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Interest Calculator
Search for an interest calculator and you could mean five different things. Rather than make you guess, this page asks what you're trying to find out and points you straight to the right tool — no data entry here.
What are you trying to find out?
Pick the goal that fits. Each card just links to the matching calculator — nothing to fill in on this page.
- See how savings grow with interest earning interestInvestments, retirement accounts, anything that compounds over years.Compound Interest Calculator
- Calculate flat interest on a principalSimple loans, promissory notes, or a fixed deposit with no compounding.Simple Interest Calculator
- Find the hidden interest rate on a loan or goalSolve for the rate baked into a payment — or the rate you need to hit a target.Interest Rate Calculator
- Plan a savings goalHow much to set aside each month, or when you will reach a dollar target.Savings Calculator
- Compare a CD's interest and early-withdrawal penaltyWeigh maturity value against the cost of breaking the term early.CD Calculator
Simple vs compound, APR vs APY
Two distinctions cause most of the confusion around interest. Get these straight and picking the right calculator above becomes obvious.
| Simple interest | Compound interest | |
|---|---|---|
| Charged on | The original principal only | Principal plus all interest earned so far |
| Growth shape | Straight line | Curved, accelerating over time |
| Common uses | Short-term loans, some bonds, car notes | Savings, investments, mortgages, credit cards |
| Tool | Simple Interest → | Compound Interest → |
APR vs APY is the other one. APR (annual percentage rate) is the nominal rate before compounding — the number usually quoted on loans. APY (annual percentage yield) folds compounding into one effective figure — the number usually quoted on savings and CDs. A 12% APR compounded monthly is about a 12.68% APY. When you compare two accounts, compare APY to APY so the compounding is already accounted for.
The types of interest — and which tool does the math
There is no single “interest formula,” which is exactly why one calculator can’t answer every interest question. This page is a router, not an engine — each calculation lives in the dedicated tool built for it, so the assumptions stay honest and the numbers stay verifiable.
- Simple interest — I = P · r · t. Interest on the principal only. Handled by the simple interest calculator.
- Compound interest — A = P(1 + r/m)m·t, plus a growing-annuity term when you add contributions. Handled by the compound interest calculator.
- An unknown rate — solved numerically from a payment or a target balance by the interest rate calculator.
- A savings goal — future value with contributions, or goal-seek for the monthly deposit or the time to a target, in the savings calculator.
- A certificate of deposit — APY-based maturity value and the cost of an early withdrawal in the CD calculator.
Reading interest without getting fooled
- Ask whether interest builds on a fixed principal (simple) or a growing balance (compound) — that alone decides the tool.
- Compare rates as APY, not APR, so compounding is already included on both sides.
- Contributions usually matter more than the rate — steady monthly deposits outrun a slightly higher yield.
- Time is the multiplier; use the Rule of 72 (72 ÷ rate) for a quick “years to double.”
- Remember the results are pre-tax and nominal — taxes and inflation both shave the real return.
Related calculators
Frequently asked questions
Which interest calculator should I use?
Match the tool to your question. If interest builds on a growing balance over years, use the compound interest calculator. If interest is charged only on the original principal, use the simple interest calculator. To uncover an unknown rate, use the interest rate calculator. To plan how much to save, use the savings calculator; to model a fixed-term deposit with a penalty, use the CD calculator. The picker above deep-links you to the right one.
What is the difference between simple and compound interest?
Simple interest is always calculated on the original principal only, so it grows in a straight line. Compound interest is calculated on the principal plus all interest already earned, so the balance curves upward and accelerates. On $10,000 at 5% for 20 years, simple interest earns $10,000, while monthly compounding earns roughly $17,100 — the gap widens the longer you wait.
How do regular contributions affect the interest I earn?
Contributions are the biggest lever for most people. Each deposit becomes new principal that starts earning interest itself, so steady monthly contributions usually outweigh small differences in rate or compounding frequency. The compound interest and savings calculators both let you add a monthly contribution and see the effect on the final balance.
How do taxes and inflation change my interest results?
These calculators show pre-tax, nominal growth. In a taxable account, interest is generally taxed as ordinary income each year, which trims your effective return. Inflation erodes buying power on top of that: 7% nominal growth with 3% inflation is closer to 4% in real terms. Treat the headline number as a ceiling and plan with a margin.
What is the Rule of 72?
The Rule of 72 is a quick mental shortcut: divide 72 by your annual rate to estimate the years it takes money to double. At 6% a balance doubles in about 12 years; at 9%, about 8 years. It is an approximation that works best for rates between roughly 4% and 12% — use the compound interest calculator when you need an exact figure.
What is the difference between APR and APY?
APR (annual percentage rate) is the nominal yearly rate before compounding, typically quoted on loans. APY (annual percentage yield) folds compounding into a single effective rate, typically quoted on savings and CDs. A 12% APR compounded monthly works out to about 12.68% APY. When comparing accounts, compare APY to APY so the compounding is already baked in.
Disclaimer: these calculators are educational tools, not financial advice. They model your inputs with published formulas, but they cannot know your full situation — for decisions with real stakes, talk to a qualified professional. Formulas and defaults last reviewed .