Why an invoice’s tax total is off by a cent

A one-cent difference in an invoice’s tax usually comes from rounding at different stages. Rounding the tax on each line and adding the results gives a slightly different figure from calculating tax once on the subtotal. Both methods are legitimate; the problem is mixing them. Choose one rule and use it everywhere.

A small example

Three items at 0.99 each, with 20% tax:

• Tax per line: 20% of 0.99 is 0.198, which rounds to 0.20. Three lines: 0.60.
• Tax on the total: the subtotal is 2.97; 20% of 2.97 is 0.594, which rounds to 0.59.

One cent apart, and neither calculation is wrong. Each rounding moves a figure by up to half a cent, and per-line rounding does that once per line. On long invoices with many cheap items, the gap can reach several cents.

Other places cents go missing

• Unit prices with more than two decimals. 0.3333 × 3 is 0.9999, which an invoice shows as 1.00 — but if the line is rounded before multiplying, 0.33 × 3 is 0.99.
• Tax-inclusive prices. Removing 20% from 9.99 gives 8.325 net, which must round to 8.33 or 8.32, leaving tax of 1.66 or 1.67. The Tax Calculator shows the split from either direction.
• Discounts. A percentage discount rounded per line differs from one rounded on the subtotal, for the same reason as tax.
• Spreadsheets. A cell formatted to two decimals still holds the full value. The sum of displayed values can differ from the displayed sum. Wrap calculations in ROUND() if what you see must add up.
• Currency conversion. Converting each line and then totalling rarely matches converting the total.

Which rule should you use?

Many tax authorities let businesses round per line or per invoice, provided they are consistent, and some set specific rounding rules. The rules depend on where you are registered, so check your tax authority’s guidance or ask your accountant. In practice:

• Per invoice (tax on the subtotal) is simple and gives the result closest to the true figure.
• Per line suits invoices where items carry different tax rates, or where each line must be complete on its own.

Whatever you choose, make the invoice show the method: list the tax on the subtotal, or show tax on each line and total those.

How this site’s invoice generator does it

The Invoice Generator works in whole cents to avoid floating-point drift. Each line is quantity × unit price, rounded to the cent. The lines are added, any discount is applied to that subtotal, and tax is calculated once on the discounted subtotal, so the totals always add up on the page. If your accounting software rounds per line, expect an occasional one-cent difference and record the invoice as issued.

Rounding half-up, half-even and truncating

Even “round to the nearest cent” has variants. Half-up turns 0.125 into 0.13. Half-even, or banker’s rounding, turns it into 0.12 and 0.135 into 0.14, rounding exact halves to the even digit. Some systems simply truncate. Two programs rounding the same tax “correctly” can therefore differ by a cent on exact halves. If your invoices and your accounts disagree only on lines ending in a half cent, this is why.

When a client’s payment is a cent short

• Compare their figure with yours line by line; it is usually rounding, not a dispute.
• Many businesses write off sub-unit differences rather than chase them. Decide a policy.
• Record the payment against the invoice and note the difference, so the books reconcile.

Mistakes to avoid

• Mixing per-line and per-invoice rounding in the same invoice.
• Rounding unit prices before multiplying by large quantities.
• Editing a total by hand so it matches, which breaks the arithmetic on the page.

Checklist

• One rounding rule, used in the invoice, spreadsheet and accounts?
• Tax shown per line or on the subtotal — clearly?
• Local tax rounding guidance checked?