Submit money without floating-point surprises

Problem

An amount changes by a cent because browser or server code treats currency as a binary floating-point number.

Before you start

Choose the currency, accepted decimal precision, rounding rule, minimum, and maximum on the server. Never infer currency only from the user’s locale.

Submit the entered decimal text

<form ng-post="/api/invoices" on-error="errors = $res">
  <label>
    Amount in EUR
    <input
      name="amount"
      inputmode="decimal"
      autocomplete="off"
      required
    />
  </label>
  <input name="currency" type="hidden" value="EUR" />
  <p ng-if="errors.amount">{{ errors.amount }}</p>
  <button type="submit">Create invoice</button>
</form>

Server contract

Parse the amount with a decimal or money type, validate its scale, and store minor units or an exact decimal together with the currency. Return formatted display text separately from the canonical value when needed.

Failure path

Reject ambiguous separators and unsupported precision with 422. Do not silently round an amount when that changes what the user authorized.

Apply it now

Trace one amount from input through JSON, application code, database, and output. Remove every binary floating-point conversion from that path.

Verify

Test 0.01, large values, too many decimal places, negative values, comma input, and a currency with a different minor-unit rule.