About Our Calculators — Pennywise Finance

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 UK tax-year rules as published by HMRC.
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 & quality assurance

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
A shared library (/js/calculator-core.js) that provides a numerically-safe annuity helper, so a zero interest rate correctly returns n rather than silently dropping contributions

The underlying JavaScript is readable. You can verify any calculator via your browser's "view source". If you spot a bug or disagree with a formula, tell us — we fix calculator issues within one working day where possible.

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.

The full Pennywise calculator estate

Thirty-five tools, grouped by what they help you decide. The Pennywise Money Plan is the central product — it takes a snapshot of your whole financial picture and routes you to the right single-topic tools from there.

Planning

Money Plan — flagship: income, spending, savings, debts, pension, goal, all on one screen

Savings

Savings interest — lump-sum projection
Compound interest — long-term with monthly compounding
Regular savings — partial-period compounding for UK monthly savers
Savings goal — solves for time to target
Emergency fund — 3-9 month target sizing
Cash ISA — tax-free growth vs taxable comparison

Tax

UK take-home pay — income tax, NI, pension, student loan
Personal Savings Allowance — tax on interest
Savings tax — full HMRC breakdown with PSA, starting rate
Dividend tax — 2026/27 rates (10.75% / 35.75% / 39.35%)

Debt

Loan repayment — fixed personal loan amortisation
Credit card repayment — the minimum-payment trap
Debt snowball — smallest balance first
Debt avalanche — highest APR first (optimal)
APR — derive true cost from monthly repayments
Balance transfer — does the fee pay back

Investing

Investment growth — portfolio projection with fee drag
Investment fee impact — compounded fee cost
ISA growth — tax-free S&S ISA projection

ISA

ISA allowance — tracker across wrapper types
LISA penalty — cost of early Lifetime ISA withdrawal

Mortgage

Mortgage overpayment — interest saved and years off

Pension & retirement

Pension pot projection — SIPP or workplace, to retirement
Pension tax relief — basic / higher / additional
Workplace pension — auto-enrolment model
Retirement income — target pot from desired income

Decision tools (side-by-side modelling)

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.