Simple Interest, Recurring Deposits & Credit Card APR Explained
Interest shows up everywhere — savings, loans, deposits, credit cards — and the same few formulas explain all of it. This guide covers the ones people actually ask about, in plain language.
See what your money becomes — compound growth at any frequency.
Simple interest: the formula and the pseudocode
Simple Interest = Principal × Rate × Time. Borrow $1,000 at 5% for 2 years: 1000 × 0.05 × 2 = $100 interest. As pseudocode (a common homework and interview question):READ principal, rate, years
interest = principal * rate * years / 100
PRINT interest
Simple interest applies to some short-term loans and is the teaching foundation for everything else — but most real savings and loans use compound interest, where interest earns interest (try our Compound Interest Calculator to see the difference).
Recurring deposits (RD), explained
A recurring deposit is a savings plan where you deposit a fixed amount every month for a fixed term (common in India and other markets), earning interest that compounds quarterly. Example: $100/month for 12 months at 6% grows to roughly $1,230 — the exact figure depends on compounding frequency. RDs suit people who save from a salary: small, regular, automatic. Our Savings Calculator models the same idea for any regular deposit.
What is a good credit card APR?
APR (Annual Percentage Rate) is the yearly cost of borrowing on a card. As a rough guide: the average card APR sits in the high teens to low 20s percent; "good" means below average for your credit profile — excellent credit can qualify for low-teens or 0% introductory rates, while building credit often means 20%+. The honest rule: if you pay the full balance monthly, the APR barely matters (no interest accrues); if you carry a balance, the APR is the most expensive number in your wallet, and paying it down beats almost any savings return.
Figuring out interest and checking income tax
To figure out interest on any loan, you need three numbers: principal, rate, and time — and whether it compounds (then use a calculator, because the math gets long). For income tax, "check tax income" means estimating tax on your income: use your country's official tax calculator or tables (rates, brackets and deductions change yearly and vary by country), and never trust a random blog's tax table without checking the current year's official figures.
Frequently asked questions
Is simple interest better than compound interest?
It depends on your side: as a saver, compound is better (interest earns interest); as a borrower, simple is cheaper. Most savings compound; some loans are simple.
How much does a recurring deposit earn?
Roughly: monthly deposit × months, plus interest at the stated rate compounded quarterly. Use a savings calculator with your bank's actual rate — advertised rates vary.
What APR should I accept on a first credit card?
Accept what you can get, pay in full monthly, and let on-time payments build your score — a better APR follows good history, not negotiation.
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