Mango Labs

30 Sep 2026

An app that adds up the cash in your wallet in one currency

This is an app that adds up travel cash in one currency. Someone types the notes in their wallet, and the app shows the total in the currency they earn, with two exchange rates beside it. It is not a trading screen.

What you would build

The screen is a wallet. They pick the currency they earn and the currency in their pocket, then type how much cash they are holding and what they have already spent. Two rates show up for that pair, then one total in the home currency. A spend keeps the rate from the day they entered it, so yesterday’s lunch does not change when today’s rate moves.

Who it is for

Someone on a trip, or about to leave, who changes one lump of cash and then spends it. They want the wallet total in the currency they earn, not a board of pairs to watch all day.

Which APIs

Two APIs, both in Currency Exchange.

Frankfurter is “Exchange rates, currency conversion and time series.” No key. Use conversion for the wallet total, and the time series only for the dates of this trip.

Currency-api is a “Free Currency Exchange Rates API with 150+ Currencies & No Rate Limits.” No key. Show it as a second quote next to Frankfurter. If the two numbers differ, show both. Do not average them into a made-up middle rate.

The source list does not list a price for Frankfurter. Currency-api’s description says it is free and has no rate limits.

How to build the first version

  1. Let them pick the currency they earn and the currency in the wallet.
  2. Let them type the cash they are holding. No bank login.
  3. Convert that amount with Frankfurter, and show the rate plus the date the response refers to.
  4. Fetch Currency-api for the same pair and label it as the second source.
  5. Add a few lines they type for money spent and what is left. Store that day’s rate with each spend.
  6. Optional and small: Frankfurter’s time series for the trip dates, as a short list of daily rates, not a chart of the major pairs.

How it could make money

Sell a trip pass. They pay once for a date range, and the pass ends when the trip ends. That matches how the cash is actually used. A monthly trading fee does not, and neither does a referral to a currency exchange.

What to leave out

Price alerts, leverage, a board of major pairs, and crypto. The first version is one wallet, two rates, and a total in the home currency.