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📐 Math Simple Interest Calculator
📐

Simple Interest Calculator

Calculate interest, principal, rate or time — all 4 variables of I = P × r × t

$
%
yrs
Simple Interest
$0
Principal$0
Total amount (P + I)$0
Daily interest$0
Monthly interest$0
Principal
$0
Interest
$0
Total Amount
$0
Effective Rate
0%
I = P × r × t
$
%
yrs
Principal (P)
—
Original investment / loan
Total amount (P + I)$0
Interest paid / earned$0
P = I / (r × t)
$
$
yrs
Annual Interest Rate (r)
—
Per year
Total amount$0
Return on principal0%
r = I / (P × t)
$
$
%
Time Required (t)
—
To earn this interest
In months—
Total amount at end$0
t = I / (P × r)
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Simple vs Compound Interest Comparison

See the difference over time

$
%
yrs
📐 Simple Interest
$0
Interest$0
📈 Compound Interest
$0
Interest$0

Simple Interest Formula Explained

Simple interest is the most straightforward way to calculate the cost of borrowing or the return on lending money. Unlike compound interest, simple interest only accrues on the original principal — it never earns interest on itself. This makes it easier to calculate but less rewarding for savers (and cheaper for borrowers) over long periods.

Students learn it first because the formula is elegant. Financial analysts use it for short-term loans, Treasury Bills, and bonds. Teachers use it to introduce the concept of interest before explaining compounding. This calculator solves for any one of the four variables given the other three.

The Four Simple Interest Formulas

I = P × r × t (find interest)
P = I / (r × t) (find principal)
r = I / (P × t) (find rate)
t = I / (P × r) (find time)
Where I = interest, P = principal, r = annual rate (decimal), t = time in years.

Practical Example

$5,000 invested at 6% annual simple interest for 3 years:
I = $5,000 × 0.06 × 3 = $900 interest. Total = $5,900.
With compound interest (monthly): $5,000 × (1.005)^36 = $5,983 — $83 more.

When to Use This Calculator

Use the simple interest calculator for short-term loans, Treasury Bills, certain auto loans, and anywhere you need to understand interest costs without the complexity of compounding schedules. It's also the right tool for back-calculating interest rates when you know what you paid but not the rate.

  • Short-term personal loans: You lend a friend $3,000 at 5% per year for 8 months. Simple interest = $3,000 × 0.05 × (8/12) = $100. They owe $3,100. Use the Interest calculator mode.
  • Understanding Treasury Bills: T-Bills use simple interest (discount basis). A 26-week $10,000 T-Bill at a 5.2% discount rate earns approximately $260 in interest over 6 months.
  • Finding an unknown rate: You paid $640 in interest on a $4,000 loan over 2 years. Rate mode: r = $640 / ($4,000 × 2) = 8%. Now you know your actual rate — useful when reviewing old loan documents.
  • Comparing simple vs compound products: Use results from this calculator alongside the compound interest calculator to quantify exactly how much more (or less) compounding produces over your timeframe.

Step-by-Step Example: Calculating a 9-Month Bridge Loan

A small business takes a $15,000 bridge loan at 7.5% annual simple interest for 9 months (0.75 years). Interest = $15,000 × 0.075 × 0.75 = $843.75. Total repayment = $15,000 + $843.75 = $15,843.75. Monthly payment if paid in equal installments = $15,843.75 / 9 = $1,760.42/month. If this were a compound interest loan (monthly compounding), the interest would be slightly higher at $871.41 — $27.66 more. For short terms, the difference between simple and compound is small.

Frequently Asked Questions

What is the simple interest formula? ▼
I = P × r × t. Interest (I) equals Principal (P) times Rate (r) times Time (t). The rate must be in decimal form (6% = 0.06) and time in years. Total amount = P + I = P(1 + rt). Example: $2,000 at 5% for 4 years → I = $2,000 × 0.05 × 4 = $400 interest, total = $2,400.
What is the difference between simple and compound interest? ▼
Simple interest is calculated only on the original principal — it never grows. Compound interest is calculated on principal plus accumulated interest — it snowballs. For short periods (under 1 year), the difference is small. For 10+ years, compound interest can produce dramatically more. Einstein reportedly called compound interest the "eighth wonder of the world."
When is simple interest used in real life? ▼
Simple interest is used for: US Treasury Bills, some auto loans (especially older ones), short-term personal loans, some bonds, payday loans (though at egregious rates), installment loans (some), and for educational purposes. Most savings accounts, mortgages, credit cards, and investments use compound interest. Verify which type applies to any financial product you use.
How do I calculate simple interest per month? ▼
Monthly interest = Principal × (Annual Rate / 12). Or use the formula with time in months: I = P × (r/12) × months. Example: $8,000 at 9% annual for 6 months → I = $8,000 × (0.09/12) × 6 = $8,000 × 0.0075 × 6 = $360 monthly-accrued simple interest.
How do I find the interest rate from simple interest? ▼
Rate = I / (P × t). Rearrange the formula: r = Interest / (Principal × Years), then multiply by 100 for percentage. Example: $480 interest on a $4,000 loan over 2 years → r = $480 / ($4,000 × 2) = $480 / $8,000 = 0.06 = 6% per year. This is useful when you know what you paid but not the rate.
How long does it take to double money with simple interest? ▼
With simple interest, doubling time = 100 / (annual rate %). At 5%, doubling takes 100/5 = 20 years. At 10%, it takes 10 years. Compare to compound interest: the Rule of 72 gives 72 / rate = doubling time. At 10% compound: 72/10 = 7.2 years — significantly faster than 10 years for simple interest.
How does simple interest apply to car loans? ▼
Most US auto loans use simple interest — but they calculate it daily and apply your payment to interest first, then principal. This means making your payment a few days early reduces the principal balance sooner, cutting the interest accruing on the remaining balance. On a $25,000 car loan at 6.9% for 60 months, making payments 5 days early each month can save $150–$300 in total interest. The simple interest structure actually rewards extra or early payments more immediately than compound-interest loans.
What is add-on interest and how does it compare to simple interest? ▼
Add-on interest calculates the total interest upfront on the full principal, then adds it to the loan before dividing into payments — even though you're paying down the principal each month. This makes the effective APR nearly double the stated rate. Example: a $5,000 loan at "10% add-on" for 2 years → interest = $5,000 × 0.10 × 2 = $1,000, total $6,000, monthly payment $250. But the effective APR is about 18.5% because you're paying interest on the original balance even as you repay it. Always ask whether interest is simple or add-on before accepting any loan offer.
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