Skip to main content

Simple Interest Calculator

Calculate simple interest, total amount and monthly interest. Supports fractional terms with year-by-year breakdown.

Total amount
$11,500.00
Interest $1,500.00
Total amount
$11,500.00
Principal $10,000.00 + Interest $1,500.00
Interest earned
$1,500.00
Monthly interest
$41.67

Balance & cumulative interest

  • 100% private

    All math runs in your browser. Nothing leaves your device.

  • Formula-verified

    Each calculator is unit-tested against authoritative sources.

  • Instant results

    Static-rendered pages. Sub-second loads on any device.

  • Works offline

    Visit once and it keeps working without an internet connection.

How to use the Simple Interest Calculator

  1. 1

    Enter your inputs

    Fill in the required fields at the top of the simple interest calculator. Each input shows a default placeholder so you can see the expected format and units before you type.

  2. 2

    Adjust assumptions and options

    Use the toggles, sliders and dropdowns to tailor the calculation to your situation — currency, country, time period, advanced options and any optional fields all change the result in real time.

  3. 3

    Review the result

    The result card updates instantly as you type. Read the headline number, then check the breakdown, chart and any per-period schedule to understand how the inputs combined to produce the answer.

  4. 4

    Compare scenarios

    Change one input at a time to see how sensitive the result is to that variable. This is how you build intuition: small changes that move the answer a lot are the levers that matter.

  5. 5

    Share or save your result

    Copy the shareable link to send the exact scenario to someone else, or use your browser to print or save the page. The URL preserves every input so the recipient sees the same answer you do.

What this calculator does

Simple Interest (SI) is interest charged or earned only on the principal amount, calculated as a flat percentage per unit of time. Unlike compound interest, it does not earn interest on previously accrued interest - so a 10-year simple-interest deposit earns linearly while a 10-year compound deposit grows exponentially. The gap between simple and compound widens dramatically with time: at 8% over 1 year, simple and compound differ by 0.3% (the difference is essentially within compounding frequency); over 30 years, simple earns 240% while monthly compounding earns 991% - more than 4x the gain. In modern finance, true simple interest is rare in long-term contracts - it shows up mostly in short-term loans (1-3 year auto loans, certain mortgages, treasury bills) and as a teaching/exam construct.

Formula

SI = P * r * t, A = P + SI = P * (1 + r * t)
SI
Total simple interest earned or paid
P
Principal (initial amount invested or borrowed)
r
Annual interest rate as decimal (8% = 0.08)
t
Time in years (can be fractional; 6 months = 0.5)
A
Maturity amount = Principal + Interest

Multiplicative and linear in both rate and time. Doubling either the rate or the time exactly doubles the interest earned. For partial periods, use fractional t directly: 18 months = 1.5 years, 7 months = 7/12 years. Monthly interest = P * r / 12 (independent of t). Reverse formulas: r = SI / (P * t), t = SI / (P * r), P = SI / (r * t). Critical distinction from compound interest: SI grows the balance linearly, CI grows it exponentially. They give the same result only for t = 1 year and identical r.

Worked examples

Example: $5,000 at 7% for 3 years

P = $5,000, r = 0.07, t = 3 years.

SI = 5,000 * 0.07 * 3 = $1,050 A = 5,000 + 1,050 = $6,050 Monthly interest = 5,000 * 0.07 / 12 = $29.17

Compare to compound interest at the same rate and term: A_compound = 5,000 * (1.07)^3 = $6,125. The compound version earns $75 more - a small premium for 3 years. Over 30 years at the same rate, simple earns $10,500 while compound earns $33,062 - 3x as much.

Example: reverse-solve for rate

You lent $10,000 for 4 years and got back $13,200. What rate did you earn?

SI = 13,200 - 10,000 = $3,200 r = SI / (P * t) = 3,200 / (10,000 * 4) = 0.08 = 8% per year

You earned 8% per year on a simple-interest basis. If the original contract was actually compound interest, the equivalent compound rate would be slightly lower: (13,200 / 10,000)^(1/4) - 1 = 7.21% CAGR.

Example: short-term loan (6 months)

A short-term business loan of $25,000 at 12% per year for 6 months on a simple-interest basis.

SI = 25,000 * 0.12 * 0.5 = $1,500 Total repayment after 6 months = $26,500 Effective monthly cost = $250, or 1% of principal per month

For short tenors like this, simple and compound interest produce nearly identical results - the difference at 12% for 6 months is just $44 ($1,544 vs $1,500). This is why short-term lending products often use simple interest - it's easier to communicate and the gap to "true" compound is negligible.

Common use cases

  • Short-term personal or business loans (under 5 years) where simple interest is used by contract
  • Calculating interest on savings accounts in jurisdictions that use simple interest (some Asian and African markets)
  • Computing accrued interest on bonds between coupon dates (day-count fraction times annual coupon)
  • Homework and exam questions in school/college finance and mathematics courses
  • Treasury bill / commercial paper yield calculations - typically quoted as discount yield using simple-interest math
  • Late payment fees - many contracts specify "X% per year simple interest" on overdue invoices
  • Comparing the bare minimum return on a deposit vs a compounding alternative to quantify the compounding benefit
  • Auto loans in the US - most are simple-interest loans, where extra payments directly reduce principal

What affects the result

  • Whether the contract specifies simple or compound interest - critical, and frequently misread
  • Day count convention - 30/360, actual/360, actual/365 all give slightly different t values for partial periods
  • Whether the rate is per annum, per month or per period - misread units are a major source of error
  • Whether the loan has any fees that should be added to principal for true cost (effective rate may differ from nominal)
  • Compounding frequency comparison - even monthly compounding can differ materially from "simple" over long horizons
  • Tax treatment - simple interest is typically taxed the same as compound interest in the year accrued
  • Pre-payment terms - simple-interest loans usually allow pre-payment to reduce future interest pro-rata; some have penalties
  • Inflation - real simple interest = nominal rate - inflation (approximately); long-horizon simple-interest deposits may have negative real returns

Tips

  • Always confirm whether a contract uses simple or compound interest - search for the phrase explicitly
  • For short-term lending (under 1 year) the difference between simple and compound is negligible; for long-term, demand compound
  • Convert all rates and times to consistent units before calculating - usually decimal annual rate and years
  • Use simple interest for any homework/exam problem unless it explicitly specifies compounding
  • For auto loans labeled "simple interest" in the US, pay extra at the start of the month to reduce the principal base for that month's interest
  • For savings, prefer compound interest accounts - the difference can double or triple your money over decades
  • When borrowing, prefer simple interest if rates are equal - your interest cost is lower than the compound equivalent
  • For Treasury bills and short bonds, accept that the quoted "yield" uses simple-interest discount math, not annual compounding - the bond-equivalent yield is slightly higher

Mistakes to avoid

  • Confusing simple interest with compound interest - the gap becomes enormous over long horizons
  • Using the rate as a percentage instead of decimal - 8 instead of 0.08 produces 100x error
  • Forgetting to convert months to years - 8% * 18 months = 144% if you forget; correct is 8% * 1.5 = 12%
  • Adding fees to interest separately - lenders typically include fees in the effective rate; comparing nominal rates without fees is misleading
  • Assuming "simple interest" on a savings account when the bank actually compounds daily (most US banks)
  • Using simple interest for long-term planning - over 20+ years the gap to compound interest is 2-4x; never use SI for retirement projections
  • Forgetting fractional years - "interest for 200 days" is t = 200/365, not t = 200
  • Not differentiating "simple interest loan" (US auto loan style, where interest accrues only on outstanding principal) from "flat interest" (where interest is calculated on original principal for entire term)

Frequently asked questions

What's the difference between simple and compound interest?

Simple interest is calculated only on the original principal: SI = P * r * t. Compound interest is calculated on principal + accumulated interest: A = P * (1 + r)^t. For 1 year they're identical. For 30 years at 8%, simple turns $10K into $34K, compound (annually) turns it into $100K.

When is simple interest actually used?

Short-term loans (under 5 years), most US auto loans, bond accrued interest calculations between coupon dates, Treasury bill discount yield, late-payment penalties in contracts, and almost all introductory finance/math textbook problems. For long-term lending, savings or investment products in modern markets, compound interest is standard.

How is monthly simple interest calculated?

Monthly interest = P * r / 12, where r is the annual rate. For $20,000 at 9% annual: monthly = 20,000 * 0.09 / 12 = $150. This is independent of total time - same dollar amount whether the loan is 1 year or 5 years (as long as principal stays constant).

How do I solve for time or rate?

Rearrange SI = P * r * t. To find rate: r = SI / (P * t). To find time: t = SI / (P * r). To find principal: P = SI / (r * t). For a maturity-amount-based reverse calculation: t = (A - P) / (P * r), etc.

Can simple interest be negative?

In standard contracts, no - rate and time are positive, so SI is non-negative. In theoretical accounting models (e.g., simple interest discount), a negative SI can represent prepaid interest or a discount. The calculator doesn't accept negative rates by design.

Is simple interest used on credit cards?

No - credit card interest is virtually always compound, typically calculated daily and added monthly. The "APR" displayed is an annual rate, but the effective annual rate (EAR) is higher due to daily compounding. A 24% APR card has an EAR of roughly 27%.

What is flat interest vs reducing balance?

"Flat interest" charges interest on the ORIGINAL principal for the entire loan term - this is what people often call simple interest in loan contexts. "Reducing balance" (the modern standard) charges interest on the OUTSTANDING principal at any time. A 10% "flat" 5-year loan has an effective rate near 18% on a reducing-balance basis - the gap is huge and often hidden in marketing.

How does simple interest apply to bonds?

Bond coupon interest accrued between coupon dates is calculated on a simple-interest basis using the bond's day-count convention (30/360, actual/365, actual/actual). This is the "accrued interest" you pay when buying a bond between coupon dates. The bond's yield to maturity, by contrast, uses a more complex compound-interest calculation.

Last reviewed:

Sources & references

Related Calculators

Mortgage Calculator

Calculate monthly mortgage payments including taxes, insurance and PMI. See full amortization schedule.

Calculate

UK Take-Home Pay Calculator

Calculate your UK 2025/26 take-home pay after income tax, National Insurance, student loan and pension.

Calculate

India Salary Calculator

Convert CTC to monthly in-hand salary. Computes income tax under new and old regime, EPF, professional tax.

Calculate

Australia Tax Calculator

Calculate Australian take-home pay for FY 2025-26 with ATO rates, Medicare Levy and Superannuation.

Calculate

US Income Tax Calculator

Federal + state + FICA take-home pay calculator for 2025. All 50 states + DC, filing status, 401(k) and HSA pre-tax support.

Calculate

Canada Income Tax Calculator

Federal + provincial + CPP + EI take-home pay calculator for 2025. All 13 provinces and territories with verified CRA brackets.

Calculate