How do you calculate late payment interest when the client makes partial payments?

Split the overdue period into segments at each payment date. For each segment, calculate interest on the balance that was outstanding during that segment: balance x (annual rate / 365) x days. Add the segments together.

Should a partial payment apply to interest or principal first?

Check your contract or the governing statute. Many contracts apply payments to accrued interest first, then to principal. If your agreement is silent, state your ordering in the invoice terms before the dispute arises.

Can you charge compound interest on late payments?

Only if your contract or the applicable statute allows it. Most late-payment interest on invoices is simple interest, calculated once against the outstanding balance rather than compounded.

Late Payment Interest on Partial Payments: The Segment Method

Q: Do you use 360 or 365 days in the interest calculation?

Most commercial and state-law calculations use 365 days. The federal Prompt Payment Act and some banking contracts use 360. Use whichever your contract or statute specifies.

Guides · Updated October 2026

A $6,000 invoice at 12% annual interest. The client pays $2,000 after 30 days, then the remaining $4,000 sixty days later. Total interest owed: $138.08. Not $177.53, which is what you get if you lazily charge 12% on the full $6,000 for 90 days, and not some smaller number either. The right answer sits between the two, and the method that gets you there works for any number of partial payments.

The segment method

The rule is one sentence: charge interest on whatever balance is outstanding during each period. Every payment splits the overdue timeline into segments. You calculate each segment separately and add them up.

For each segment the formula is the same as always: balance times the daily rate times days. The daily rate is your annual rate divided by 365. At 12%, that is 0.12 divided by 365, or about 0.0003288.

Worked example: $6,000, paid in two chunks

Segment	Balance	Days	Interest
Mar 1 to Mar 31	$6,000	30	$59.18
Mar 31 to May 30	$4,000	60	$78.90
Total interest			$138.08

Segment one: $6,000 x 0.0003288 x 30 = $59.18. The $2,000 payment lands on March 31, dropping the balance to $4,000. Segment two: $4,000 x 0.0003288 x 60 = $78.90. Add them: $138.08. The final payoff is $4,138.08.

Notice the asymmetry. The first $2,000, paid early, killed interest on a third of the balance for the long 60-day stretch. If the client had instead paid $2,000 at the very end, total interest would have been close to the full $177.53. When you negotiate a payment plan with a late payer, earlier chunks are worth more to you than the same dollars paid later. That is worth saying out loud in the negotiation.

The payment-ordering question

There is one wrinkle the math above glosses over. When a partial payment arrives, does it reduce the principal first, or does it pay off accrued interest first? The worked example applied the whole $2,000 against the balance, which is the simple approach, and the one I would use unless the contract says otherwise.

But many contracts say otherwise: payments apply to accrued interest first, then to principal. Under that ordering, the March 31 payment would first wipe out the $59.18 of accrued interest, leaving $1,940.82 to reduce the balance to $4,059.18, and the second segment would be slightly larger. The difference on this invoice is a few dollars. On a large balance over many months, it is not.

Decision rule: read your contract. If it specifies an ordering, follow it. If it is silent, your state's law may supply one. And if neither does, write the ordering into your invoice terms now, before the next dispute, because this is the first thing a debtor challenges.

The worksheet. For each payment: (1) write the balance outstanding and the date it started; (2) count the days to the next payment; (3) interest for that stretch is balance x (annual rate / 365) x days; (4) subtract the payment from the balance (after clearing accrued interest, per your terms); (5) repeat. If you are doing this in a spreadsheet, one row per segment is the whole system.

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Frequently asked questions

How do you calculate late payment interest on partial payments?

Split the overdue period into segments at each payment date. For each segment, calculate balance x (annual rate / 365) x days on the balance outstanding during that segment. Add the segments together.

Do partial payments reduce the interest owed?

Yes. Each payment lowers the outstanding balance, and interest accrues only on the smaller balance afterward. Money paid early in the overdue period saves more interest than the same amount paid later.

Does the payment apply to interest or principal first?

Follow your contract or statute. Many contracts apply payments to accrued interest first, then principal. If your agreement is silent, add an ordering clause to your invoice terms.

Should I use 360 or 365 days?

Most commercial and state-law calculations use 365. The federal Prompt Payment Act and some banking contracts use 360. Use whichever your contract or governing law specifies.

Can interest compound across partial payments?

Only if your contract or statute allows it. Standard invoice late-payment interest is simple interest, calculated against the outstanding balance per period, not interest on interest.

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