Can I charge a late fee if my contract never mentioned one?

Usually not enforceably. Late fees come from the agreement, not from being annoyed. Put a rate in every contract and on every invoice footer going forward, it costs nothing and changes payment behavior.

What late fee rate is normal for freelancers?

1–1.5% per month (12–18% annually) is the standard range in the US. Higher rates risk hitting state caps and feel punitive; the point is nudging payment, not earning interest.

Do late fees actually get paid?

Often no, and that's fine. The fee's real job is making your invoice the one that gets paid first next month. Many freelancers waive it once payment lands, in writing, as a goodwill move.

Calculators

Invoice late fee calculator

A late fee isn't about the money, it's about making "I'll pay you eventually" cost something. This computes simple interest by days overdue, the defensible method, plus the per-day figure that makes reminder emails concrete.

Your numbers

Unpaid invoice amount

The overdue amount, excluding any fees.

Annual interest rate

12–18%/year (1–1.5%/month) is the common contractual range in the US. Some states cap it, check yours.

Days past due

Counted from the due date on the invoice, not the send date.

Flat late fee (if contracted)

A one-time fee your contract sets. Zero if none.

What the ledger says

Total now due$5,049.32

Interest accrued: $49.32
Accruing per additional day: $1.64
Total fees: $49.32
Implied monthly rate: 1%

Estimate, not advice

The formula

Interest = amount × (annual rate ÷ 365) × days late, plus the flat fee. Simple interest, no compounding, the version that holds up when a client's accountant pushes back.

What this ignores

State usury caps and jurisdiction rules (a few states limit interest on commercial invoices), compounding (avoid it unless your contract explicitly allows it), and collection costs. A late fee only works if it was in the contract before the invoice.

Field notes

Can I charge a late fee if my contract never mentioned one?: Usually not enforceably. Late fees come from the agreement, not from being annoyed. Put a rate in every contract and on every invoice footer going forward, it costs nothing and changes payment behavior.
What late fee rate is normal for freelancers?: 1–1.5% per month (12–18% annually) is the standard range in the US. Higher rates risk hitting state caps and feel punitive; the point is nudging payment, not earning interest.
Do late fees actually get paid?: Often no, and that's fine. The fee's real job is making your invoice the one that gets paid first next month. Many freelancers waive it once payment lands, in writing, as a goodwill move.