Simple Interest Calculator
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Calculate interest using the simple interest formula I = P × r × t. Shows total interest earned and final amount.
Total Amount
$0
📐 Formula
I = P × r × t, where I = interest, P = principal, r = annual rate (decimal), t = time in years
How to Use the Simple Interest Calculator
Enter the principal
Input the original loan or investment amount — the starting balance before any interest applies.
Enter the annual rate
Input the interest rate as a percentage. For periods under one year, the calculator prorates the annual rate.
Set the time period
Enter duration in years. For partial years: 6 months = 0.5, 3 months = 0.25, 90 days ≈ 0.247.
Compare to compound
Note how the total diverges from compound interest over longer periods — this is why compound interest is so much more powerful for investment growth.
Simple Interest vs Compound Interest: The Key Difference
Simple interest calculates only on the original principal. Compound interest calculates on principal plus all previously accumulated interest. On a $5,000 principal at 8% for 3 years: simple interest = $5,000 × 0.08 × 3 = $1,200 interest, $6,200 total. Compound (annual): $6,298 — only $98 more over 3 years. Over 20 years: simple = $13,000; compound = $23,305 — a $10,305 difference on the same principal and rate. The divergence grows exponentially with time.
Where Simple Interest Is Used in Practice
Most US auto loans use daily simple interest on the outstanding balance — meaning early payments reduce interest charges, and late payments increase them, even within the same month. Short-term personal loans and some bridge loans also use simple interest. Student loans accrue simple interest during school, which then capitalises (converts to principal) at repayment start. Understanding which type of interest applies to a loan determines how to optimise your payment strategy — for simple-interest loans, even paying a few days early each month reduces total cost.
Simple Interest in Auto Loans: Pay Earlier, Pay Less
Most US auto loans use daily simple interest on the outstanding balance. This means every day you hold the loan, interest accrues on the current balance. Paying your car payment three days early each month eliminates three days of daily interest charges — a small but meaningful reduction over a 60-month loan. Conversely, paying three days late adds three days of interest. Unlike some mortgage structures, there is no fixed amortisation schedule that determines interest allocation — your actual payment date determines the exact interest and principal split for that payment.
How to Calculate Simple Interest by Hand: Worked Example
Simple interest formula: I = P × r × t. On $8,000 at 6% for 3.5 years: I = $8,000 × 0.06 × 3.5 = $1,680.00 in interest, for a total repayment of $9,680.00.
Compare the same numbers under annual compounding: A = $8,000 × 1.06³·⁵ ≈ $9,809.81, meaning $1,809.81 of interest — $129.81 more than simple interest over the same period. The gap exists because compound interest earns interest on previously earned interest, while simple interest only ever applies to the original principal. Over short periods and small principals the gap is modest; it widens quickly with longer terms or higher balances.
How do you calculate simple interest for a partial year?
Use a daily or per-diem rate: annual rate ÷ 365 × principal × number of days. On $18,000 at 5.5% for 40 days: daily interest = $18,000 × 0.055 ÷ 365 = $2.71/day; over 40 days that is $108.36. Auto loans and many personal loans use exactly this per-diem method, which is why paying even a few days early measurably reduces the interest charged on your next statement.
Where Is Simple Interest Actually Used?
Which loan types use simple interest in practice?
Most auto loans, many personal loans, and some short-term consumer loans are structured as simple interest amortized loans — interest for each period is calculated on the current outstanding balance using the per-diem method above, not on the original principal for the whole term. This means paying a few days early or making a payment slightly ahead of the due date reduces the interest portion of your next payment, since fewer days accrued interest on the pre-payment balance.
Does paying an auto loan early with simple interest actually help?
Yes, directly. Because interest accrues daily on the outstanding balance, any extra payment reduces principal immediately and every subsequent day's interest calculation uses the smaller balance — unlike some older "Rule of 78" loans (largely phased out but still worth checking for) which front-load interest and reduce the benefit of early payoff.
Why do bonds and CDs sometimes quote simple interest?
Short-term instruments (under one year) commonly quote simple interest because the compounding difference over such a short period is negligible, and it keeps the quoted yield easy to verify by hand — exactly the calculation performed above.
Frequently Asked Questions
Sources & Methodology
Calculations are based on the most current publicly available data from authoritative government and industry sources: