Design Philosophy

The 3-Way Calculator is built on four principles. Together they define what the app is — and what it is not.

1. Currency-agnostic

The calculator does not assume any currency. The user can think in USD, EUR, GBP, JPY, SAR, or any other currency they work with.

The Decimal Places setting in Settings lets you choose between 0, 0.0, 0.00, and 0.000 decimal places. Whether you need whole numbers, tenths, cents, or millimes, the calculator adapts. Support for 5 and 8 decimal places (for cryptocurrencies) is planned.

This keeps the interface flexible while letting every user work in their preferred precision.

2. Universal labels

The three display fields are labelled Before, After, and Amount, with a Direction toggle (+ / −). These terms fit any use case:

Label	Meaning
Before	The starting amount
After	The resulting amount
Amount	The difference
Direction	Addition or subtraction

A VAT filer reads them as net, gross, and tax (additive). A Zakat payer reads them as wealth, remaining, and Zakat due (subtractive). A discount shopper reads them as original, sale price, and savings (subtractive).

The same labels, infinite interpretations. There are no industry-specific or use-case-specific label presets — and no plan to add them.

3. User-set rate

The flat rate defaults to 15%, but the app does not ship with pre-configured rates for any specific country, industry, or tax type.

This is deliberate:

• Tax rates change. A hardcoded rate becomes wrong the moment a government adjusts it — and the user blames the app, not the government.
• There are thousands of rate × jurisdiction combinations. Covering them all is impractical. Covering a subset creates an uneven experience, and maintaining an exhaustive list is a burden neither practical nor desirable for a focused calculator.
• Every user already knows what rate applies to their situation. Asking them to type it once is trivial. Maintaining a rate database for them forever is not.

The calculator accepts any rate from 0% up to the direction-dependent maximum (1000% in additive mode, 99.99% in subtractive mode). The rate resets to 15% on each page load — this ensures you always start from a clean baseline.

4. No presets

The app does not have modes, profiles, or presets for “VAT mode”, “Zakat mode”, “discount mode”, or any other named use case.

A preset system would need to encode every use case — and every country’s variant of every use case — into the app. That creates:

• A maintenance burden — every rate change, every new tax law, every new country requires a code change and a release.
• False certainty — a preset labelled “VAT” might be wrong for a specific transaction in a specific jurisdiction, but the user trusts it anyway.
• Scope creep — once presets exist, users naturally ask for more. The app would need to track every rate change and jurisdiction, pulling it away from its core purpose: simple, fast percentage calculations.

Instead, the calculator stays focused on one job: given a flat rate and one value, compute the other two. The user decides what the rate is, what the values mean, and which direction the calculation goes.

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What this means for you

• You do not need to tell the app what you are calculating.
• You do not need to select a country, industry, or use case.
• You type the rate you know, enter the value you have, and read the result.

The app meets you where you are. It does not guess, and it does not get in the way.