Quick tricks for calculating simple and compound interest

Interest questions in exams are really just percentage questions wearing a costume. If you're already comfortable building percentages from 10%/1% blocks, simple interest is nearly free — and compound interest just adds one repeatable step on top.

Simple interest: it's one percentage, repeated

Simple interest is calculated fresh on the original principal every year — it never compounds. The formula:

The mental shortcut: calculate one year's interest as a percentage of the principal, then just multiply by the number of years. No need to redo the percentage calculation each year.

Worked example: SI on ₹8,000 at 5% for 3 years

• One year's interest: 5% of 8,000 = 400
• Since simple interest doesn't compound, just multiply: 400 × 3 = 1,200

Compound interest: repeat the percentage on the new total

Compound interest recalculates the percentage on the growing amount each period, not the original principal. The formula:

Rather than computing this with powers directly, the fast mental approach is to apply the percentage year-by-year and add each year's interest to the running total — this is usually quicker than computing a power by hand for 2–3 year periods, which is what most exam questions use.

Worked example: CI on ₹8,000 at 5% for 3 years

• Year 1: 5% of 8,000 = 400 → new total = 8,400
• Year 2: 5% of 8,400 = 420 → new total = 8,820
• Year 3: 5% of 8,820 = 441 → new total = 9,261

Compare that to simple interest on the same numbers (₹1,200) — the ₹61 difference is entirely from interest earning interest in years 2 and 3.

Shortcut for 2-year compound interest

Two-year compound interest comes up often enough in exams to deserve its own shortcut. The difference between CI and SI over 2 years has a clean formula:

This means once you know the simple interest for 2 years, you can get compound interest by adding just one small correction term — no need to run the full year-by-year process.

Worked example using the 2-year shortcut

P = ₹5,000, R = 10%, T = 2 years.

• SI for 2 years: 10% of 5,000 = 500 per year × 2 = 1,000
• Correction: 5,000 × (10/100)² = 5,000 × 0.01 = 50

Doubling time: the Rule of 72

To estimate how long it takes money to double at a given compound rate, divide 72 by the rate:

At 8% compound interest, money roughly doubles in 72/8 = 9 years. This is an approximation, but it's accurate enough for the estimate-and-eliminate style of exam question.

Where this actually helps

Simple and compound interest are staple topics in bank exam quant sections, often paired with percentage and ratio questions. Being able to run a 2–3 year compound interest calculation in your head — instead of expanding a power by hand — saves real time on a timed paper.