What is an interest rate?
Key takeaways
04 · ideas- Interest rate is the annual cost of borrowing, expressed as a percentage
- APR includes fees and is always higher than the stated rate
- 1% rate difference on a $300k mortgage = $70,000+ over 30 years
- Central banks set benchmark rates that ripple through every loan
Basic level
The essentials · 1 min
An interest rate is the price you pay to borrow money, expressed as a percentage per year.
Simple example: You borrow $10,000 at 5% annual interest.- After 1 year: you owe $10,500
- The $500 is the interest — the lender's fee for letting you use their money
Interest rates appear in everything: mortgages, car loans, credit cards, savings accounts. When you borrow money, you pay interest. When you save money, you earn interest.
The three big factors that determine your rate:- Your credit score — higher score = lower rate (you're less risky)
- The loan type — mortgages have lower rates than credit cards (secured vs unsecured)
- The economy — central banks raise/lower benchmark rates, affecting all loans
- Rate: the raw annual percentage
- APR: rate PLUS fees — always higher, always more honest
Intermediate level
How it applies · 1 min
How rates differ by loan type
| Loan type | Typical rate range | Why |
|---|---|---|
| Mortgage (30yr fixed) | 6–8% | Secured by property, low risk |
| Car loan | 6–12% | Secured by vehicle, medium risk |
| Personal loan | 10–20% | Unsecured, higher risk |
| Student loan (federal) | 5–8% | Government-subsidized |
| Credit card | 18–29% | Unsecured, revolving, high risk |
| Payday loan | 300–400%+ | Predatory, should be avoided |
The compounding effect over time
The real damage of a high rate shows when you look at total cost:
| Loan amount | Rate | Term | Monthly payment | Total interest |
|---|---|---|---|---|
| $10,000 | 5% | 5 yr | $189 | $1,322 |
| $10,000 | 15% | 5 yr | $238 | $4,274 |
| $10,000 | 25% | 5 yr | $296 | $7,753 |
| $300,000 | 6% | 30 yr | $1,799 | $347,515 |
| $300,000 | 7% | 30 yr | $1,996 | $418,527 |
| $300,000 | 8% | 30 yr | $2,201 | $492,386 |
A 2% higher mortgage rate on $300,000 costs an extra $144,871 over 30 years.
Fixed vs. variable rates
| Fixed rate | Variable rate | |
|---|---|---|
| Definition | Rate stays the same | Rate changes with market |
| Predictability | Perfect — same payment forever | Changes over time |
| Starting rate | Usually higher | Usually lower |
| Best for | Long-term loans, rising rate environments | Short-term loans, falling rate environments |
| Risk | None | Payment could rise significantly |
Advanced level
The math, step by step · 1 min
How interest rates are set
The Federal Reserve (US) sets the federal funds rate — the rate at which banks lend to each other overnight. This cascades:- Fed funds rate → affects SOFR (benchmark for variable loans)
- SOFR → affects HELOC rates, ARM mortgages, credit card rates
- 10-year Treasury yield → affects 30-year fixed mortgage rates
Mortgage rates aren't directly controlled by the Fed but correlate strongly with the 10-year Treasury.
The real vs. nominal rate
The Fisher equation separates the inflation component:
Where \pi is the inflation rate. A 6% mortgage when inflation is 3% has a real cost of ~3% — the lender is only earning 3% in purchasing power terms.
Implications for borrowers: During high-inflation periods, fixed-rate debt becomes cheaper in real terms. A 4% mortgage signed in 2020 with inflation at 8% in 2022 had a negative real interest rate — borrowers were effectively being paid to borrow.
Effective annual rate (EAR)
When interest compounds more frequently than annually:
This is why credit card "24% APR" actually costs more than 24% — daily compounding makes the effective rate ~26.8%.