Late Payment Interest on Net 30 Invoices: Day 31 Is When the Clock Starts
Late payment interest on net 30 invoices confuses people in exactly one place. They know net 30 means 30 days to pay, but they are unsure when late starts and how much interest that lateness is actually worth. The answer to the first question is day 31. The answer to the second is usually less money than the argument about it costs, which is worth knowing before you send the email.
Work it with real numbers. You invoice $4,800 on March 1 with net 30 terms. The due date is March 31. The client pays on May 17, which is 47 days after the due date. At a 12 percent annual rate, the interest is $4,800 times 0.12 times 47 divided by 365, which comes to $74.17. That is the whole calculation for simple interest, and simple interest is what most freelancer contracts use. Principal times rate times days over 365.
The counting trips people up more than the formula. Days late are calendar days, including weekends and holidays, unless your contract says otherwise. A client who says the weekend should not count is usually bluffing or confused, since interest clauses almost never exclude weekends. What does stop the clock is payment, and a partial payment shrinks the principal that later days run on. If that same client had paid $2,400 on April 15, interest after April 15 would run on the remaining $2,400, not the full $4,800.
The rate is where you need a contract clause before the invoice goes out, not after it goes late. Without an agreed rate, you fall back on whatever your state statute allows, which varies and may be lower than you expect. With a clause, you can set a rate like 1 percent per month, which is 12 percent annually, and state that interest runs from the due date. One percent a month is common in freelancer contracts, it is easy to explain to a client, and it stays clear of usury arguments that higher rates invite.
Late payment interest on net 30 invoices starts the day after the due date
Grace periods need a careful sentence in your contract. Some freelancers give clients a 7 day grace period as a courtesy, but a grace period is not the same as a free week. If your clause says interest runs from the due date even if you do not chase it during the grace week, the interest still accrues. If it says interest starts after the grace period, it does not. Write the sentence that matches what you mean, because clients will read it the way that favors them.
Compounding is the other clause to settle in advance. Simple interest charges on the principal only. Compound interest charges on principal plus accrued interest, which grows the bill faster and annoys clients faster too. Unless your contract explicitly compounds, assume simple interest and calculate it that way. The extra dollars from compounding on a typical freelance invoice are small, and the goodwill cost of surprising a client with them is not.
Here is the part most guides skip. Seventy four dollars and seventeen cents is rarely worth losing a client over, and sometimes it is worth waiving entirely to get the principal paid this week instead of next month. My rule is to charge interest consistently on paper, so the clause has teeth, and waive it selectively in practice when waiving gets the money faster. Put the interest line on the reminder invoice, show the math, and let the client see that paying now stops the meter. The clause earns you the payment. The waiver, used sparingly, earns you the next project.
Try it. Run your own numbers before you decide. Try the free late payment interest calculator
Frequently asked questions
When does interest start on a net 30 invoice?
On day 31 after the invoice date, unless your contract sets a different start. Net 30 means payment is due 30 days after invoicing, so a payment on day 31 is one day late.
How do I calculate interest on a late net 30 invoice?
Multiply the unpaid principal by the annual rate and by days late divided by 365. A $4,800 invoice at 12 percent for 47 days late is $4,800 times 0.12 times 47 over 365, which is $74.17.
Do weekends count toward days late?
Yes in most contracts, because interest runs on calendar days. Weekends and holidays only stop the clock if your contract or the governing statute says so.
Does a partial payment stop interest?
It reduces it going forward, because interest after the payment runs on the smaller remaining balance. It does not erase interest already accrued before the payment arrived.
Should interest compound on overdue invoices?
Only if your contract says so. Simple interest is the default most freelancers use, and compounding without a contract clause invites a dispute that costs more than the extra interest is worth.
Related: Late Payment Interest With Partial Payments · Do Weekends and Holidays Count? · What Is a Reasonable Late Payment Interest Rate?
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