Simple Interest Calculator
A $1,000 balance at 5% simple interest for 3 years earns $150, so the ending total is $1,150. This simple interest calculator estimates interest from three inputs: principal, annual rate, and time in years. Use it when a loan, note, or deposit uses a flat non-compounding structure and you want the interest amount, total amount, and average monthly interest without writing the formula yourself. Because simple interest stays tied to the original principal, the growth pattern is linear. That makes this page useful for quick classroom checks, private loan estimates, short-term lending scenarios, and deposit comparisons where the terms clearly state simple interest instead of compound interest.
Quick answer
Simple interest uses the starting principal only, so the balance that earns interest does not grow from one period to the next.
What this tells you
- •Simple interest uses the starting principal only, so the balance that earns interest does not grow from one period to the next.
- •If the rate and term stay fixed, each additional year adds the same dollar amount of interest.
- •The tool converts the percent rate to a decimal, multiplies by the principal, then multiplies by time in years.
- •The total amount equals principal plus interest, which makes it easy to compare the cost of borrowing or the return on a deposit.
- •Monthly interest on this page is an average monthly equivalent across the full term, not a promise about how a bank or lender posts interest each month.
How to Use
- 1Enter the principal, which is the original amount borrowed or deposited before any interest is added.
- 2Enter the annual interest rate as a percentage. Type 7.5 for 7.5%, not 0.075.
- 3Enter the time in years. For partial years, use a decimal such as 0.25 for 3 months or 1.5 for 18 months.
- 4Click Calculate to view the total interest, final amount, original principal, and average monthly interest for the term entered.
- 5Review the result against your actual agreement if the account uses fees, changing rates, day-count rules, or compounding, because those terms can change the real amount due or earned.
How It Works
Formula
Simple Interest = Principal x Rate x TimeThe formula in `lib/formulas/simple-interest-calculator.ts` calculates interest as `principal x (ratePercent / 100) x years`. That means a 5% rate becomes 0.05 before the multiplication happens. After the tool finds the interest amount, it adds that interest back to the original principal to get the total amount. It also calculates average monthly interest by dividing the total interest by the number of months in the term. This method assumes a constant annual rate, a fixed original principal, and no compounding during the period you enter.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Three-year savings deposit
The formula is 1000 x 0.05 x 3 = 150, so the deposit earns $150 in simple interest. Add that to the original $1,000 principal and the ending amount is $1,150. The tool's monthly equivalent is $150 divided by 36 months, which rounds to $4.17.
Eighteen-month personal note
Convert 6.5% to 0.065, then multiply 2500 x 0.065 x 1.5 to get $243.75 in interest. Add the interest to the principal and the total payoff becomes $2,743.75. Because the term is 18 months, the average monthly interest is $243.75 divided by 18, which rounds to $13.54.
Two-year fixed-rate loan estimate
This example uses 5000 x 0.04 x 2 = 400, so the interest cost over the full two years is $400. Add the interest to the original $5,000 balance and the total amount is $5,400. Spread across 24 months, the average monthly interest works out to $16.67 after rounding.
Five-year deposit quote
Write 3.2% as 0.032 and multiply 12000 x 0.032 x 5 to get $1,920 of total interest. The final amount is the original $12,000 plus $1,920, which equals $13,920. Over 60 months, the average monthly interest is exactly $32.
Ninety-day short-term advance
A 90-day period is one quarter of a year, so the time value is 0.25. The interest is 800 x 0.09 x 0.25 = 18, which brings the total amount to $818. Since 0.25 years equals 3 months, the average monthly interest is $18 divided by 3, or $6.
Nine-month loan between friends
Jordan lends a friend $4,000 to cover a car repair and they agree on 5% simple interest, repaid in one lump sum after 9 months. Nine months is 0.75 of a year, so the interest is 4000 x 0.05 x 0.75 = $150 and the payoff is $4,150. Spread over 9 months that averages $16.67 of interest per month. If they count the actual 273 days instead, 273 divided by 365 is 0.748 years and the interest is $149.59, 41 cents less. Writing the rate, the day-count rule, and the due date into a one-page note keeps a friendly loan from turning into an argument.
Simple interest on $1,000 and $10,000 by rate and term
Interest earned or owed under I = P x r x t, with t in years and no compounding. Doubling the rate, the principal, or the term doubles the interest, so every cell scales in a straight line.
| Principal and rate | 1 year | 3 years | 5 years | 10 years |
|---|---|---|---|---|
| $1,000 at 3% | $30 | $90 | $150 | $300 |
| $1,000 at 5% | $50 | $150 | $250 | $500 |
| $1,000 at 7% | $70 | $210 | $350 | $700 |
| $1,000 at 10% | $100 | $300 | $500 | $1,000 |
| $10,000 at 3% | $300 | $900 | $1,500 | $3,000 |
| $10,000 at 5% | $500 | $1,500 | $2,500 | $5,000 |
| $10,000 at 7% | $700 | $2,100 | $3,500 | $7,000 |
| $10,000 at 10% | $1,000 | $3,000 | $5,000 | $10,000 |
Computed with the formula above, I = P x r x t with t in years. The same formula drives daily simple interest on many consumer loans, where t is the number of days divided by 365. Regulation Z (12 CFR part 1026) requires lenders to disclose the resulting finance charge, which is why loan paperwork shows a dollar figure and not just a rate.
Simple vs compound interest, day counts, and daily accrual
Simple interest only ever looks at the original principal. Compound interest adds each period's interest to the balance and charges or pays interest on that larger amount next time. On $10,000 at 5% for 10 years the simple interest in the table above is $5,000, while the same deposit compounded once a year would earn about $6,289. The gap is small in year 1 and widens every year, which is why the two methods feel interchangeable for short terms and very different for long ones.
Lenders and banks rarely count time in whole years. They convert days to a fraction of a year using a day-count convention, and two conventions are common in US lending. The 365-day convention divides the annual rate by 365 to get a daily rate. The 360-day convention divides by 360, which produces a slightly higher daily rate, and then usually charges that rate for every actual day. Both are conventions set by the contract, not by the formula, so read the agreement to see which one applies before you check a statement against this tool.
Daily accrual applies the formula one day at a time. Each day the lender multiplies the outstanding principal by the daily rate and adds the result to an interest bucket, and the next day it does the same thing again on the same principal. Payments go to the accrued interest first and then to principal. A $4,000 loan at 5% with a 365-day year accrues about $0.55 per day, so paying 10 days late costs roughly $5.48 more and paying 10 days early saves about the same. Deposit accounts in the US work on the same daily principle: Regulation DD requires institutions to calculate interest on the full principal in the account each day (12 CFR 1030.7).
Three limits matter when you compare this page with real paperwork. It never compounds, so any product that adds interest to the balance will cost or earn more than the figure shown. It has no fee field, while the finance charge a lender must disclose under Regulation Z includes fees imposed as a condition of the credit, not only the interest (12 CFR 1026.4). And it measures time in years, so a contract using a 360-day year or counting actual days will differ by a small, predictable amount from what you see here.
Common mistakes
- Entering the rate as a decimal in the percent field, such as 0.05 instead of 5, which makes the result 100 times too small.
- Typing months as whole years, such as entering 18 instead of 1.5 for an 18-month term.
- Assuming the monthly interest value is a lender's exact posted monthly charge instead of an average monthly equivalent for the full term.
- Using the tool for compound interest products where interest is added back to the balance and future interest grows from that larger amount.
- Ignoring fees, taxes, penalties, or required payments that may change the real payoff amount on a note or loan agreement.
Limitations
This tool assumes one original principal amount, one constant annual rate, and a term measured in years with no compounding. Any account that adds interest to the balance will earn or cost more than the figure shown. It has no fee field, and the finance charge a lender must disclose under Regulation Z includes fees imposed as a condition of the credit as well as interest (12 CFR 1026.4), so a real loan's disclosed cost can be higher than this estimate even at the same rate. It measures time in whole or fractional years rather than actual days, while lenders and deposit institutions accrue daily under a 365-day or 360-day convention set by the contract, and Regulation DD requires deposit interest to be calculated on the full principal each day (12 CFR 1030.7). It does not model changing balances, periodic payments, late fees, origination charges, taxes, or minimum finance charges, and it treats monthly interest as a simple average across the term.
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