Simple Interest Calculator

Principal:

Enter dollars only, e.g. 15000 for a $15,000 quote or note.

Annual simple rate:

Use percent form, e.g. 6.25 for 6.25% per year.

% / yr

Term:

Enter a positive value and choose years, months, weeks, or days, e.g. 18 months.

Day-count basis:

Use Actual / 365 by default; choose Actual / 360 or 30 / 360 only when the quote says so.

Check the inputs:

Payout cadence:

Choose monthly, quarterly, semiannual, annual, or maturity-only checkpoints.

Start date:

Pick the quote start date in YYYY-MM-DD format, or leave blank to omit dates.

Interest withholding:

Enter 0-100; use 0 when you want gross and net interest to match.

Scenario label:

Use a short quote name such as Treasury note quote or Bank A 270-day offer.

Metric	Value	Copy
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Checkpoint	Coverage	Projected date	Period interest	Gross interest	Withheld	Net interest	Net total	Copy
{{ row.label }}	{{ row.coverage }}	{{ row.projectedDateLabel }}	{{ moneyLabel(row.periodInterest) }}	{{ moneyLabel(row.grossInterestAccrued) }}	{{ moneyLabel(row.withheldAccrued) }}	{{ moneyLabel(row.netInterestAccrued) }}	{{ moneyLabel(row.netTotal) }}

Priority	Action	Why it matters	Copy
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Marker	Value	Detail	Copy
{{ card.label }}	{{ card.value }}	{{ card.detail }}

No chart-ready checkpoints are available for the current inputs.

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Simple interest prices time without letting earlier interest become part of the balance. That makes it useful when a note, deposit, classroom problem, late-payment estimate, or short quote is meant to stay flat: the original principal is the base, the annual rate is the price of one year, and the term says how long that price applies.

The word "annual" does a lot of work in a simple-interest quote. A borrower may see a 9% rate on a 60-day advance, or a saver may compare a six-month certificate with a one-year offer, but the rate still has to be scaled to the fraction of a year actually used. A six-month quote at 8% is not an 8% return for the period. Before fees, tax effects, or withholding, it is roughly half that amount because the money is exposed for half a year.

Line diagram comparing flat simple interest with a curved compound-interest path.

Day-count basis is the detail that separates a casual estimate from a contract-shaped estimate. A daily quote can divide by 365, 360, or a 30-day-month convention. The headline percentage may be identical, but a smaller denominator gives a larger year fraction for the same number of calendar days. The difference can look tiny on a short, low-rate example and become meaningful when principal, rate, or term grows.

Flat interest is a poor fit for products that amortize, compound, charge recurring fees, capitalize unpaid interest, or change principal after payments. A loan can still be called a simple-interest loan while its outstanding balance changes after each payment, so read the agreement before treating a quick straight-line estimate as a payoff statement. The practical job is narrower: isolate the cost or earning created by principal, rate, time, basis, and withholding when those are the only moving parts.

Term	Plain meaning	Common mistake
Principal	The base amount used for the interest calculation.	Using principal plus unpaid interest when the quote is meant to stay flat.
Annual rate	The yearly percentage applied to the principal.	Reading a six-month result as though the full annual percentage was earned.
Term	The length of time the principal accrues interest.	Mixing months, days, and weeks without converting them to a year fraction.
Day-count basis	The convention used to turn days into part of a year.	Comparing offers that use different denominators as if they were identical.
Withholding	A portion removed from interest before the net amount is kept.	Treating the gross maturity amount as spendable cash when withholding applies.

How to Use This Tool:

Enter the quote in the same units used by the agreement, then compare the maturity amount with the schedule and warning messages before relying on the number.

Enter the Principal as a positive dollar amount. If the page asks you to check the inputs, fix the principal before reading the results.
Enter the Annual simple rate as a percent, such as 6.25 for 6.25% per year. Do not enter the decimal form.
Set the Term and choose years, months, weeks, or days. Check the converted years beside the selected day-count basis before comparing offers.
Choose the Day-count basis that matches the quote. Use Actual / 365 for a general estimate unless the agreement says Actual / 360 or 30 / 360.
Open Advanced when you need payout checkpoints, a start date, interest withholding, or a scenario label. The start date only affects projected checkpoint dates, not the interest formula.
Read Interest Snapshot first, then compare the Payout Ladder, Decision Guide, Guide Markers, and Accrual Curve when the schedule or sensitivity checks matter.
Copy or download the table, chart, or JSON output after the warnings, withholding value, and basis choice match the agreement you are checking.

If the result disappears or a warning appears, start with the required inputs: principal above zero, a non-negative annual rate, and a term above zero.

Interpreting Results:

Gross interest is the amount earned or owed before withholding. Gross maturity amount adds that interest to principal. When withholding is entered, net interest subtracts the withheld portion, and net maturity amount adds only the remaining interest to principal.

Daily accrual is an average under the selected basis, not a daily compounding rate. Gross yield on principal is interest for the entered term divided by principal, so it should not be read as annual percentage yield. The Accrual Curve should rise evenly because simple interest adds the same amount for equal fractions of time.

Verify the day-count basis before comparing two offers with the same headline rate.
Use the Payout Ladder as a checkpoint schedule, not as proof that interest is reinvested or paid out.
Treat high-rate and very-long-term warnings as stop-and-check signals because many real products use different math.
Use the Decision Guide and Guide Markers to see how a one-point rate move, one extra term unit, withholding, and basis changes affect the result.

Technical Details:

Simple interest has a linear shape because principal stays fixed during the calculation. The key technical step is the year fraction: the entered term is converted to years, the annual rate is converted from a percentage to a decimal, and the product of principal, rate, and time gives gross interest.

Formula Core:

I = P \times r \times t

A = P + I

N = P + I \times (1 - w)

P is principal, r is annual rate as a decimal, t is the year fraction, I is gross interest, A is gross maturity amount, w is withholding as a decimal, and N is net maturity amount.

For a $15,000 principal at 6.25% for 18 months, the year fraction is 1.5. Gross interest is 15,000 x 0.0625 x 1.5, or $1,406.25. With 10% withholding, $140.63 is removed from interest after cent rounding, leaving $1,265.63 of net interest and a net maturity amount of $16,265.63.

Basis or unit	Year-fraction rule	Result effect
Years	Entered years are used directly.	Two years means t = 2.0000 before rounding for display.
Months	Months are divided by 12.	Month terms are calendar-neutral estimates, so 18 months means t = 1.5000.
Weeks	Weeks become seven-day spans, then are divided by 365 or 360.	The selected day-count basis changes the year fraction.
Days with Actual / 365	Days are divided by 365.	Common for consumer-style daily accrual estimates.
Days with Actual / 360	Days are divided by 360.	The same day count produces slightly more interest than Actual / 365.
30 / 360	Months are treated as 30 days and years as 360 days.	Useful only when the agreement, worksheet, or market convention calls for it.

The schedule divides the final gross and net interest across the selected number of checkpoints. Each row uses a fraction of the total term, so a halfway checkpoint shows roughly half of the final simple interest. The chart samples the same checkpoint path and draws gross interest, net interest, and withheld interest as separate line series.

Derived value	How it is derived	Use with care when
Daily accrual	Gross interest divided by the rounded day count.	The quote compounds daily or uses lender-specific posting rules.
Monthly-equivalent accrual	Gross interest divided by total years times 12.	The product actually has unequal calendar months or payment dates.
Gross yield on principal	Gross interest divided by principal.	You need an annualized yield or annual percentage yield.
Rate-point impact	Principal times year fraction times 1.00%.	You are comparing quotes with different compounding or fee terms.
Basis shift	Interest difference between the selected basis and the alternate actual-day basis.	The agreement uses a convention outside Actual / 365, Actual / 360, or 30 / 360.

Displayed money values are rounded to cents. Percent displays can show extra decimals so the year fraction and yield are easier to audit. Inputs are bounded for stability: principal is capped at $1,000,000,000,000, annual rate at 1,000%, term at 1,000,000 units, and withholding from 0% to 100%.

Warning or validation	Reason to check it
Principal above 0 is required	A zero or negative base amount cannot produce a useful maturity estimate.
Annual rate cannot be negative	The calculation covers ordinary positive simple interest, not negative-yield products.
Term above 0 is required	No time means no meaningful accrual path.
Annual rates at or above 50%	High rates often signal fees, penalties, risk pricing, or terms that deserve closer review.
Terms of about 30 years or longer	Long horizons are commonly modeled with compounding, amortization, or contract-specific rules.

Privacy and Accuracy Notes:

The calculation uses the numbers entered in the browser. It does not look up a bank account, pull a credit report, or verify a contract. If you share or bookmark a URL after changing inputs, visible scenario values may be included in the URL, so avoid placing account numbers or private customer details in the scenario label.

Use the written agreement when a lender, bank, school, or agency specifies a day-count convention.
Do not use this result for amortizing loans, revolving credit, late fees, capitalized interest, or compounding investments.
Treat withholding as a planning adjustment only. Tax rules, exemptions, and reporting rules are outside the calculation.
Confirm currency, rounding, and payment timing before using the estimate in a contract or payoff request.

Worked Examples:

Eighteen-month deposit

A $10,000 principal at a 6.5% annual simple rate for 18 months has a year fraction of 1.5. Gross interest is $975.00, so the gross maturity amount is $10,975.00 before any withholding or product fees.

Forty-five-day quote

A $25,000 principal at 8.25% for 45 days on Actual / 365 produces about $254.28 of gross interest. On Actual / 360, the same principal, rate, and day count produces about $257.81 because the denominator is smaller.

Withholding check

If $975.00 of gross interest has 10% withholding applied, $97.50 is withheld and $877.50 remains as net interest. The net maturity amount is $10,877.50 because the principal is returned with interest after withholding.

FAQ:

Is simple interest the same as compound interest?

No. Simple interest applies the rate to the original principal. Compound interest adds earned interest back into the balance, so later interest can be calculated on a larger amount.

Why does Actual / 360 produce more interest than Actual / 365?

The same number of days is divided by 360 instead of 365. That creates a larger year fraction, which increases interest when principal and rate stay the same.

Does payout cadence change the final interest?

No. Payout cadence changes the checkpoint schedule and chart samples. It does not compound, reinvest, or subtract interim payouts from principal.

Can this estimate a loan payoff?

Only when the payoff is truly flat simple interest with no amortizing payments, late fees, capitalized interest, or changing principal. Most real payoff statements need lender-specific terms.

Glossary:

Principal: The original amount on which interest is calculated.
Annual simple rate: The yearly percentage applied to principal without compounding.
Year fraction: The term converted into part of a year for the simple-interest formula.
Day-count basis: The convention used to convert days, weeks, or some month assumptions into a year fraction.
Gross maturity amount: Principal plus gross interest before withholding.
Net maturity amount: Principal plus interest after the entered withholding percentage.

References:

What is the difference between a simple interest rate and precomputed interest on an auto loan?, Consumer Financial Protection Bureau.
12 CFR 1030.7 Payment of interest, Consumer Financial Protection Bureau.
Interest Rates Frequently Asked Questions, U.S. Department of the Treasury.