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Compute simple interest and total amount on loans, deposits and investments with I = P × R × T.
Enter the principal amount, the annual interest rate and the time in years. The calculator applies the simple interest formula and shows the interest earned and the total amount.
Formula: I = P × r × t. Example: $10,000 × 5% × 3 years = $1,500 interest; total $11,500.
Quick answer: $10,000 at 5% simple interest for 3 years earns $1,500 in interest, for a total of $11,500. The formula is I = P × r × t (interest = principal × annual rate × years), and the total amount is A = P + I. So: 10,000 × 0.05 × 3 = $1,500. Enter your own numbers in the calculator above, then read the full guide below for step-by-step instructions, how simple interest works, the formula with fully worked examples, two detailed real-life scenarios, the five mistakes that produce wrong answers, and related tools.
The Simple Interest Calculator needs just three inputs — principal, annual rate, and time in years — and returns the interest earned and the total amount. Here is how to use it precisely.
Step 1 — Enter the principal (P). Type the starting amount of money: the loan amount you borrow or the deposit you invest. Example: for a $10,000 certificate of deposit, enter 10000. Do not include commas; the calculator formats the result for you.
Step 2 — Enter the annual interest rate (r) as a percentage. Type the rate exactly as quoted — 5 for 5%, 6.5 for 6.5%. The calculator converts it to a decimal internally (5% → 0.05). Double-check whether your quote is annual: a “1.5% per quarter” rate is 6% annually, and entering 1.5 would understate three years of interest by a factor of four.
Step 3 — Enter the time (t) in years. Use years, including fractions: 6 months = 0.5, 18 months = 1.5, 90 days ≈ 0.2466 (90 ÷ 365). Example: $10,000 at 5% for 3 years → interest = 10,000 × 0.05 × 3 = $1,500.00; total = $11,500.00.
Step 4 — Read both results. The calculator shows Total amount (principal + interest) and Interest earned separately, plus the formula with your numbers substituted — e.g., “I = P × r × t = $10,000.00 × 5.00% × 3 years.” Use the interest figure for cost comparisons and the total for payoff or maturity planning.
Step 5 — Compare scenarios. Run the numbers for different rates and terms before committing. Example: $10,000 at 5% for 3 years earns $1,500; at 6% it earns $1,800 — a single percentage point is worth $300. Stretching to 5 years at 5% earns $2,500. Small input changes, very visible output changes.
Step 6 — Know when simple interest does NOT apply. If your loan or investment compounds (most savings accounts, mortgages, credit cards), switch to the Compound Interest Calculator — simple interest will understate multi-year growth. Auto loans, some personal loans, and short-term notes are the classic simple-interest cases.
Simple interest is interest calculated only on the original principal — the initial sum borrowed or invested — never on accumulated interest. Each year (or period) earns the same dollar amount: at 5% on $10,000, every year adds exactly $500, whether it is year 1 or year 10. This linearity is simple interest’s defining trait, and it is captured in the time period variable t, the interest rate r, and the principal P.
Contrast this with compound interest, where each period’s interest is added to the balance and the next period earns interest on the larger amount — growth accelerates. On $10,000 at 5% compounded annually for 10 years, simple interest yields $5,000 (total $15,000) while compound yields $6,288.95 (total $16,288.95) — a $1,288.95 gap that widens every year. The maturity value (the final amount A) under simple interest is just P + I, a straight line; under compounding it is P(1 + r)t, a curve.
Where is simple interest actually used? Auto loans in the U.S. typically use simple interest computed daily on the remaining balance. Many short-term personal loans, student loan subsidies, and some bonds and Treasury bills quote simple-interest-equivalent yields. Retail “90-days-same-as-cash” financing and many intra-family loans also use it. What does not use simple interest: credit cards (daily compounding), mortgages (monthly amortization, which behaves like compounding), and savings accounts (compounding — compare via annual percentage yield, APY, which bakes compounding in).
The mathematics has deep roots: simple interest is humanity’s oldest formal financial calculation, appearing in Babylonian and Roman loan tablets. Its modern importance is practical rather than theoretical — it is the legally required disclosure basis for many consumer loans under truth-in-lending rules, because a borrower can verify I = P × r × t with a pocket calculator. When a lender quotes “7% simple interest,” the total cost is fully determined by three numbers you can see; when they quote a compounding product, you need the compounding frequency too.
Two formulas cover everything:
Interest: I = P × r × t
Total amount: A = P + I = P(1 + rt)
Where P = principal (dollars), r = annual rate as a decimal (5% = 0.05), t = time in years. Every other quantity can be rearranged: P = I ÷ (rt); r = I ÷ (Pt); t = I ÷ (Pr).
Worked Example 1. Invest $5,000 at 4% simple interest for 2 years. Step 1: convert the rate: 4% = 0.04. Step 2: I = 5,000 × 0.04 × 2 = $400. Step 3: A = 5,000 + 400 = $5,400. Each year earned an identical $200 — the hallmark of simple interest.
Worked Example 2. Borrow $12,000 at 6.5% simple interest for 5 years. Step 1: 6.5% = 0.065. Step 2: I = 12,000 × 0.065 × 5 = 12,000 × 0.325 = $3,900. Step 3: A = 12,000 + 3,900 = $15,900. The interest is 32.5% of the principal — which makes sense: 6.5% per year × 5 years = 32.5%. That cross-check (rate × years = interest as % of principal) catches most arithmetic slips.
Rearrangement check: if you earned $1,500 at 5% over 3 years, what was the principal? P = I ÷ (rt) = 1,500 ÷ (0.05 × 3) = 1,500 ÷ 0.15 = $10,000 — matching the calculator’s default example exactly.
Meet Bilal, 29, buying a used car. He borrows $18,000 at 7% simple interest for 4 years (48 months) — a typical U.S. auto loan structure where interest accrues daily on the declining balance but the total finance charge is well approximated by the simple-interest formula.
The math: I = 18,000 × 0.07 × 4 = 18,000 × 0.28 = $5,040 in total interest. Total repayment A = 18,000 + 5,040 = $23,040. Monthly payment = 23,040 ÷ 48 = $480.00. The interest represents 28% of the amount borrowed (7% × 4 years) — Bilal pays $1.28 for every $1.00 of car.
Bilal’s decision point: the dealer also offers 8.5% for 5 years (60 months). That alternative: I = 18,000 × 0.085 × 5 = $7,650; A = $25,650; monthly = $427.50. The longer loan cuts the payment by $52.50/month but costs $2,610 more in interest and keeps him paying a full extra year. Because his $4,200 monthly take-home pay absorbs the $480 payment at 11.4%, he takes the 4-year loan — a textbook case of using the calculator to price the term trade-off before signing.
Now meet Ayesha, 45, comparing a $25,000 three-year certificate of deposit at 4.5% simple interest against a compounding alternative.
Simple interest path: I = 25,000 × 0.045 × 3 = 25,000 × 0.135 = $3,375. Maturity value = $28,375. Each year adds an identical $1,125.
Compound alternative (annual compounding): A = 25,000 × (1.045)3 = 25,000 × 1.141166 = $28,529.16; interest = $3,529.16.
The compounding edge is $154.16 over three years — modest here, but it scales brutally with time and rate: over 10 years at 7%, simple interest on $25,000 yields $17,500 while annual compounding yields $24,178.75, a $6,678.75 gap. Ayesha’s rule: for terms under 2 years or rates under 5%, the simple-vs-compound difference is small enough that convenience wins; beyond that, always compare with the Compound Interest Calculator. Her CD choice: the compounding account, because $154.16 is $154.16.
Mistake 1 — Entering the rate as a whole number instead of a decimal. Computing 10,000 × 5 × 3 = $150,000 instead of 10,000 × 0.05 × 3 = $1,500 — a 100× error. It happens constantly when people do the math by hand or in a spreadsheet. Fix: always convert first — 5% → 0.05, 6.5% → 0.065 — or use the calculator, which handles the conversion.
Mistake 2 — Mixing time units. Using t = 18 for 18 months instead of t = 1.5 years inflates the answer 12×: $10,000 at 6% for “18” → $10,800,000 vs the correct $10,000 × 0.06 × 1.5 = $900. For days, divide by 365 (90 days = 0.2466 years); for months, divide by 12. Fix: convert every time input to years before multiplying.
Mistake 3 — Using simple interest where compounding applies. Projecting a savings account at “5% simple” for 10 years predicts $5,000 of interest on $10,000; annual compounding actually pays $6,288.95 — you would understate your money by $1,288.95. Fix: confirm the product’s interest type; if it compounds, use the Compound Interest Calculator.
Mistake 4 — Quoting the interest instead of the total. “The loan costs $3,900 in interest” is true but incomplete — the borrower repays $15,900, and affordability depends on the $15,900 (or the monthly slice of it), not the $3,900. Fix: always compute A = P + I and budget from the total.
Mistake 5 — Ignoring fees that raise the effective rate. A $10,000 loan at 5% simple for 3 years ($1,500 interest) with a $400 origination fee really costs $1,900 on $9,600 received — an effective rate of about 6.6%, not 5%. Fix: add fees to the interest before comparing offers, or compare by APR.
Simple interest is the foundation; these tools extend it. The Interest Calculator handles general interest problems, the Compound Interest Calculator shows what happens when interest earns interest — essential for savings and investments over multiple years — and the Loan Calculator converts any loan into monthly payments and amortization schedules. Start with the Simple Interest Calculator above for the basics, then graduate to compounding when the product requires it.
Entity: Simple Interest Calculator — computes interest (I) and total amount (A) for principal P at annual rate r over t years with no compounding. Core formulas: I = P × r × t; A = P + I = P(1 + rt); rearrangements: P = I ÷ (rt), r = I ÷ (Pt), t = I ÷ (Pr). Reference example: $10,000 at 5% for 3 years → I = $1,500.00; A = $11,500.00. Second example: $12,000 at 6.5% for 5 years → I = $3,900; A = $15,900. Key properties: linear growth (equal interest each period); interest as % of principal = r × t × 100. Simple vs compound: $10,000 at 5% for 10 years → simple $5,000 vs annually-compounded $6,288.95. Typical uses: auto loans, short-term personal loans, some bonds/T-bills, intra-family loans. Not used for: credit cards, mortgages, savings accounts (these compound). Input rules: r as decimal (5% = 0.05); t in years (months ÷ 12, days ÷ 365).
Interest calculated only on the original principal — I = P × r × t. Unlike compound interest, earned interest does not earn more interest.
Multiply principal × annual rate (as a decimal) × years. $10,000 at 5% for 3 years: 10,000 × 0.05 × 3 = $1,500.
Simple interest grows linearly; compound interest grows exponentially because interest compounds. Savings accounts compound; many short-term loans use simple interest.
P = I ÷ (r × t). If you earned $1,500 at 5% over 3 years: 1,500 ÷ 0.15 = $10,000.
Auto loans, short-term personal loans and some bonds use simple interest. Mortgages and credit cards compound instead.
Also try our Interest Calculator, Compound Interest Calculator and Loan Calculator.
Sources: Federal Reserve — consumer credit and interest rate data; U.S. Securities and Exchange Commission — investor guidance on interest calculations; Truth in Lending Act (Regulation Z) disclosure standards.