The short version
Chase in stages, counted from the due date rather than the invoice date. A reminder in the first week, a statement in the first month, a phone call after thirty days, and a decision about supply after sixty.
Work out how late it actually is
Before anything else, count from the due date. An invoice raised six weeks ago on net 30 terms is twelve days past due, and chasing it as though it were six weeks late is how you sound unreasonable to somebody who has done nothing wrong.
The days past due calculator does this and names the aging bucket, which is useful because what you should do changes at each boundary rather than gradually.
Week one: a reminder, not a complaint
Short, factual, and quoting the invoice number, the amount and the due date. Most invoices at this stage are not being withheld; they are sitting in an inbox behind a hundred others, or they went to the person who booked the work rather than to accounts.
Ask one question that is easy to answer: is there anything you need from me to get this processed. A missing purchase order reference is the single most common reason an invoice sits untouched in a company of any size, and it is a two-minute fix once you know.
Weeks two to four: send a statement instead
Chasing the same invoice a second time invites a conversation about that invoice: whether it arrived, whether it was right, whether somebody approved it. A statement changes the subject to the balance, and a balance is much harder to have an opinion about.
It also catches the case where they genuinely have lost it. If an invoice is on your statement and not on their ledger, that is found in a minute rather than after a month of chasing something they were never holding.
Send it to the accounts address, not to the person who booked the job.
After thirty days: pick up the phone
Email has stopped working by this point, and sending a third one mostly signals that nothing will happen if they keep ignoring you.
Ask for a payment date rather than for payment. A date is something a person can give you, it commits them to something specific, and it gives you a reason to call again that is not a complaint.
After sixty days: decide about supply
This is the point where the question stops being when you will be paid and starts being whether to keep working. Continuing to supply an account that has stopped paying is how a manageable debt becomes an unmanageable one.
Past ninety days, collection rates fall sharply and the decision is about what the debt is worth rather than when it arrives. The bad debt calculator will show you what writing it off actually costs, which is usually several times the amount itself once you account for the sales needed to replace the lost profit.
Late fees, used carefully
A late fee you never mentioned before the invoice went out is a fee you will probably not collect and may resent having asked for. A late fee stated in your terms and applied consistently is a normal commercial term that most clients accept.
What you can charge is limited by law in many places. The late fee calculator gives you the amount and the wording for both the invoice clause and the reminder.
Questions people ask
When should I start chasing an invoice?
The day after it is due, counted from the due date rather than the invoice date. A short reminder at that point is normal and is not an accusation.
Is it better to email or call?
Email for the first week or two, because it is easy to answer and leaves a record. After about thirty days email has usually stopped working and a call is the next step.
What should I send after the first reminder?
A statement rather than another reminder about the same invoice. It moves the conversation from one invoice they might dispute to a balance they have to account for, and it catches invoices they never received.
Should I charge a late fee?
Only if it was in your terms before the invoice went out, and only if you are willing to apply it consistently. What you can charge is limited by law in many places.
When should I stop supplying?
Most businesses stop extending further credit somewhere around sixty days past due. Continuing to supply an account that has stopped paying turns a manageable debt into an unmanageable one.
What does writing off an invoice really cost?
More than the invoice. A write-off costs profit, not revenue, so at a 20% margin $4,200 written off needs $21,000 of new work to replace.