How to Calculate Daily Interest on a Loan: Formulas
Learn to calculate daily interest on a loan with clear formulas and real examples. A guide for lenders and daily-collection operators. Automate the math with CobrApp.
Let CobrApp calculate the interest for you
Set the daily rate once and CobrApp works out the interest, the installments, the balances and the days overdue on its own. No manual formulas, no errors, no disputes. Free for Android and iOS.
1.What daily interest is and when it is used
Daily interest is the rate applied each day to a loan balance. It is the most common model in daily-payment lending and short-term microcredit.
Daily interest is the most transparent and direct way of pricing a short-term loan. Unlike traditional bank loans that work in monthly or annual rates, daily-payment lenders and informal microcredit across Latin America generally charge a fixed rate for each day the customer holds the money.
It shows up mainly in these contexts:
- Daily-payment lending: the customer pays a daily installment that already includes principal and interest.
- Weekly or fortnightly loans: interest days accumulate until the payment date.
- Emergency credit: when the term is short and the cost has to be clear immediately.
- Rural microcredit: where the payment frequency is daily or weekly depending on the harvest or the sale.
The advantage of daily interest is that both the lender and the customer know exactly what each additional day costs, which makes negotiation and portfolio control easier.
An app like CobrApp calculates daily interest automatically, with no need to do the math by hand, removing both errors and arguments.
2.The simple daily interest formula, step by step
The basic formula for daily interest on a loan is straightforward and works on paper, on a calculator or in a collections app.
The simple daily interest formula is the one most used by informal lenders and daily collectors:
A worked example:
- Principal lent: $1,000
- Daily rate: 1% per day
- Term: 30 days
To work out the daily installment on a daily-payment loan:
This is exactly the logic CobrApp uses internally, but calculated automatically and with a record of every payment. It removes the need to do the math by hand each time a customer pays.
3.How to convert rates: daily, monthly and annual
One of the most common mistakes is confusing interest rates across different periods. Learn to convert between daily, monthly and annual rates correctly.
Converting rates between periods is essential for comparing loans and communicating the terms correctly to your customers. There are two methods: simple conversion and compound conversion.
Simple conversion (the method most used in informal lending):
| Conversion | Formula | Example (1% daily) | | --- | --- | --- | | Daily → monthly | Daily rate × 30 | 1% × 30 = 30% monthly | | Daily → annual | Daily rate × 365 | 1% × 365 = 365% annual | | Monthly → daily | Monthly rate ÷ 30 | 3% ÷ 30 = 0.1% daily | | Annual → daily | Annual rate ÷ 365 | 36.5% ÷ 365 = 0.1% daily |
Compound conversion (mathematically more precise):
In practice, most informal lenders use the simple conversion because it is easier to explain to the customer. What matters is being consistent: always use the same method with every customer.
4.Worked table: daily-interest loans at different rates
Real examples of how the total cost and the daily installment change with the rate applied, so you can compare and choose the structure that fits your business.
The table below shows what a customer pays on a $2,000 loan at different daily rates and terms:
| Daily rate | Term | Total interest | Total to pay | Daily installment |
|---|---|---|---|---|
| 0.5% | 30 days | $300 | $2,300 | $76.67 |
| 1% | 30 days | $600 | $2,600 | $86.67 |
| 1% | 60 days | $1,200 | $3,200 | $53.33 |
| 2% | 15 days | $600 | $2,600 | $173.33 |
| 0.3% | 90 days | $540 | $2,540 | $28.22 |
Notice how lowering the daily rate but extending the term can keep total interest similar while the daily installment becomes far more manageable for the customer. That flexibility helps you keep customers and cut arrears.
CobrApp runs all of these calculations automatically once you set the rate and term of each loan, and it also generates the complete payment calendar with the exact date of every installment.
5.Daily interest on the balance vs flat interest: the key differences
There are two main ways to apply daily interest: on the outstanding balance, or as a flat amount worked out at the start. Here is how they differ.
This is one of the most frequent questions among lenders looking to professionalize:
1. Flat interest (the most common method in daily-payment lending):
Interest is calculated once on the initial principal and spread across equal installments. The customer always pays the same amount per day.
- Advantage: simple to understand and to explain to the customer.
- Advantage: the installment never changes, which is easy to budget for.
- Drawback: the effective rate ends up higher than the nominal one, because the customer pays interest on principal they have already repaid.
2. Interest on the outstanding balance (the banking method):
Interest is recalculated each period on the principal still owed. As the customer pays down principal, the interest falls.
- Advantage: fairer to the customer — they pay interest only on what they still owe.
- Advantage: if the customer pays extra principal, the interest drops.
- Drawback: a more complex calculation, hard to explain without tools.
Which one to use? For short loans of 15 to 60 days, flat interest is the standard in the daily-payment market. For medium and long terms, interest on the balance is more transparent and more professional.
CobrApp supports both models and lets you choose which to apply per customer or per portfolio, recording every payment and recalculating the balance in real time.
6.Common mistakes calculating daily interest, and how to avoid them
Interest calculation errors are the leading cause of customer disputes and unrecorded losses. Here are the most frequent ones and how to prevent them.
Research on informal lending shows that a significant share of disputes between lenders and customers originates in errors or misunderstandings about interest calculations. These are the most common:
- ❌ Confusing the monthly rate with the daily one: charging 10% monthly as though it were 10% daily is a catastrophic error. Always state the rate’s period: “1% daily” or “10% monthly”.
- ❌ Not recording partial payments: if a customer pays irregularly and you keep no exact record of each payment, it is impossible to know what they really owe. Use an app that logs every payment with date and time.
- ❌ Calculating on the original amount when principal has been repaid: if the customer has paid down principal and you keep charging interest on the original loan, you are overcharging and you may lose the customer.
- ❌ Not stating the rate clearly at the start: many disputes arise because the customer never really understood the terms. Always hand over a receipt or contract showing the rate, the term and the total to pay.
- ❌ Using spreadsheets or paper for complex calculations: formula errors in a spreadsheet or mistakes in manual addition are expensive. An app designed for lending removes these risks automatically.
CobrApp has these checks built in: it will not save a payment without first calculating the updated balance, and it automatically generates the customer’s receipt with the remaining balance.