An invoice due date calculator does one small thing that a surprising number of invoices get wrong: it adds the agreed terms to the issue date and writes the resulting date on the document, so the reminder, the aging and the customer all work from the same day. An invoice calculator does the rest of the arithmetic on the page, the lines, the discount, the tax on the goods, the shipping and the total, and after a part payment, the balance still due. This page is about both calculations as a seller on account needs them, and where the free proforma, statement and tracker on this site do them.
The due date from the terms
Net 30 means thirty days from the invoice date; end of month plus 30 means the last day of the invoice month and then thirty days; a discount for early payment means a second date, the last day the discount applies. Write the terms on the customer's account once and the due date on every invoice follows from the issue date; type it by hand and the aging drifts. The tracker on this site reads days overdue from the due date the invoice carries, so the date has to be right on the invoice.
Totalling the invoice
Quantity times unit price per line, less any line or invoice discount, plus the tax on the taxable goods, plus shipping, gives the total. The order matters: discount before tax, shipping after or before tax according to the state, which is why the calculator asks rather than assumes. The free proforma on this site totals a priced offer in that order and the invoice at shipment carries the same total.
The balance after a part payment
When a payment arrives that does not cover the invoice, the open amount is the total less the payment, and it keeps its original due date; a part payment does not restart the clock. The statement shows the invoice with its open amount; the tracker ages it from the original due date. Applying the payment against the named invoice is what makes both figures right.
Days from a set of invoices
Across the account the calculation the owner wants is days sales outstanding: the open balance divided by the period's credit sales, times the days in the period. The free invoice tracker on this site works it from the seller's own invoices and payments, alongside the outstanding and overdue totals.
Questions people ask about invoice due date calculator
How is an invoice due date calculated?
Add the customer's terms to the invoice date: net 30 is thirty days on; end of month plus 30 is the last day of the invoice month and thirty days. Record the terms on the account and the date follows.
Does a part payment change the due date?
No. The open amount falls and the due date stays, so the aging runs from the original date.
What does an invoice calculator total?
Lines, discount, tax on the taxable goods and shipping into the total, and after payments, the balance due. Discount before tax; shipping according to the state's rule.
Is there a free invoice calculator here?
The proforma invoice and the billing statement on this site do the totals with no account, and the invoice tracker works the outstanding, overdue and days figures.