About Our Calculators

They run in your browser, not on our server

Every Pennywise calculator is a self-contained JavaScript application running on your device. The numbers you type into the inputs never leave your browser. We don't see them, we don't store them, and we don't pass them to any third party.

This is a deliberate choice. UK personal finance calculations sometimes need numbers you'd reasonably want to keep private — your income, savings balance, mortgage size. There is no engineering reason for those numbers to touch a server, so they don't.

Privacy in practice

• Inputs and results live only in the browser tab
• Refreshing the page clears them
• Closing the tab clears them
• No cookies, local storage or backend logging captures calculator inputs
• Site analytics count page visits, not interactions with calculator inputs

The calculators work fine in private/incognito mode, and they work fine if you reject cookies on the site.

Sources of the maths

Each calculator uses standard published formulas:

• Compound interest, savings, ISA growth — geometric compounding adjusted from AER to the relevant periodic rate, matching UK convention.
• Mortgage and loan amortisation — standard annuity payment formula with the periodic rate derived from the published APR or annual rate.
• ISA allowance and bonuses — current-tax-year rules as published by HMRC (£20,000 main allowance and £4,000 LISA sub-allowance for 2025/26).
• Tax bands and allowances — current-tax-year figures from gov.uk.
• Credit score factors — illustrative weightings based on the published guidance of the three UK credit reference agencies (Experian, Equifax, TransUnion).

Assumptions and limitations

Every calculator is an illustration. Real-world outcomes vary because:

• Interest rates change. We model a constant rate; real accounts have variable rates.
• Tax rules change. We use the current tax year; future years may differ.
• Provider mechanics vary. Different lenders calculate interest, charges and timing slightly differently.
• Life happens. Withdrawals, missed payments, job changes and product switches all affect outcomes.

Use calculator results as estimates. For decisions involving large sums or complex tax situations, confirm with the provider and consider talking to an FCA-authorised adviser.

Testing

Before publication, every calculator is checked against:

• A known-correct manual calculation for a representative input set
• Comparable outputs from the equivalent tool at gov.uk, the MoneyHelper service, or a major UK provider (whichever applies)
• Edge cases including £0 inputs, zero interest rates, and very long/short time horizons

The calculator's underlying JavaScript is readable. You can verify the maths yourself via your browser's "view source" if you want.

When they're updated

Calculators are updated at the start of every UK tax year (6 April) to reflect new allowances and rate changes. Mid-year updates happen if HMRC, the FCA or the Bank of England announce a change that materially affects the calculation. The "Last reviewed" date on the corresponding article reflects the most recent check.

Disclaimer

Pennywise Finance is not authorised by the Financial Conduct Authority. Calculator outputs are general information, not personalised financial advice. For advice tailored to your situation, consult an FCA-authorised adviser or the free, government-backed MoneyHelper service.