How Our Calculators Work

This page explains where MoneyZio's figures come from, how they're kept current, and what each calculator does and doesn't account for.

One central tax configuration

Every MoneyZio calculator — Salary, Self Assessment Tax Estimate, Employer Tools, Payslip Generator, Salary Comparison, Hourly Wage — reads its Income Tax, National Insurance, and student loan figures from a single shared configuration for the current tax year, rather than each tool keeping its own separate copy. This is meant to stop different calculators quietly drifting out of sync with each other as rates change year to year.

Sources

Tax and National Insurance figures are sourced from HMRC and GOV.UK's published rates, primarily:

• gov.uk/guidance/rates-and-thresholds-for-employers
• gov.uk/income-tax-rates
• gov.uk/national-insurance/how-much-you-pay
• gov.uk/self-assessment-tax-returns/deadlines

Where a figure needed independent cross-checking (for example, Class 2/Class 4 National Insurance history), we've cross-referenced multiple independent accountancy sources alongside the underlying legislation, but GOV.UK and HMRC remain the primary reference.

Tax year and review policy

Each tax-year configuration carries a "last reviewed" date and a status (confirmed or draft). MoneyZio updates its calculators each tax year using official HMRC and government sources, and figures are reviewed again if HMRC publishes a mid-year change. Results are always estimates based on the information you enter — we don't promise perfect or real-time accuracy, and we don't have access to your actual HMRC record. See What's Changed for the current tax year for the specific figures currently in use.

What the calculators assume

• Salary Calculator — standard tax code by default (or the exact code you enter), 5% pension auto-enrolment unless you turn it off, no benefits-in-kind or salary sacrifice.
• Self Assessment Tax Estimate — Income Tax and Class 4/Class 2 National Insurance on the profit figure you enter; does not account for Payments on Account, the trading allowance, capital allowances, or other Self Assessment adjustments.
• Employer Tools — standard Class 1 employer National Insurance and Employment Allowance eligibility; doesn't verify your business actually qualifies for the Allowance, and doesn't include employer pension contributions or other employment costs.
• Payslip Generator — an illustrative document only, not a real payroll simulation; its "year to date" figures assume identical pay every month, which is rarely how a real payslip's YTD actually behaves.
• Mortgage Calculator and Budget Planner — general illustrative estimates based on common income-multiple and budgeting conventions, not a lending decision or personalised financial plan.

Limitations

None of MoneyZio's calculators can see your actual tax code, payslip, or HMRC record. They can only work with the numbers you type in. Your real position may differ — check your payslip, your HMRC personal tax account, or a qualified adviser for anything that matters financially.