Invoice Tools
Small utilities that handle the fiddly parts of invoicing — numbering, due dates, tax arithmetic and writing amounts in words. Every calculation runs locally in your browser.
Invoice Number Generator
Create a consistent, sequential invoice number using a prefix, the current year and zero-padded digits.
Due Date Calculator
Work out the payment due date from an issue date and a credit period in days.
Tax & Total Calculator
Apply a percentage discount and a tax rate to a subtotal and see every figure broken out.
Amount in Words
Convert a numeric amount into words for the "amount in words" line some invoices require.
Why Consistent Invoice Numbering Matters
Invoice numbers look trivial until an accountant asks you to reconcile a payment. A numbering scheme that is unique, sequential and predictable makes it obvious when something is missing, and it lets both you and your client refer to an invoice unambiguously. The most common pattern is a short prefix, the year, and a zero-padded counter — INV-2026-0042 — which sorts correctly in spreadsheets and stays readable when spoken over the phone.
Due dates matter for the same reason. "Net 30" means thirty days from the issue date, not thirty days from when the client received it, and the difference matters when you are chasing a late payment. Calculating the date up front and printing it on the invoice removes any ambiguity later.
Tax arithmetic is the third place small mistakes creep in. Discounts should be applied before tax, not after, and the tax should be calculated on the discounted amount. Use the calculators on this page to check your figures, then build the full invoice with the invoice generator.