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Banking and Insurance - Laws and Practice · Calculation of Interest and Annuities

Interest Calculation in Banking Practice: Methods and Numericals

Updated 11 October 2026 · Fact-checked

Banks calculate interest on the balance outstanding for the days it stayed in the account. On savings accounts they multiply each balance by the days it was held (the product), add the products, then apply rate ÷ 365 (or 366). Loans use daily or monthly rests; FDs use compounding; RDs use instalment-wise compounding.

Understand Interest Calculation in Banking Practice

Interest is the price of using money. A bank pays it on deposits and charges it on loans. In both cases the amount depends on three things: the balance, the rate and the time it stays.

The balance in a running account changes often. So banks cannot use one simple principal. They use the product method. For each balance, multiply the balance by the number of days it stayed unchanged. Add all these products. Then interest = total products × rate ÷ (100 × days in year). This is the same as simple interest applied to each balance and added up.

A rest is the interval at which the bank totals the account and adds interest to the principal. With a daily rest, interest is worked out on the day-end balance and the borrower pays interest only on the actual outstanding. With a monthly rest, the bank uses the lowest or the opening balance for the month, or charges on the monthly balance, so a payment made mid-month may not reduce interest until the next month. Daily rest is usually better for the borrower. Where interest is added to principal at each rest, interest on interest arises, which is compounding.

For savings accounts, RBI has left rates to banks, but interest must be calculated on a daily product basis on the end-of-day balance and is usually credited quarterly or at shorter intervals. For fixed deposits, interest is compounded at quarterly rests in common bank practice, unless the deposit is a payout type, where simple interest is paid out at the chosen interval. For recurring deposits, each monthly instalment earns interest for its own remaining period, and banks usually compound quarterly.

In exams, the question always states the rate, the dates and the method. Follow the method given. If none is stated, say your assumption (365 days, simple interest on products) and proceed.

Key rules to remember

Simple interest
I = P × R × T ÷ 100 (T in years)
Use T = days ÷ 365 when the period is in days.
Product (balance × days)
Product = Balance × Number of days the balance remained unchanged
Count the day of deposit but not the day of withdrawal, or follow the convention stated in the question.
Interest by product method
Interest = Σ Products × R ÷ (100 × 365)
Use 366 for a leap year if the question says so. Rate is annual.
Compound interest (FD with periodic rest)
A = P × (1 + r ÷ n)^(n×t), where r is annual rate in decimals and n is compounding periods per year
Quarterly rest means n = 4 and rate per quarter = annual rate ÷ 4.
Recurring deposit maturity (quarterly compounding)
M = Σ of each instalment × (1 + i)^(remaining quarters), with i = annual rate ÷ 4; a simple-interest RD uses I = P × n(n+1) ÷ 24 × R ÷ 100
Here P is the monthly instalment, n the number of months, R the annual rate in %. The simple version assumes instalments at the start of each month.

How to solve Interest Calculation in Banking Practice questions

Use this order for any banking interest numerical, whether deposit or loan.

  1. 1Read the question and note the rate, the account type, the period and the rest (daily, monthly, quarterly).
  2. 2Check whether the year has 365 or 366 days, or whether the question fixes 360. Use what is stated.
  3. 3List every transaction with its date in a table: date, deposit or withdrawal, running balance.
  4. 4Find the number of days each balance stays unchanged. The days must add up to the full period.
  5. 5Compute each product (balance × days) and total them. For a loan with monthly rest, use the balance fixed by the rest rule.
  6. 6Apply the rate: interest = total products × rate ÷ (100 × days in year). For FD or RD, apply the compounding formula instead.
  7. 7Add the interest to the balance if asked for closing balance, and state the final answer with rupee units and Indian grouping.

Quickest way: Product table with rate applied once

When to use it: Savings accounts and running loan accounts with several transactions in the period.

  1. Make three columns: balance, days, product.
  2. Check that the days column totals the period length.
  3. Add the products and apply rate ÷ (100 × 365) only once at the end.
  4. Round off at the end only, not after each product.

Common mistakes in Interest Calculation in Banking Practice

  • Applying the rate to the balance without multiplying by days

    Students treat the annual rate as if it were for each balance period.

    Fix: Always convert to a per-day basis. Interest = product × rate ÷ (100 × 365).

  • Days in the table do not add up to the period

    Counting both the start and end dates, or missing a month's length.

    Fix: Add the days column and compare it with the total days in the period before computing.

  • Using compound interest for a savings account product

    Mixing FD rules with savings account rules.

    Fix: Savings interest is simple on daily balances for the period. It is credited, and the credit then forms part of balance.

  • Taking quarterly rate as the annual rate for FD

    Forgetting to divide the rate by the number of compounding periods.

    Fix: Rate per period = annual rate ÷ n, and periods = years × n.

  • Ignoring the rest type for loans

    Using the closing balance for the whole month by default.

    Fix: Under monthly rest, apply the stated rule for which balance is used. Under daily rest, use each day's balance.

  • Switching between 365 and 360 days within one answer

    Habit from other interest problems.

    Fix: Choose one denominator at the start and use it throughout.

Worked examples

Example 1

A savings account at 3% p.a. had these balances in April (30 days): ₹40,000 from 1 to 9 April, ₹55,000 from 10 to 19 April, ₹35,000 from 20 to 30 April. Calculate the interest for April using the product method, taking 365 days a year.

Show the solution
  1. Days: 1 to 9 April = 9 days; 10 to 19 April = 10 days; 20 to 30 April = 11 days. Total 9 + 10 + 11 = 30 days, which matches.
  2. Products: 40,000 × 9 = 3,60,000; 55,000 × 10 = 5,50,000; 35,000 × 11 = 3,85,000.
  3. Total products = 3,60,000 + 5,50,000 + 3,85,000 = 12,95,000.
  4. Interest = 12,95,000 × 3 ÷ (100 × 365) = 38,85,000 ÷ 36,500 = ₹106.44 (rounded).

Answer: The interest for April is about ₹106.44.

Example 2

Meera deposits ₹1,00,000 in a bank fixed deposit for 1 year at 8% p.a., with interest compounded quarterly. Find the maturity amount and the interest earned. Also find what the interest would be on simple interest for the same year.

Show the solution
  1. Rate per quarter = 8 ÷ 4 = 2%. Number of quarters = 4.
  2. Maturity amount A = 1,00,000 × (1.02)^4.
  3. (1.02)^2 = 1.0404. (1.0404)^2 = 1.08243216.
  4. A = 1,00,000 × 1.08243216 = ₹1,08,243.22 (rounded).
  5. Interest earned = 1,08,243.22 − 1,00,000 = ₹8,243.22.
  6. Simple interest for the year = 1,00,000 × 8 × 1 ÷ 100 = ₹8,000.

Answer: Maturity amount is about ₹1,08,243.22 and interest is about ₹8,243.22. Simple interest would be ₹8,000, so quarterly compounding gives ₹243.22 more.

Exam tips

  • Draw the balance, days and product table every time. Marks are often given for the method and the table.
  • State your assumptions (365 days, simple interest on products, day of withdrawal excluded) in one line if the question is silent.
  • Check that days add up to the period before you multiply anything.
  • For theory parts, explain why daily rest is better for the borrower than monthly rest, using a short example.
  • Show the compounding formula with values substituted before the final figure.

Practice questions from Calculation of Interest and Annuities

Interest Calculation in Banking Practice: frequently asked questions

How do banks calculate interest on a savings account?

Banks work out the end-of-day balance, multiply it by the days it stays, and add up these products. The total is then multiplied by rate ÷ (100 × days in year). Interest is credited at the interval the bank follows.

What is the difference between daily rest and monthly rest on loans?

Under daily rest, interest is charged on the outstanding balance each day. Under monthly rest, interest is worked on a monthly balance, so a mid-month repayment may not reduce that month's interest. Daily rest generally costs the borrower less.

How is interest on a recurring deposit calculated?

Each monthly instalment earns interest only for the months it remains with the bank. Many banks compound quarterly, so each instalment is grown at the quarterly rate for its remaining quarters. A simple version uses P × n(n+1) ÷ 24 × R ÷ 100.

Should I use 365 or 360 days in the exam?

Use what the question states. If it is silent, use 365 days and write that assumption. Do not change the number of days within one answer.