Authentication

Create a key in your account. Send it only from your server in the Authorization header. Every key can be revoked immediately.

Authorization: Bearer YOUR_REKENBOX_API_KEY
Content-Type: application/json

Run a calculation

POST https://rekenbox.winstwaker.nl/api/v1/calculate

{
  "tool": "btw",
  "input": { "amount": "100", "rate": "21", "mode": "add" },
  "options": { "year": 2026, "version": "2026.1", "locale": "nl" }
}

All input fields are strings. Input money amounts are euros; result fields of type money are whole cents as strings. The response contains result and usage.

Tools and field definitions

GET https://rekenbox.winstwaker.nl/api/v1/tools

This returns supported tool IDs, fields, selection values, limits, years, versions, sources and assumptions. Unknown years or versions are rejected; Rekenbox does not silently substitute them.

Bulk

POST https://rekenbox.winstwaker.nl/api/v1/bulk

{ "calculations": [
  { "tool": "btw", "input": { "amount": "100" } },
  { "tool": "iban", "input": { "iban": "NL91ABNA0417164300" } }
] }

At most ten calculations per request and 10 MB of source files. Each entry has ok and result or error. Failed entries do not count towards quota. Processing stops when the time limit is reached; unprocessed entries report bulk_time_limit.

Quota and errors

Quota are shared by all keys and bulk checks for the billing account per UTC calendar month. At most 120 requests per key per minute. No automatic overage charges. Supply source files in input.file as XML or CSV text; they are not retained after processing. Keep the result version with your application.