Interest Calculator Compare simple, compound, and continuous interest — with 2025 benchmarks, visual timelines, and educa...
Interest Calculator
Compare simple, compound, and continuous interest — with 2025 benchmarks, visual timelines, and educational guidance.
Simple Interest**:
$$I = P \times r \times t$$ $$A = P + I = P(1 + rt)$$
Compound Interest**:
$$A = P \left(1 + \frac{r}{n}\right)^{nt}$$
Continuous Compounding**:
$$A = Pe^{rt}$$
Where: • $P$ = principal • $r$ = annual rate (decimal) • $t$ = time in years • $n$ = compounding periods/year • $e$ = Euler’s number (~2.71828)
Example** ($10K @ 5% for 10 yrs): • Simple: $10,000 × (1 + 0.05 × 10) = **$15,000** • Monthly: $10,000 × (1 + 0.05/12)120 = **$16,470** • Continuous: $10,000 × e0.5 = **$16,487**
⚠️ Avoid these common errors:
- Simple interest on auto loans** — Many “simple interest” loans *actually compound* (read amortization schedule!)
- Ignoring compounding frequency** — 5% APR compounded daily = 5.13% effective
- Using nominal rate for long-term projections** — Always use effective annual rate (EAR)
- Teaser rates without cliff warnings** — 0% for 12 mos → 29.99% after
✅ Pro Tips**:
- For savings: maximize **APY** (not APR) — includes compounding
- For debt: minimize **APR** — but verify how interest is calculated
- Use the **Rule of 72**: 7% return → doubles in ≈10.3 years
| Product | Nominal APR | Effective APY | Compounding |
|---|---|---|---|
| HYSA (Top) | 5.00% | 5.13% | Daily |
| 1-Yr T-Bill | 4.80% | 4.92% | Semi-Annual |
| Personal Loan | 10.50% | 11.03% | Monthly |
| Credit Card | 20.40% | 22.54% | Daily |
📉 Rule of 72 Examples**:
- 3% inflation → Purchasing power halves in **24 years**
- 7% stock market return → $100K → $1M in **~31 years**
➡️ Simple Interest
“$10K at 5% for 3 years” → $1,500 interest, $11,500 total.
➡️ Compound Interest
Model monthly vs. daily compounding — see how frequency affects growth.
➡️ Continuous Compounding
Theoretical max growth — useful for math modeling or comparing APYs.
You’ll get:
- Future value and interest earned
- Effective annual rate (EAR)
- Doubling time (Rule of 72)
- SVG growth curve
Note: All calculations assume fixed rate and no withdrawals. For variable-rate products, use average expected rate.