Invoice Interest Calculator

Calculator Inputs

Invoice Amount

Partial Payment

Annual Interest Rate (%)

Fixed Late Fee

Grace Days

Currency Code

Invoice Due Date

Payment Date

Interest Method

Compounding Frequency

Day Count Basis

Example Data Table

Scenario	Invoice	Rate	Due Date	Payment Date	Grace	Method	Fee
Consulting Invoice	USD 2,500.00	12.00%	2026-03-01	2026-04-10	5 Days	Simple	USD 25.00
Agency Retainer	USD 8,000.00	18.00%	2026-02-15	2026-04-15	0 Days	Compound Monthly	USD 75.00
Wholesale Goods	USD 14,500.00	10.50%	2026-01-20	2026-03-05	10 Days	Simple	USD 0.00

Use these examples to test different payment delays, methods, and fee structures.

Formula Used

1. Net Principal
Net Principal = Invoice Amount - Partial Payment

2. Calendar Days Late
Calendar Days Late = Payment Date - Due Date

3. Chargeable Days
Chargeable Days = max(Calendar Days Late - Grace Days, 0)

4. Simple Interest
Interest = Principal × Rate × (Chargeable Days ÷ Days In Year)

5. Compound Interest
Amount = Principal × (1 + Rate ÷ n)^(n × Time In Years)

6. Final Amount Due
Total Due = Principal + Interest + Fixed Late Fee

This page supports 365, 360, and actual-year bases. Compound mode also lets you choose daily, weekly, monthly, quarterly, or yearly accrual frequency.

How to Use This Calculator

Enter the original invoice amount.
Add any partial payment already received.
Type the annual interest rate from your agreement.
Select the due date and the actual payment date.
Enter grace days and any fixed late fee.
Choose simple or compound interest.
Pick a compounding frequency and day-count basis.
Press Calculate Interest to view totals, schedule, and chart.
Use the CSV and PDF buttons to save the report.

Frequently Asked Questions

1. What does this invoice interest calculator do?

It estimates late-payment charges on unpaid invoices. You can include partial payments, grace periods, fixed fees, several compounding options, and different day-count conventions in one calculation.

2. What is the difference between simple and compound interest?

Simple interest grows only on the net principal. Compound interest grows on principal plus previously accrued interest, so the charge increases faster when the overdue period becomes longer.

3. Why would I use a 360-day basis?

Some finance agreements use a 360-day convention for easier calculations and consistent monthly estimates. Others use 365 days or the actual year length. Use the basis stated in your contract.

4. How are grace days applied?

Grace days reduce the overdue period before interest starts. If an invoice is ten days late and the agreement allows five grace days, the calculator charges interest on five days only.

5. Can I include partial payments?

Yes. The calculator subtracts the partial payment from the original invoice first. Interest is then applied to the remaining balance, which gives a more realistic late-charge estimate.

6. Does this replace legal or accounting advice?

No. It provides planning estimates only. Always confirm allowable rates, fees, and notice requirements under your contract, internal policy, and local law before billing interest.

7. What does the chart show?

The chart plots projected balance against chargeable overdue days. It helps you see how quickly the total rises under your selected method, fee setting, and compounding frequency.

8. What is included in the downloadable files?

The CSV contains the summary metrics and accrual schedule. The PDF provides a clean report of the result section, making it easier to archive, review, or share internally.