Simple Interest Calculator
The original financial formula: I = P × r × t. Simple interest shows up in more real-world lending than most people realize — car loans, personal loans, Treasury bills. Know the difference between this and compound interest, and you know which side of the transaction you want to be on.
Calculate simple interest
The formula — and why it is simpler than it looks
Simple interest calculates interest on the original principal only. Interest never earns interest. $10,000 at 6% for 3 years earns 10,000 × 0.06 × 3 = $1,800. Every year adds an identical $600. The calculation is the same whether you check it after year one, year two, or year three — the interest grows in a perfectly straight line. No surprises, no acceleration.
Compare that to compound interest at the same 6% for 3 years: $10,000 grows to about $11,910 — roughly $110 more. The gap is interest-on-interest, and it widens meaningfully with time and higher rates. At 6% for 30 years: simple interest returns $28,000 total; compound interest returns about $57,435. The formula is humble, but the long-run difference is enormous.
Where simple interest actually shows up in your life
Auto loans: Most US car loans accrue simple daily interest on the outstanding balance. Each payment covers accrued interest first, then whatever remains reduces principal. This means paying a few days early genuinely reduces your total cost — because less interest has accrued since your last payment. This is different from credit cards, which compound daily and are much more expensive for carrying balances.
Personal loans: Most fixed-rate personal loans use simple interest amortization. Same daily-interest structure as auto loans. Extra principal payments reduce balance immediately and cut future interest accrual.
Mortgage loans: Standard US mortgages use simple daily interest on the declining balance. Interest is collected monthly in your payment. Paying your mortgage even a week early can save a small but real amount in accrued daily interest over a 30-year loan.
Treasury bills: T-bills are sold at a discount and redeemed at face value — the implied interest is simple, not compounded, calculated on the face value and term.
Short-term lending and personal notes: Loans between individuals and many short business notes use simple interest because it is transparent — everyone can verify the math without a spreadsheet.
Simple vs. compound — know which side you are on
The rule is important: as a borrower, you want simple interest. Your cost grows linearly, and early payments hit principal directly. As a saver or investor, you want compound interest — exponential growth beats linear at any rate, given enough time.
5 years at 8%: Simple returns $14,000; compound returns $14,693. Difference: $693.
10 years at 8%: Simple returns $18,000; compound returns $21,589. Difference: $3,589.
20 years at 8%: Simple returns $26,000; compound returns $46,610. Difference: $20,610.
The gap starts small and becomes enormous. This is why credit cards — which compound interest daily on your balance — are so expensive to carry. The stated APR and the actual yearly cost diverge dramatically as balances persist.
APR vs. APY — what compounding does to the stated rate
APR (Annual Percentage Rate) is the stated rate before compounding frequency is applied. APY (Annual Percentage Yield) is the true yearly return including compounding. A savings account paying 5.00% APR compounded daily has an APY of about 5.13%. When comparing savings accounts, always compare APY to APY. A bank advertising 5% interest might mean APR; another advertising 5% APY is giving you the accurate comparable figure.
Frequently asked questions
What is simple interest?
Interest calculated on the original principal only — interest never earns interest. Formula: I = P × r × t. The total grows in a straight line, not exponentially. Good for borrowers; less effective for long-term savers than compound interest.
Is my car loan simple or compound interest?
Almost certainly simple daily interest. Each payment covers accrued interest since the last payment, then reduces principal. This means paying early genuinely saves money. Read your loan agreement to confirm, but most US auto loans work this way.
How do I calculate for months or days?
Convert to years: 6 months = t = 0.5. 90 days = t = 90/365 which is approximately 0.2466. Some lenders use 360 days instead of 365 (banker's convention), which slightly increases the charge. Your loan documents will specify the day-count convention.
Why is simple interest better for borrowers?
It grows linearly instead of exponentially. At the same stated rate, simple interest always costs less than compound interest over the same period. Early or extra payments hit principal directly without penalty, reducing future interest accrual immediately.
Does APY include compounding but APR does not?
Yes. APR is the stated rate before compounding. APY reflects the actual annual return after accounting for compounding frequency. Always compare savings products using APY — it is the honest number. A 5% APR compounded daily equals about 5.13% APY.
What real-world loans use simple interest?
Auto loans, most personal loans, mortgage loans (on the daily accruing balance), and many student loans. Treasury bills use simple interest for discount calculations. Credit cards use compound daily interest, which is why carrying a balance is far more expensive than the stated rate suggests.
Simple interest vs. compound interest — the core difference
Simple interest calculates on the original principal only: Simple Interest = P × r × t. Borrow $5,000 at 8% simple interest for 3 years: interest = 5,000 × 0.08 × 3 = $1,200 total, regardless of unpaid balance timing. Compound interest accrues on the growing balance: same $5,000 at 8% compounded annually for 3 years = $5,000 × (1.08)³ − $5,000 = $1,298 interest — $98 more, with the gap widening over longer periods. Most real-world loans and investments use compound interest; simple interest appears primarily in short-term and informal arrangements.
Where simple interest actually appears
- Promissory notes and private-party loans: Easy to calculate manually — common in informal loans between family members or private real estate transactions in Nevada
- Many auto loans: Auto loans typically use simple interest that accrues daily on the outstanding balance. Paying late allows more interest to accrue before each payment; paying early reduces the accrual period.
- Short-term business notes: 30, 60, 90-day business loans use simple interest for transparency
- US Treasury bills: T-bills under one year are priced on a simple interest discount basis
Real scenarios
Scenario 1 — Private party loan, Las Vegas: Family member lends $12,000 for down payment supplement at 5% simple interest, 2 years. Total interest: 12,000 × 0.05 × 2 = $1,200. Monthly repayment: $13,200 ÷ 24 = $550/month. Clear, simple, verifiable — which is why simple interest is common for family loans and private transactions in Nevada.
Scenario 2 — Auto loan timing: $18,000 at 7% simple interest (daily accrual), 48 months. Daily rate: 7% ÷ 365 = 0.01918%. If you pay 5 days late every month: 5 × $9.46/day × 48 months = $2,270 extra over the loan term. On daily-accrual auto loans, payment timing genuinely matters — unlike mortgages with fixed monthly accrual and grace periods.
Frequently asked questions
Do mortgages use simple or compound interest?
Mortgages use compound interest — calculated monthly on the outstanding balance. Each month, interest is charged on the remaining principal (which decreases as you pay). The total interest over 30 years far exceeds simple interest on the original balance. A $400,000 mortgage at 6.5% over 30 years costs over $510,000 in total interest. See the Mortgage Calculator.
When is simple interest better than compound for a borrower?
Simple interest is better for the borrower when rates and terms are equal — you pay less total. However, you rarely get to choose: the interest method is set by the loan product. Simple interest is standard for short-term informal arrangements; compound interest is standard for all long-term financial products.
How do I calculate total interest on a simple interest loan?
Total interest = Principal × Annual Rate × Years. For a 30-month loan: Years = 30/12 = 2.5. Monthly payment = (Principal + Total Interest) ÷ Number of Months. This assumes true simple interest on the original principal — most amortizing loans calculate monthly on the declining balance, which is slightly different but close for shorter terms.
Why does paying my auto loan late cost more than my mortgage late?
Many auto loans accrue interest daily on the outstanding balance (simple interest method). Every day between your last payment and next payment, interest accumulates. Mortgage interest, by contrast, is calculated on a set monthly schedule — paying on day 5 vs. day 10 of the grace period costs the same. This is why auto loan payment timing matters more than mortgage payment timing (within grace period).
Disclaimer: This calculator provides general educational information only. Actual loan interest calculations depend on lender methods and applicable terms. This does not constitute financial advice. Stanley King (NV BS.0143719) is not a licensed financial advisor.