How interest works on informal personal loans (with examples)

Charging interest on a personal loan to a friend feels strange to a lot of people — and for many small, short loans, it is entirely reasonable to skip it. But sometimes interest makes sense: the amount is large, the timeline is long, or the money would otherwise have been earning a return for you. When that happens, the question is no longer whether to charge interest but how to compute it fairly so the final number is something both people can verify rather than argue about.

This guide explains how interest on a personal loan typically works between individuals, with worked examples you can follow along with. No jargon, no amortization tables — just the actual mechanics.

Simple interest vs compound interest

There are two broad ways interest is calculated, and for informal loans you almost always want the simpler one.

• Simple interest is charged only on the original principal (and on whatever principal remains outstanding). It does not charge interest on previously accrued interest.
• Compound interest charges interest on the principal plus previously accumulated interest, so the balance snowballs.

For a loan between people who know each other, simple interest is the fair, transparent default. Compounding is how the balance on an unpaid loan can quietly balloon, and it is exactly the kind of thing that breeds resentment. Stick with simple interest unless you have a strong reason not to.

Daily accrual: the rate ÷ 365 trick

When someone quotes an interest rate, it is almost always an annual rate — say, 12% per year. But an informal loan does not get repaid in neat one-year blocks. It gets repaid whenever, in whatever amounts. So the practical way to handle interest is to accrue it daily on the outstanding principal.

The math is straightforward. Take the annual rate, divide by 365 to get a daily rate, and apply it to the current outstanding principal for each day the money is out:

Daily interest = outstanding principal × (annual rate ÷ 365)

Accruing daily is the fairest approach because it charges interest for exactly the number of days the money was actually borrowed — no more, no less. Pay it back in 40 days and you owe 40 days of interest, not a full month's or a full year's.

Worked example: a simple loan

Suppose you lend a friend ₹100,000 at 12% per year, with simple interest accruing daily.

• Daily rate = 12% ÷ 365 = 0.0329% per day.
• Daily interest on day one = ₹100,000 × 0.000329 = ≈ ₹32.88 per day.

If they hold the full amount for 30 days before paying anything:

• Interest accrued = ₹32.88 × 30 = ≈ ₹986.
• Outstanding pool after 30 days = ₹100,000 principal + ₹986 interest = ₹100,986.

Notice the interest is modest and completely predictable. There is no mystery in the number — it is just the daily rate times the days elapsed.

Worked example: a partial repayment (interest-first)

Now the part people get wrong. Say that on day 30, with ₹986 of interest accrued, your friend pays back ₹20,000. How does that payment apply?

The fair convention is interest-first: a payment clears the accrued interest before it touches the principal.

• Payment = ₹20,000.
• First, clear the ₹986 of accrued interest → ₹986 used, interest now ₹0.
• The remaining ₹19,014 reduces the principal → new principal = ₹100,000 − ₹19,014 = ₹80,986.

From day 31 onward, interest accrues on the new, lower principal:

• New daily interest = ₹80,986 × 0.000329 = ≈ ₹26.64 per day.

So each repayment does two things: it clears the interest meter back to zero, and it shrinks the principal so future interest accrues more slowly. This is why paying down an informal loan early genuinely saves the borrower money — fewer days, and a smaller base.

Why interest-first is the fair default

You might ask why interest comes first rather than principal. The logic is simple: interest is the cost of the time the money was outstanding, so it should be settled for the period that just passed before you start reducing what is owed going forward. It mirrors how virtually all formal loans work, and it prevents a situation where the principal drops but unpaid interest lingers and quietly compounds. Interest-first keeps the loan honest in both directions.

Keep the calculation out of your hands

Here is the catch with all of this: even though the math is simple, keeping it current is not. The correct balance changes every single day, and it has to be recomputed every time a payment lands. Do it by hand and you will either drift out of date or make an arithmetic slip — and a wrong interest number is precisely the kind of thing that turns a friendly loan sour.

This is the strongest argument for using a tracker over a spreadsheet: a good lending tracker accrues the daily interest automatically up to today, applies each payment interest-first, and shows both people the live outstanding pool. Nobody has to trust anybody's arithmetic, because the calculation is right there and identical on both screens.

Before you charge interest at all

A few non-mathematical notes worth saying out loud:

• Agree on the rate before the money moves. Interest discovered after the fact feels like a penalty. Interest agreed upfront feels like a deal.
• Keep the rate reasonable. This is a favor with a fair cost, not a payday loan.
• Write down the start date and rate the day the loan happens — it is the part everyone forgets.

For the bigger picture on tracking informal loans, see why you need a peer-to-peer lending tracker. And if you are the one being owed, here is how to track money owed to you.

Create a free tracker on P2P Track — enter the principal, the annual rate, and the start date, and it accrues the daily interest and applies interest-first payments for you. Share the link so the borrower sees the exact same number. No login needed to start.