Interest on overdue invoices: clause first, maths second

Interest applies when the contract or the law allows it. The calculation is simple interest on the outstanding amount for the days late — but the rate comes from the agreement.

Try it on your own invoice

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Where the rate comes from

The contract: a fixed percentage, or a reference rate plus a margin.

If silent, statutory rules may apply — that is a legal question, not a formula.

The formula

Interest = amount × annual rate × days late ÷ 365.

On €2,400 at 8% for 30 days: 2,400 × 0.08 × 30 ÷ 365 ≈ €15.79.

When it helps and when it hurts

Helps: a chronic late payer changes behaviour when interest appears on the statement.

Hurts: adding interest to a client who is late because of your own invoice error turns a small delay into a dispute.

How to state it

One line, once: the clause, the rate, the days, the amount.

Then continue asking for the principal — that is what you actually want.

Frequently asked

Can I add interest without a clause?

Sometimes statutory interest applies, but relying on it without advice is risky.

Should I actually collect it?

Often the leverage is worth more than the amount. Waiving it to get paid today is a legitimate trade.