💰 Interest Caclulator
Interest Calculator — Simple Interest
Calculate simple interest instantly. Enter your principal, annual rate, and time period — get total interest, future value, and a full year-by-year breakdown with charts.
⚡ Instant Results
📅 Years / Months / Days
📊 Year-by-Year Schedule
⏱️ Rule of 72
Simple Interest Calculator
I = P × R × T · A = P(1 + RT)
Enter Your Values
$
%
$0
Principal
$0
Total Interest
$0
Future Value
0%
Interest / Principal
ComponentAmount% of Total
Principal$00%
Total Interest$00%
Future Value (Total)$0100%
📊 Principal vs Interest Breakdown
Rule of 72 — Time to Double
Divide 72 by your interest rate to estimate how long it takes money to double.
—
years to double
📋 Year-by-Year Accumulation Schedule
| Period | Opening Balance | Interest Earned | Closing Balance |
|---|---|---|---|
| Enter values above to see schedule | |||
💡 Simple Interest Tips
📈For long-term savings, compound interest grows faster — see our Compound Interest Calculator
🏦Most bank accounts, mortgages, and credit cards use compound interest, not simple
⏰Simple interest is commonly used for short-term loans, bonds, and treasury bills
💳For loan repayment calculations with amortization, use our Loan Calculator
⚠️
Note: This calculator computes simple interest only. Most real-world financial products (mortgages, credit cards, savings accounts) use compound interest. Results are estimates for planning purposes.
What Is Simple Interest?
Simple interest is the most basic form of interest calculation. It is computed only on the original principal — not on any accumulated interest. This makes it predictable and easy to calculate, and it is commonly used for short-term loans, bonds, and treasury bills.
The formula is: I = P × R × T, where:
- I = Interest earned or owed
- P = Principal (original amount)
- R = Annual interest rate (as a decimal)
- T = Time in years
The future value (total amount) is: A = P(1 + RT)
Simple Interest Examples
| Principal | Rate | Time | Interest | Future Value |
|---|---|---|---|---|
| $5,000 | 4% | 3 years | $600 | $5,600 |
| $10,000 | 6% | 5 years | $3,000 | $13,000 |
| $25,000 | 8% | 2 years | $4,000 | $29,000 |
| $1,000 | 5% | 6 months | $25 | $1,025 |
| $50,000 | 3.5% | 10 years | $17,500 | $67,500 |
Simple Interest vs Compound Interest
The key difference: compound interest earns interest on interest, while simple interest does not. Over short periods the difference is small, but over decades compound interest grows dramatically more.
| Period | $10,000 @ 6% Simple | $10,000 @ 6% Compound (Monthly) | Difference |
|---|---|---|---|
| 1 year | $10,600 | $10,617 | $17 |
| 5 years | $13,000 | $13,489 | $489 |
| 10 years | $16,000 | $18,194 | $2,194 |
| 20 years | $22,000 | $33,102 | $11,102 |
| 30 years | $28,000 | $60,226 | $32,226 |
The Rule of 72
The Rule of 72 is a quick shortcut to estimate how long it takes money to double at a given interest rate. Simply divide 72 by the annual rate:
| Interest Rate | Years to Double |
|---|---|
| 2% | 36 years |
| 4% | 18 years |
| 6% | 12 years |
| 8% | 9 years |
| 10% | 7.2 years |
| 12% | 6 years |
Frequently Asked Questions
Simple interest is calculated only on the original principal amount, never on accumulated interest. Formula: I = P × R × T. It is the most straightforward form of interest and is used for short-term loans, bonds, and some savings products.
Simple Interest = Principal × Rate × Time (I = P × R × T). Future Value = Principal + Interest = P(1 + RT). Rate should be in decimal form (6% = 0.06) and Time should be in years. For months, divide by 12; for days, divide by 365.
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all accumulated interest — so your interest earns interest. Over 30 years, $10,000 at 6% simple interest grows to $28,000. The same amount at 6% compound interest (monthly) grows to over $60,000.
Simple interest is used for short-term personal loans, auto title loans, US Treasury bills, some bonds, and certain installment loans. Most long-term financial products — mortgages, credit cards, savings accounts, and investment accounts — use compound interest instead.
Convert to years first: months ÷ 12 = years; days ÷ 365 = years. Our calculator handles this automatically — just select Months or Days and enter your time value. For example, 180 days = 180/365 = 0.4932 years; $10,000 at 6% for 180 days = $10,000 × 0.06 × 0.4932 = $295.89 interest.
The Rule of 72 estimates how long it takes to double your money: divide 72 by the annual interest rate. At 6%, money doubles in approximately 12 years. At 9%, approximately 8 years. It works best for rates between 2% and 20% and is accurate within about 1–2 years of the exact answer.