Why we built this.
Most UK tax calculators give you a single, static number and leave you to figure out what it means. Serenity Tax exists because that number is often the least useful part.
Serenity Tax is a calculator for UK sole traders, company directors, and employees who want to understand their income tax, National Insurance, student loan repayments, and Self Assessment payments on account — without wading through jargon or guessing what happens next.
The two things other calculators get wrong
Lumpy income within a tax year.If you take a large one-off dividend or bonus partway through the year, your tax should be worked out on your cumulative position — not a flat annual average. That's what actually captures being pushed into a higher rate band.
The multi-year payments-on-account chain. What HMRC asks you to pay on account this year is based on lastyear's actual liability, not this year's real income — so a good year now doesn't hit your cash flow until the balancing payment around 18 months later. Most calculators show you today's bill and stop there. Serenity Tax projects the whole timeline forward, so the surprise never arrives as a surprise.
What this deliberately doesn't cover
We'd rather be upfront about scope than have you rely on a number that quietly leaves something out. Serenity Tax currently doesn't model:
- Personal Allowance tapering for total income above £100,000
- Capital Gains Tax
- Marriage Allowance, Blind Person's Allowance, and other less common reliefs
- Class 2 National Insurance (voluntary since April 2024)
Everything Serenity Tax does calculate is covered by an automated test suite checked against real HMRC figures, and the rates it uses are reviewed each tax year. It's built to be a clear, honest estimate for planning — not a replacement for an accountant or HMRC's own tools, and not tax advice.