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Tool 7

Investment calculators · Explicit time conventions

Simple Interest Calculator

Created by Arunangshu DasReviewed

Calculate interest on an unchanged principal. Choose years, model months or supplied days, inspect the time fraction and separate interest from ending value.

A fixed-principal educational model, before fees and taxes. No compounding, changing balances, forecast or personalized financial advice.

Preparing calculator…

Principal, annual simple rate and time convention

Keep the time basis visible

Interest is calculated on an unchanged principal only. This is not compounding, a repayment schedule or a forecast.

Formatting only; no currency conversion or tax rules.

One nonnegative amount, unchanged for the whole term. Explicit zero is allowed; blank is not zero. Maximum 10^15.

Enter −100% through 1000% as an annual simple percentage, not a monthly rate, APY/AER or certified APR. Use an ordinary minus sign for a negative rate.

Changing units clears duration so 2 years cannot silently become 2 days. Months are equal fractions of a model year, not inferred calendar dates.

Used only for supplied days. This is not date counting, 30/360 date adjustment or actual/actual leap-year accrual. Confirm your agreement’s convention.

0–100 model years, including decimals. Use ungrouped values with a point and at most 12 fractional places.

Simple interest amount

Enter all three numeric assumptions, or load the verified six-month example.

50 significant decimal digits internally; half-even currency display at 2 places. Tiny nonzero amounts are marked rather than silently shown as zero. Inputs remain in browser memory. No per-period posting or cent-rounding rule is assumed.

Check the time fraction first

Six months means 6/12 of a model year. Supplied days require a stated divisor; choosing 360 does not transform dates using 30/360 rules.

Read the complete simple interest guide

Formula and methodology

I = P × R × d / (100b)
A = P + I = P × (100b + Rd)/(100b)
b = 1 for years, 12 for model months, or 365/360 for supplied days

Interest I=P×R×d/(100b). Principal plus interest A=P+I. The original principal does not grow inside the interest calculation, and no interest is earned on earlier interest. The engine calculates I directly and evaluates A as P×(100b+Rd)/(100b), forming the small growth numerator before multiplying principal to protect near-total-loss scenarios.

Verified worked examples

For principal 1,000 at an annual simple 5% over six model months, time is 6/12=0.5 years. Interest is 1,000×5×6/(100×12)=25, and principal plus interest is 1,025. For 73 supplied days, the same principal/rate gives interest 10 with divisor 365, or approximately 10.13888889 with divisor 360 (10.14 at two-place display).

Inputs and interpretation

Choose the display currency, then enter fixed principal and annual simple percentage. Select the duration unit and re-enter duration after a unit change. For supplied days, explicitly choose 365 or 360 and enter whole days. Review the time fraction, interest and principal plus interest together. Clear or reload to remove the in-memory scenario.

Assumptions and limitations

One unchanged principal, fixed annual simple rate and supplied duration. Twelve equal model months make one year. Supplied days use exactly the chosen divisor, with no actual-date counting, calendar endpoint inclusion, 30/360 adjustment or actual/actual leap splitting. No per-period interest posting or contractual cent-rounding rule is inferred.

Supported domain and common mistakes

Principal from 0 through 10^15; annual simple rate −100% through 1000%; at most 100 normalized model years. Decimal strings use an ungrouped point, up to 12 fractional places and 48 characters. Days must be integer-valued. No extra cash flows, inverse solving, compounding, loan amortization, certified APR/APY/AER, fees, taxes, inflation or FX. Negative ending balances are outside this initial model.

Using 6 as years when the intended term is six months multiplies the interest by 12. Supplying 5 as an annual rate when the quote is monthly changes its meaning. Changing a day divisor is not the same as counting calendar days or applying 30/360 date adjustments. Prt on a fixed principal does not automatically model an installment loan with changing balances.

Frequently asked questions

Can I calculate simple interest for months or days?
Yes. Years use d/1, model months use d/12, and supplied whole days use d/365 or d/360 according to your explicit selection. Fractional years/months are supported; partial days and automatic date calculations are not.
Why does changing the duration unit clear my term?
A number such as 2 means very different things in years, months and days. Clearing duration asks you to confirm the intended term rather than silently reinterpreting it. Your principal and annual simple rate stay unchanged.
How do I choose between 365 and 360?
Use the divisor required by the problem or agreement; country selection does not choose it. These are fixed divisors for a supplied day count. Selecting 360 does not perform a 30/360 date adjustment, and 365 is not an actual/actual leap-year calculation.
Does this calculator count leap days between two dates?
No dates are entered or counted. If you supply 366 days and divisor 365, the year fraction is 366/365, slightly more than one model year. Actual agreements may use other conventions; verify them separately.
What happens with zero or negative interest?
Zero principal, rate or duration gives zero interest. Negative annual simple rates are supported within the stated bounds. Exact total loss may produce a zero ending balance; scenarios producing a negative ending balance are refused, not clipped to zero.
Can I use this for a loan with monthly repayments?
Not as a complete repayment calculation. This model keeps principal unchanged for the entire term. Payments, withdrawals, new advances, fees and changing balances need a separately defined schedule; this is not a certified APR or total-cost quote.
Does currency selection convert money or apply tax rules?
No. USD, GBP, CAD, AUD and INR change display formatting only. Keep one currency basis for the scenario. No exchange rates, tax allowances, inflation adjustment or bank-specific rounding are included.

Sources and educational scope

Educational fixed-principal arithmetic, not personalized financial advice, a forecast or a certified loan/account disclosure. Verify the agreement’s rate basis, balance changes, day-count method, fees, taxes and posting rules. No future return or suitability is promised.

Reviewed . AI-assisted content under operator editorial responsibility; no independent expert financial review is claimed.

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