How late fees on invoices work
A late fee is a charge you add when a client pays after the agreed due date. It has two jobs: compensating you for the wait, and giving the client a reason to pay on time. Most freelancers state their payment terms on every invoice — "Net 30" means payment is due 30 days after the invoice date, "Net 15" means 15 days — and a late fee only makes sense once those terms are in writing and the client has accepted them.
Put the fee in writing before work starts
A late fee you invent after an invoice goes overdue is very hard to enforce. The reliable approach is a late-payment clause in your contract or proposal: the due date, the grace period if any, and exactly how the fee is calculated — a percentage per month, a fixed amount, or both. When the client signs, they have agreed to the terms, and the calculator above simply applies the numbers you both accepted.
Percentage or fixed amount?
A monthly percentage scales with the invoice size, so it stays fair on both small and large projects — many freelancers use something in the low single digits per month, applied pro-rata for each overdue day. A fixed amount per month is simpler to explain and works well for smaller invoices, but can look steep on a tiny bill. Some jurisdictions also set statutory interest rates for commercial debts that apply by default; check the rules where you and your client are based before choosing your numbers.
Chasing payment without burning the relationship
Most late payments are disorganisation, not malice. A polite reminder shortly after the due date resolves the majority of cases — mention the invoice number, the amount, the due date and the late-fee clause calmly. Escalate gradually: a second reminder, a phone call, then a formal notice stating the fee now applies. Keep every message factual and in writing. For a reliable long-term client with one slip, waiving the fee once can be a sensible goodwill gesture; for a repeat late payer, enforcing it protects your cash flow.