year without paying tax is subject to a maximum limit. This limit is £40,000 unless you are a high earner when it can taper down to a minimum limit of £10,000 (£4,000 in 2020-2021). Any contributions you make above your allowance will be taxed as if it was additional income.
The calculation of your pension allowance is quite complex, and is explained in full on this HMRC page. It relies on your threshold and adjusted incomes, which are also estimated in this calculator.
This calculator simplifies the calculation it by considering only your total taxable income, and your pension contributions. In terms of the pension contributions it is important to seperate them into contributions by your employer, and then contributions via net pay or relief at source. Net pay contributions are contributions taken from your salary before it is taxed, whereas relief at source contributions are paid from your post tax income with tax relief paid after.
For example if you earn £160,000 in 2019-2020 (before tax) and pay 5% (£8,000) net pay (salary sacrifice) directly to your pension with your employer contributing 10% (£16,000). Your total taxable income is £152,000, net pay contribution is £8,000, relief at source contribution is £0, and the employer contribution is £16,000.
Alternatively if you earn £160,000 in 2019-2020 (before tax) and pay 10% (£16,000) to your pension from your post taxed income. Your total taxable income is £160,000, net pay contribution is £0, relief at source contribution is £16,000, and the employer contribution is £0.
Roughly speaking the threshold income is your taxable income after deducting your personal pension contributions and the adjusted income is your total income i.e. income plus all pension contributions.