Private lending guide
Private Loan Interest Calculator: Choose the Right Method
A private loan can be simple to calculate when the balance never changes, or much more involved when payments arrive on irregular dates. Start by matching the calculator to the actual loan activity.
Free calculator
Need the number now?
Get the number first, then return to the guide for the explanation and next steps.
Windows software
Need to save or repeat the work?
Move from a one-time browser calculation to saved, printable, repeatable work.
For one fixed balance, calculate the per diem
Use principal × annual rate ÷ day-count basis to find interest per day. Multiply by the number of days when the balance remains unchanged.
For exact dates, count the interest days
Use the Interest Between Dates Calculator when the start and end dates matter or when you need to control whether the first or last day is included.
For real payment history, rebuild each transaction
If payments are late, partial, skipped, or principal-only, use the Irregular Payment Loan Calculator so the balance and interest are updated in chronological order.
For ongoing servicing, save the loan instead of rebuilding it
A browser calculator is ideal for a one-time answer. Ongoing private lending is easier when payments, balances, due dates, and payoff information are maintained in one saved loan record.
A $40,000 private loan at 8% using Actual/365 accrues about $8.77 per day while the principal remains $40,000. Once a borrower payment reduces principal, the per diem should be recalculated from the new balance.
Frequently asked questions
What interest calculator should I use for a private loan?
Use Daily Interest for a fixed balance, Interest Between Dates for one exact date range, or Irregular Payment Loan for actual changing payment history.
Can a private loan use a 360-day year?
Yes if the governing note uses that convention. Use the method stated in the loan documents.
How do late payments affect a private loan balance?
The prior principal can remain outstanding for additional days, increasing accrued interest before the late payment is applied.
What is the difference between a calculator and loan servicing software?
A calculator solves a particular scenario. Servicing software keeps the loan record over time, including payments, balances, due dates, and payoff information.