Methodology & Data Sources
This page sets out exactly where the numbers on prisontakehome.co.uk come from, the order the calculation runs in, how it is tested, and what it does not cover. If you want to check our working, this is the page to read.
Last updated: 26 September 2026
Where the pay data comes from
| What | Source | Updated |
|---|---|---|
| Pay bands and contract variants | PSPRB annual report (Appendix E pay tables) | Annually, from April |
| Payment Plus and on-call rates | HMPPS pay and allowances guidance | Annually |
| alpha pension contribution tiers | Civil Service Pensions | Annually, from April |
| Income tax & National Insurance | HMRC rates and thresholds | Annually, from April |
| Student loan thresholds | Student Loans Company / HMRC | Annually, from April |
Figures are transcribed from the published documents by hand and checked against a second source where one exists. Pay data is held in a single canonical file per site and mirrored into the calculator, with an automated check that fails the build if the two ever disagree.
How the calculation runs
The order matters: change it and the answer changes. The calculator works through the following steps.
- Basic pay is looked up from the published scale for your grade and pay point, then scaled if you work less than full time.
- Allowances and additions are applied, with each one classified as pensionable or non-pensionable, because that distinction changes both the pension deduction and the taxable figure.
- Pension contributions are deducted next. Prison Take-Home Pay models alpha, the Civil Service career-average scheme, with member contributions of 4.60% to 8.05% by pensionable earnings. Contributions come off before income tax, which is why a higher pension tier reduces your tax as well as your take-home.
- Salary sacrifice, where entered, is deducted before both tax and National Insurance.
- Income tax is calculated on what remains, using your tax code and the bands for your country. The Personal Allowance tapers by £1 for every £2 of adjusted net income above £100,000, so it is fully withdrawn by £125,140.
- National Insurance is calculated on gross pay after salary sacrifice but before pension contributions under a net-pay arrangement — a common source of error in generic calculators.
- Student loan repayments are applied per plan against the relevant threshold, with postgraduate loans running alongside rather than instead of an undergraduate plan.
Scottish taxpayers are calculated on the Scottish bands and rates throughout. National Insurance is UK-wide and does not change.
Testing and verification
The calculator is checked in three ways. First, a set of fixed reference scenarios per site — an entry-level grade inside the basic-rate band, and a senior grade that crosses the higher-rate threshold and the National Insurance upper earnings limit — are recomputed on every data change and must match a hand-worked figure to within a few pounds.
Second, a scenario above £125,140 is kept specifically to catch the Personal Allowance taper. That calculation is easy to get subtly wrong, and a wrong taper overstates take-home pay for high earners while looking entirely plausible.
Third, the canonical pay data and the copy embedded in the calculator are compared automatically, so an update to one that misses the other is caught rather than shipped.
Known limitations
- Scotland and Northern Ireland run separate prison services with their own pay arrangements and are not covered.
- Detached duty, temporary promotion and site-specific allowances are locally applied and not modelled.
- Private-sector prisons set their own pay and are outside the PSPRB framework.
- Figures assume a full tax year on the same pay point. Mid-year promotions, acting-up periods and pay-award back pay are not applied automatically — a month containing back pay will not match the monthly average.
- Emergency and non-cumulative tax codes are not modelled. The calculator assumes a cumulative code, which is what most people are on but not everyone.
- Local variations agreed at employer level are not reflected where they are not published nationally.
- Benefits in kind beyond the salary sacrifice options provided are not modelled, and the site does not give pension transfer, annual allowance or retirement-planning advice.
Current pay year: The site uses the PSPRB pay tables in force for 2026/27, including the seven contract variants and the London location additions.
Spotted something we have got wrong? Email contact@prisontakehome.co.uk. See also the about page and our terms of use.