About Threshold Wise

A calculator for PAYE employees whose pay is not the same every month, built around the one figure that decides several expensive questions at once.

What it does

It works out your adjusted net income for the 2026/27 tax year, tells you whether that figure is above or below £100,000, and shows three consequences: the income tax caused by the withdrawal of your personal allowance, what childcare eligibility is worth in your circumstances, and the gross pension contribution that would bring adjusted net income back to the threshold.

It exists because the people most affected by these rules tend to be the least well served by generic salary calculators. If your income arrives as a base salary plus a bonus plus a vesting schedule, a tool that asks for one number cannot tell you anything useful.

What it deliberately does not do

• It does not give advice. It states arithmetic and stops. There is no recommendation anywhere in it, no product is named, and nothing suggests a course of action. That is a hard boundary, enforced by an automated check that fails the build if advice-shaped wording appears anywhere in the codebase.
• It does not cover Scotland. Scottish income tax rates and bands differ. Modelling them approximately would produce numbers that look right and are not, so the tool declines to produce a figure rather than produce a wrong one.
• It does not model capital gains, the High Income Child Benefit Charge, the pension annual allowance or its taper, student loan repayments, or anything outside the £100,000 question.
• It does not store your figures. See the privacy page.

Where the tax figures come from

Every threshold, rate and allowance used by the calculator lives in a single configuration file, one per tax year. Each figure carries the gov.uk or HMRC page it was taken from and a flag recording whether it has been checked against that source. Nothing in the calculation reads a number from anywhere else, and an automated test fails the build if a tax figure is typed into code or copy directly.

That structure exists for two reasons. It makes the annual update a single, reviewable change rather than a hunt through the codebase. And it makes the figures auditable: the list of every constant, with its source, is maintained as a checklist to work through with an accountant before launch.

The figures are published rates, not opinions, but published rates get transcribed wrongly and Budgets change them. Check anything that matters against the source.

How the calculation is built

The tax logic is a standalone module with no dependency on the user interface. It has full unit test coverage, and the four scenarios the product was specified around are written out as tests with their expected figures stated longhand, so that the arithmetic can be checked by reading rather than by running.

Some of the detail in there is subtle. Relief at source pension contributions reduce adjusted net income but not taxable income, extending the tax bands instead. Salary sacrifice reduces National Insurance where the other pension types do not. The personal allowance is rounded up in the taxpayer’s favour. The childcare test is a strict inequality, so exactly £100,000 keeps eligibility. Each of those is a place where a plausible-looking shortcut gives the wrong answer.

Limits worth knowing

• National Insurance is annualised. Real Class 1 National Insurance is assessed per pay period, so for anyone with uneven pay the figure shown is an estimate.
• The childcare assessment covers one parent. A partner’s adjusted net income is tested separately against the same limit.
• RSU vests are taken at the value you enter. If a share price moves before the vest date, the income moves with it.
• Everything assumes the standard personal allowance and no allowances or reliefs beyond those in the form.

Getting in touch

Corrections to the tax treatment are particularly welcome. If something here is wrong, hello@thresholdwise.com reaches a person.

The notice, in full

Threshold Wise provides information about how UK tax rules apply to the figures you enter. It is not financial, tax or investment advice, and it does not take account of your personal circumstances. All figures are estimates. Speak to a qualified adviser or accountant before acting on anything you read here.