Need help building a Pounds to Dollars calculator app

General
1

I’m trying to build a simple Pounds to Dollars calculator app so I can quickly convert GBP to USD with the latest exchange rates, but I’m running into issues with both the conversion logic and keeping the rates updated. I’m not sure which API, data source, or approach is best for accurate, real-time conversions on mobile. Can anyone walk me through how to structure this app and recommend reliable tools or example code so I can get it working correctly?

2

First fix the conversion logic, then worry about live rates.

  1. Basic formula

If 1 GBP = 1.27 USD, then:

usd = gbp * rate

Example in JavaScript:

const gbpInput = document.querySelector(‘#gbp’);
const usdOutput = document.querySelector(‘#usd’);

function convert(gbp, rate) {
return gbp * rate;
}

gbpInput.addEventListener(‘input’, async () => {
const gbp = parseFloat(gbpInput.value) || 0;
const rate = await getRate();
usdOutput.value = convert(gbp, rate).toFixed(2);
});

  1. Getting live rates

Pick one free API to start. A few common ones:

OpenExchangeRates
ExchangeRate.host
Frankfurter.app

Simple example with ExchangeRate.host:

async function getRate() {
const res = await fetch(‘https://api.exchangerate.host/latest?base=GBP&symbols=USD’);
const data = await res.json();
return data.rates.USD;
}

  1. Cache the rate

Do not hit the API on every keystroke. Cache it for some minutes.

let cachedRate = null;
let lastFetch = 0;
const CACHE_MS = 5 * 60 * 1000;

async function getRate() {
const now = Date.now();
if (cachedRate && now - lastFetch < CACHE_MS) {
return cachedRate;
}
const res = await fetch(‘https://api.exchangerate.host/latest?base=GBP&symbols=USD’);
const data = await res.json();
cachedRate = data.rates.USD;
lastFetch = now;
return cachedRate;
}

  1. Handle API errors

You do not want your app to die when the API fails.

async function getRateSafe() {
try {
return await getRate();
} catch (e) {
// fallback to a default or last known rate
return cachedRate || 1.25;
}
}

Then in your converter, call getRateSafe instead.

  1. Simple HTML mockup

Key problems you likely hit:

• Using wrong direction of rate. Some APIs give USD base, so you need usd = gbp / rate.
• Calling the API on every input event.
• Not handling parseFloat and NaN.

Start with a hardcoded rate, verify the math, then plug in the API and caching. That keeps the bugs isolated and easier to track.

3

You’re on the right track, and @cacadordeestrelas already nailed the “fetch a rate and multiply” angle, so I’ll skip re-explaining that same path.

Couple of extra things you might be tripping over:

  1. Direction of conversion with different APIs

Not all APIs want base=GBP. Some only give you USD as base, like:

{
  'base': 'USD',
  'rates': {
    'GBP': 0.79
  }
}

If that’s the case, your logic is:

// rate is how many GBP for 1 USD
// you want USD, so:
usd = gbp / rate;  // note the division, not multiplication

So:

  • If the API tells you “1 GBP = X USD”, use gbp * rate.
  • If it tells you “1 USD = Y GBP”, use gbp / rate.

Print base and rates to console once and verify which you’re getting instead of guessing.

  1. Decouple UI from rate fetching

Instead of grabbing the rate inside the input handler like @cacadordeestrelas did, I’d separate concerns:

let currentRate = null;

async function refreshRate() {
  const rate = await getRateSafe();  // your API + caching here
  currentRate = rate;
  convertAndRender();
}

function convertAndRender() {
  const gbp = parseFloat(gbpInput.value) || 0;
  if (!currentRate) {
    usdOutput.value = '…';
    return;
  }
  usdOutput.value = (gbp * currentRate).toFixed(2);
}

gbpInput.addEventListener('input', convertAndRender);

This way:

  • Typing is instant.
  • Rate updates happen on a timer or button (like “Refresh rate”) instead of every keystroke.
  • Your conversion logic is one tiny, testable function.
  1. Update strategy

Instead of only time-based caching, I like a combo:

  • Fetch on:
    • App load
    • Manual “Refresh rate” button
    • Optional: every N minutes in the background

Example:

setInterval(refreshRate, 10 * 60 * 1000); // every 10 minutes

User can still hit a “Refresh” button if they’re paranoid and want the very latest.

  1. Show the user what rate you’re using

You avoid a ton of “is this wrong?” confusion by showing:

  • Current rate (e.g. “1 GBP = 1.27 USD”)
  • When it was last updated
rateLabel.textContent = `1 GBP = ${currentRate.toFixed(4)} USD`;
lastUpdatedLabel.textContent = new Date().toLocaleTimeString();

You’ll also instantly notice if you accidentally reversed the rate, because it will look insane like “1 GBP = 0.0008 USD”.

  1. Quick sanity test

Before you trust the app:

  • Pick a random GBP value, like 100.
  • Check your app’s result.
  • Compare with a site like XE or Google’s converter.
  • If you’re off by a factor of ~100 or more, you’ve probably inverted the rate.

Ironically, most of the “logic issues” end up being the direction of the rate plus some sloppy UI coupling. Get a tiny “pure” function working first:

function gbpToUsd(amountGbp, rateGbpToUsd) {
  return amountGbp * rateGbpToUsd;
}

Test just that in the console, then plug in whatever fancy rate fetching you want.

4

Skip the wiring for a second and think about how your Pounds to Dollars calculator app should behave as a “little FX tool” rather than just “fetch JSON, multiply.”

I’ll disagree slightly with @cacadordeestrelas on one thing: you don’t always want to chase “latest” every time. For most people, a stable rate that is a few minutes or even a few hours old is fine, as long as you are honest about it.

1. Treat it like a mini FX snapshot, not a trading terminal

Instead of obsessing over constant updates:

  • Decide what “fresh enough” means. For a simple converter, every 15 or 30 minutes is usually plenty.
  • Store the last rate and timestamp in localStorage (or AsyncStorage / on-disk cache in native apps).
  • On app start:
    • Load cached rate.
    • If it’s younger than your freshness window, use it.
    • In the background, pull a new rate and swap it in silently.

This gives instant results even offline and still keeps the app “good enough” for daily use.

2. Guardrails on the rate itself

To catch weird API or logic issues beyond the base/rate direction that @cacadordeestrelas and the other answer covered:

  • Put sanity bounds on GBP → USD:
    • If the rate is below, say, 0.5 or above 2.5, treat it as invalid and show an error like “Rate looks wrong, try again.”
  • This also helps if the API suddenly returns 0 or null or a string.

Logic-wise:

function safeGbpToUsd(gbp, rate) {
  if (typeof rate !== 'number' || !Number.isFinite(rate)) return null;
  if (rate < 0.5 || rate > 2.5) return null;
  return gbp * rate;
}

Then your UI can check for null and show a warning instead of silently doing nonsense.

3. Handle user input like a human would

A lot of “logic issues” in these tiny apps turn out to be formatting problems, not math:

  • Accept commas: '1,000.50' should work.
  • Prevent weird states like '.', '-', '1..2'.
  • Immediately format output to a fixed number of decimals (e.g. 2 or 4) so people can eyeball it quickly.

Example pattern:

function parseGbp(value) {
  const cleaned = value.replace(/,/g, ');
  const amount = Number(cleaned);
  return Number.isFinite(amount) ? amount : 0;
}

Then your convert function always deals with a clean number, not raw user text.

4. Be explicit about what the app actually is

If you ever plan to ship this, treat your Pounds to Dollars calculator app more like a “reference tool” than a trading engine:

  • Show a short note: “Rates are approximate and may differ from your bank.”
  • Display the data provider name: “Source: ECB / some-API-name”.
  • Prefer free reliable sources that update daily or hourly, not weird 5-second feeds that will get you rate limited quickly.

5. UI ideas that save debugging time

Two tiny touches reduce your own confusion:

  • Show both GBP → USD and USD → GBP rates just under the main input:
    • “1 GBP = 1.27 USD”
    • “1 USD = 0.79 GBP”
  • If those numbers do not look like inverses of each other, your logic is probably flipped or the API is giving you something unusual.

You can derive the inverse yourself:

const rateGbpToUsd = currentRate;
const rateUsdToGbp = 1 / currentRate;

If 1 / rate does not match what the API claims for the reverse, you know the provider is doing something funky (like mid-market vs. bank sell/buy spreads).

6. About the “product” angle: pros & cons

If you package this as a simple “Pounds to Dollars calculator app” product:

Pros:

  • Incredibly fast: one input, one output, no clutter.
  • Perfect for travel planning or quick budget checks.
  • Can work with cached rates so it is still usable on flaky connections.
  • Easy to maintain since it only tracks one currency pair.

Cons:

  • Very narrow scope: if users suddenly want EUR or JPY, you have to extend it.
  • Depends completely on whatever FX API you choose.
  • If you do not expose timestamp and source, people will mistrust the numbers.
  • Needs good handling for offline / stale data to avoid misleading conversions.

Compared with what @cacadordeestrelas suggested, I would lean harder on caching, sanity checks, and UX transparency instead of just “always get the latest and convert.” Both approaches are valid; yours should depend on whether this is a fun personal tool, or something you expect other people to rely on regularly.

If you share your current API shape (just the JSON structure, no keys), it is very easy to write a tiny adapter layer so the rest of your app never has to care which provider you use.