What is an interest rate?
An interest rate is the percentage charged by a lender to a borrower for the use of money, or the percentage earned on money deposited in a savings account or investment. It represents the cost of borrowing or the reward for saving.
Interest rates are fundamental to the economy and affect everything from mortgages and car loans to savings accounts and investments.
How do interest rates work?
When you borrow money, you pay back the original amount (the principal) plus an additional percentage — the interest. When you save money, the bank pays you interest for holding your funds.
Simple example
If you borrow $10,000 at a 5% annual interest rate for one year:
- Principal: $10,000
- Interest: 500
- Total repayment: $10,500
Types of interest rates
Fixed interest rate
A fixed rate stays the same for the entire loan or investment term. This provides predictability — you know exactly how much you'll pay or earn.
Best for:
- Long-term mortgages when rates are low
- Personal loans
- Certificates of deposit (CDs)
Variable interest rate
A variable (or adjustable) rate can change over time, usually based on a benchmark rate like the prime rate or SOFR.
Best for:
- Short-term borrowing
- When rates are expected to decrease
- Credit cards (though you have no control)
Prime rate
The prime rate is the interest rate that commercial banks charge their most creditworthy borrowers. It serves as a benchmark for many consumer lending products.
Federal funds rate
Set by the Federal Reserve, this is the rate at which banks lend to each other overnight. Changes in this rate ripple through the entire economy, affecting mortgage rates, savings rates, and more.
What affects interest rates?
Several factors influence interest rates:
1. Inflation
Higher inflation typically leads to higher interest rates. Lenders need to charge more to compensate for the decreased purchasing power of future payments.
2. Central bank policy
The Federal Reserve (and equivalent institutions in other countries) sets benchmark rates that influence all other rates in the economy.
3. Supply and demand
When there's high demand for loans, rates tend to rise. When there's excess capital looking for borrowers, rates tend to fall.
4. Credit risk
Borrowers with higher credit risk (lower credit scores) pay higher interest rates to compensate lenders for the increased chance of default.
5. Loan term
Longer-term loans generally have higher rates because there's more uncertainty over a longer period.
Interest rates and your financial life
Mortgages
Even a small difference in mortgage rates can mean tens of thousands of dollars over the life of a 30-year loan.
Example on a $300,000 mortgage:
| Rate | Monthly Payment | Total Interest (30yr) |
|---|---|---|
| 4.0% | $1,432 | $215,609 |
| 5.0% | $1,610 | $279,768 |
| 6.0% | $1,799 | $347,515 |
| 7.0% | $1,996 | $418,527 |
A 3% difference (4% vs 7%) costs over $200,000 in additional interest.
Savings accounts
Higher rates mean your money grows faster. At 1% vs 5% APY:
| Years | 1% on $10,000 | 5% on $10,000 |
|---|---|---|
| 5 | $10,510 | $12,763 |
| 10 | $11,046 | $16,289 |
| 20 | $12,202 | $26,533 |
| 30 | $13,478 | $43,219 |
Credit cards
Credit card rates are among the highest consumer interest rates, typically 18–25% APR. Carrying a balance at these rates can quickly lead to a debt spiral.
Example: $5,000 balance at 20% APR, making only minimum payments (~2% of balance):
- It would take 30+ years to pay off
- You'd pay over $10,000 in interest alone
How to get the best interest rates
- Improve your credit score — Higher scores qualify for lower rates
- Shop around — Compare rates from multiple lenders
- Consider shorter terms — Shorter loans typically have lower rates
- Make a larger down payment — Reduces the lender's risk
- Lock in rates — When rates are favorable, consider fixed-rate products
- Pay down debt — Lower debt-to-income ratio = better rates
Using our Interest Rate Calculator
Our calculator helps you determine:
- Loan mode: The interest rate you're paying on a loan or mortgage, using the Newton-Raphson method for precision
- Savings mode: The rate of return on your investments or savings, accounting for compounding
Understanding your actual interest rate empowers you to make better financial decisions, whether you're comparing loan offers or evaluating investment returns.