Rounding on invoices: why a total can be a cent out

Ruth runs a small stationery supply business in Leeds. A retailer's accounts team writes back: “Your invoice says £7.17, our system says £7.16. Which is right?” Both are. Ruth's software worked out the VAT on each line and rounded each one. Their system worked it out once on the total. Nobody made a mistake. They rounded at different points.

Why it happens

Money is written to two decimal places, but tax and quantities produce more. 20% of £1.99 is £0.398. That has to become £0.40 at some point, and where you round changes the answer by a cent or a penny.

Per line or on the total

There are two common methods for tax.

• Per line. Work out the tax on each line, round it, then add the lines. Each line can be read on its own, which suits invoices with different tax rates and credit notes that take back one line.
• On the total. Add the net amounts for each rate, work out the tax once, round once. Fewer roundings, so a result closer to the exact figure.

Many tax offices accept either, as long as you use one method consistently, and some publish detailed rules. In the UK, HMRC sets out how VAT may be rounded on invoices in its VAT guidance. Check your own tax office's rules if your figures must match exactly, for example for large invoices with many lines.

Quantities with decimals

Hours, kilos and metres often come with decimals, and a decimal quantity times a price can land on half a cent. 2.5 hours at €33.33 is €83.325. Most software rounds half a cent up, to €83.33. Some systems round half to the nearest even cent, giving €83.32. If the gap bothers a customer, a rate that divides neatly, like €33.40, removes it.

Prices that include tax

When a price already includes tax, the net and the tax are worked out of it, and each part is rounded. €10.00 including 21% VAT is €8.2645 net and €1.7355 tax. Rounded, that is €8.26 and €1.74, which adds back to €10.00. But if the tax is then worked out again as 21% of €8.26, it comes to €1.73, and the line adds up to €9.99. The usual fix is to keep the price the customer saw and let the tax absorb the cent. See prices with or without tax.

Rounding the total to a whole unit

In some places it is common to round the amount to pay to a whole unit, so nobody pays ₹1,178.82. The invoice shows the exact total, a “Round off” line of +₹0.18, and ₹1,179.00 to pay. This is common practice in India. The tax itself stays exact; only the amount to pay is rounded. Cash rounding is also used in countries that have withdrawn their smallest coins, such as Australia, Canada and New Zealand, though there it is usually done at the till rather than on an invoice.

Currencies without cents

Not every currency has two decimals. The Japanese yen and the Korean won are written in whole units, so every line and every tax figure is rounded to a whole yen or won. Treat such an invoice in another currency the same way: the amount to pay should be one the customer can actually send.

Conversion between currencies

Converting an invoice into your own currency for your records adds another rounding, and the rate itself is rounded too. Record the rate you used and the converted total once, on the invoice date, and do not convert line by line. See also billing clients abroad.

When a customer pays a cent less

If the payment is a cent short because their system rounded differently, do not chase it. Record what arrived and settle the rest with a small credit note, or note it however your accountant prefers. Chasing a penny costs more than the penny, and it costs some goodwill. For larger gaps, see why a client paid less.

The short version

• A cent or penny difference usually means two systems rounded at different points. Neither is wrong.
• Tax can be rounded per line or on the total. Pick one and keep to it.
• Decimal quantities and tax-inclusive prices create half cents. Round them the same way every time.
• Rounding the amount to pay to a whole unit is shown as a separate Round off line.
• Do not chase a payment that is a cent short. Record it and close the invoice.