Are my financial numbers uploaded to a server?

No. CalcTools is 100% client-side. Every calculation — mortgage, loan, compound interest, tax — runs in your browser as plain JavaScript. There is no backend and no /api route. The site's Content-Security-Policy (connect-src 'self') even restricts which hosts the page may contact, so the privacy guarantee is enforced by the browser itself.

Are the calculations accurate?

Yes — each calculator uses the standard, published formula for its problem (the loan PMT formula, compound-interest future value, marginal tax bands, the HKMA stress-test rule, and so on). The verify suite hand-checks reference values against the implemented math before every build. Still, these are decision aids, not financial advice; confirm with a professional for a binding decision.

How is the currency converter's exchange rate sourced?

The converter ships with a build-time snapshot of exchange rates (taken from a public, no-key exchange-rate API at build time) bundled same-origin — it never makes a runtime request to any server in this MVP build. So currency conversion works offline and exposes no browsing data to a third party.

Do the Hong Kong calculators reflect the current year?

They use the published 2024/25 HK rates: Salaries Tax progressive bands (2%/6%/10%/14%/17%) and 15% standard rate with a $132,000 basic allowance; property Stamp Duty on the smoothed ad valorem band scale; MPF at 5% employee + 5% employer capped at the $30,000 monthly relevant-income limit. Always confirm the current figures on gov.hk before relying on a number for a real filing or purchase.

Does it work offline?

Yes. After the first load the app shell is cached, so every calculator works with no internet connection. Install CalcTools as a PWA and it opens and runs offline.

How do I calculate what percent X is of Y?

Divide the part by the whole and multiply by 100: (X ÷ Y) × 100. For example, 15 is what percent of 60: (15 ÷ 60) × 100 = 25%. The first mode on this page does exactly that, live as you type.

How do I calculate X% of a number?

Multiply the number by the percentage as a decimal: X% of Y = (X ÷ 100) × Y. For example, 15% of 60 = 0.15 × 60 = 9. Use the second mode.

How do I calculate percentage increase or decrease?

Subtract the starting number from the new number, divide by the STARTING number, and multiply by 100: ((new − old) ÷ old) × 100. Positive is an increase, negative is a decrease. So 80 → 100 is a 25% increase, but 100 → 80 is a 20% decrease — the change is measured relative to where you started.

Why is percentage change from 0 undefined?

Percentage change divides by the starting number, and division by zero has no defined value. A rise from 0 to 50 is an infinite percentage increase, so the calculator asks for a non-zero starting number instead of showing a wrong answer.

Are my numbers uploaded anywhere?

No. This percentage calculator runs entirely in your browser as plain JavaScript — no backend, no account, no tracking of your inputs.

General

Percentage Calculator

Three percentage modes: X is what % of Y, X% of Y, and the % change between two numbers — live as you type.

Percentage Calculator

Pick the question you're asking — the answer updates live as you type.

X (the part)Y (the whole)

15 is what % of 60 → 25%

Your numbers never leave your device — all math runs in your browser.

Result

X is this % of Y

25%

“% of” multiplies (15% of 60 = 0.15 × 60). “% change” divides the difference by the STARTING number — that's why 80 → 100 is +25% but 100 → 80 is −20%.

How to use this calculator

Pick the mode that matches your question: 'X is what % of Y', 'X% of Y', or '% change from X to Y'.
Type your two numbers.
Read the live answer — it updates as you type.

Bookmark Percentage Calculator — it works offline once loaded. Figures are decision aids, not financial advice.