Use our Simple Interest Calculator to easily compute interest, total amount & savings. Fast, accurate & free online tool.
Calculate Simple Interest.
The Interest Calculator helps you understand how interest works on both loans and savings. Whether you're calculating simple interest on a short-term loan, comparing it to compound interest, or figuring out how much your savings will grow, this tool provides instant, accurate results with a complete breakdown of your interest earnings or costs.
Understanding interest is fundamental to making smart financial decisions. Simple interest is calculated only on the original principal, making it predictable and easy to understand. While compound interest is more common in modern banking, simple interest is still used for certain auto loans, personal loans, and some investment calculations. This calculator shows you both to help you compare and make informed choices.
I = Interest earned or paid
P = Principal (initial amount)
R = Annual interest rate (as decimal, e.g., 5% = 0.05)
T = Time period in years
Total Amount = Principal + Interest = P + (P × R × T) = P(1 + RT)
A = Final amount
P = Principal
r = Annual interest rate (decimal)
n = Compounding frequency per year (12 for monthly, 365 for daily)
t = Time in years
Compound Interest = A - P = P[(1 + r/n)^(nt) - 1]
$10,000 invested at 5% annual interest:
| Time Period | Simple Interest | Compound (Annual) | Compound (Monthly) | Difference |
|---|---|---|---|---|
| 1 year | $10,500 | $10,500 | $10,512 | $0 - $12 |
| 5 years | $12,500 | $12,763 | $12,834 | $263 - $334 |
| 10 years | $15,000 | $16,289 | $16,470 | $1,289 - $1,470 |
| 20 years | $20,000 | $26,533 | $27,126 | $6,533 - $7,126 |
| 30 years | $25,000 | $43,219 | $44,677 | $18,219 - $19,677 |
*The longer the time period, the greater the advantage of compound interest for savings (or higher cost for loans)
| Financial Product | Interest Type | Notes |
|---|---|---|
| Savings Accounts | Compound (Daily) | APY shows true annual return |
| Certificates of Deposit (CDs) | Compound (Daily/Monthly) | Fixed rate for fixed term |
| Mortgages | Compound (Monthly) | Amortized with fixed payments |
| Credit Cards | Compound (Daily) | On average daily balance |
| Auto Loans | Usually Simple | Calculated on original principal |
| Personal Loans | Simple or Compound | Varies by lender |
| Student Loans (Federal) | Simple | During school/grace periods |
| Treasury Bonds | Simple | Fixed coupon payments |
Example: You invest $5,000 at 6% for 3 years.
Simple Interest: I = $5,000 × 0.06 × 3 = $900. Total: $5,900
Compound Interest (annual): A = $5,000(1.06)³ = $5,955. Interest: $955
Compound Interest (monthly): A = $5,000(1 + 0.06/12)^36 = $5,983. Interest: $983
| Account Type | Average Rate | Best Available | Where to Find |
|---|---|---|---|
| High-Yield Savings | 4.5% APY | 5.0-5.25% APY | Online banks (Marcus, Ally, Discover) |
| Traditional Savings | 0.01-0.50% APY | 0.50% APY | Major banks (Chase, BofA, Wells Fargo) |
| 1-Year CD | 4.5% APY | 5.0-5.5% APY | Credit unions, online banks |
| Money Market | 4.0% APY | 5.0% APY | Online banks, brokerages |
| I Bonds | 5.27% | 5.27% | TreasuryDirect.gov |
| Credit Cards (APR) | 20-24% | 15-18% | Best rates require excellent credit |
*Rates fluctuate based on Federal Reserve decisions. Always verify current rates before opening accounts.
• At 6% interest: 72 ÷ 6 = 12 years to double
• At 8% interest: 72 ÷ 8 = 9 years to double
• At 12% interest: 72 ÷ 12 = 6 years to double
This works for compound interest and helps you quickly estimate growth. Works best for rates between 6-10%.
Sources & Methodology: Interest calculations follow standard financial formulas used by banks and financial institutions. Rate references based on FDIC national rate data and Federal Reserve Economic Data (FRED). APY/APR definitions per Truth in Savings Act and Truth in Lending Act regulations. For personalized financial advice, consult a qualified financial advisor. Calculator updated January 2026.
To calculate simple interest, use the formula I = P × R × T where I is the interest earned, P is the principal (starting amount), R is the annual interest rate as a decimal (divide percentage by 100), and T is time in years. For example, to find simple interest on $5,000 at 6% for 3 years: I = $5,000 × 0.06 × 3 = $900. Your total amount is Principal + Interest = $5,900. For months, convert to years (6 months = 0.5 years). Simple interest is linear and predictable—the interest amount stays constant each period because it's only calculated on the original principal, never on accumulated interest.
Simple interest is calculated only on the original principal using I = P × R × T, while compound interest is calculated on both principal AND previously earned interest, creating exponential growth. For example, $10,000 at 5% for 10 years: simple interest yields $5,000 total interest ($15,000 final), but compound interest (annual) yields $6,289 interest ($16,289 final)—a $1,289 difference. At 20 years, compound interest produces $16,533 vs simple's $10,000. Simple interest benefits borrowers (lower cost), while compound interest benefits savers (higher returns). Auto loans typically use simple interest; savings accounts, mortgages, and credit cards use compound interest.
Use simple interest calculations for: auto loans (most use simple interest structure), short-term personal loans, Treasury bonds and fixed-coupon investments, federal student loans during grace periods, and quick mental math estimates. Use compound interest calculations for: savings accounts and CDs (daily/monthly compounding), mortgage and home loan payments, credit card debt (daily compounding on balances), and long-term investment projections. Key insight: simple interest is easier to calculate and always costs less for borrowers, but compound interest reflects how most modern financial products actually work—especially savings vehicles where you want interest-on-interest working in your favor.