Simple Interest Calculator Widget

Add a simple interest calculator that handles days properly. Readers enter the principal, the rate and the time in years, months or days, pick a 365- or 360-day year, and see the interest next to what yearly compounding would have earned.

Finance Calculator Runs in your browser Free · no ads

Customize your widget

Theme
Auto follows the visitor's light/dark setting.
Style
Attribution on your page
Optional and entirely your choice. The exact line is shown in the code below; it links to the tool with rel="nofollow".
More options
Starting values
Leave blank to use the widget's defaults. Visitors can still change every value.

Live preview

Exactly what your visitors will see

Embed code

<iframe src="https://a2z.tools/embed/w/simple-interest-calculator" title="Simple Interest Calculator by A2Z Tools" width="100%" height="710" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>

A plain iframe. Works everywhere, including site builders that strip scripts. Adjust height if your content needs more room.

Works with

How it works

The widget uses the same engine as the A2Z Simple Interest Calculator. Interest is charged on the original principal only, so it grows in a straight line. Months are converted to years by dividing by 12, and days by the day-count basis you choose: 365 (actual), 360 (the banker's year used on many loans and money-market deals) or 366. The comparison line compounds the same principal once a year for the same fraction of years, which shows why simple interest is cheaper for a borrower over several years but slightly dearer over a few months.

Calculation method

  • I = P x r x t; P = principal, r = annual rate / 100, t = time in years
  • t: months / 12, or days / 365, 360 or 366
  • Total amount = P + I
  • Comparison: P x (1 + r)^t compounded once a year
  • Rounding: displayed to 2 decimals; t shown to 4 decimals

Worked examples

Three-year loan

Inputs: 10,000 at 5% for 3 years

Result: Interest 1,500.00; total 11,500.00; yearly compounding would give 11,576.25

76.25 more with compounding, the interest-on-interest.

90 days on a 360-day basis

Inputs: 10,000 at 5% for 90 days, 360-day year

Result: Interest 125.00; total 10,125.00

The same 90 days on a 365-day basis gives 123.29.

Eighteen-month note

Inputs: 2,500 at 8% for 18 months

Result: Interest 300.00; total 2,800.00; yearly compounding 2,805.92

18 / 12 = 1.5 years.

Limitations

  • Day counts are entered by the reader; the widget does not count days between two dates or handle leap years automatically.
  • No partial repayments: interest is on the full original principal for the whole period.
  • Statutory late-payment interest rules (reference rate plus a margin) are not built in; enter the resulting rate yourself.
  • Rates are annual; a rate quoted per month (common on pawn and gold loans) must be multiplied by 12 before entering.

Where publishers use it

  • School and college maths pages teaching I = PRT
  • Short-term lending, bridging loans and invoice-finance explainers
  • Accounting blogs calculating interest on late payments
  • Pages comparing simple and compound interest side by side
  • Pawnbroking, gold-loan and microfinance explainers where interest is quoted per month on the principal
  • Trade-credit and supplier-terms explainers pricing an early-settlement or late-payment charge

Questions

What is the difference between a 365 and a 360-day year?

A 360-day year (actual/360) makes each day's interest slightly larger: 90 days at 5% on 10,000 is 125.00 on a 360 basis and 123.29 on a 365 basis. Check your loan agreement for the basis it uses.

When is simple interest used?

Short-term loans, some car loans, certificates that pay out interest, and late-payment interest often use simple interest. Savings accounts and most investments compound.

Why is the compound figure lower for 90 days?

For less than a year, P(1 + r)^t grows more slowly than P(1 + rt): 10,000 at 5% for 90/360 of a year is 10,122.72 compounded against 10,125.00 simple. Over several years the order flips.

How do I enter 18 months?

Choose months and type 18: the widget uses 1.5 years, so 2,500 at 8% earns 300.00 of interest, 200.00 a year. Yearly compounding over the same 18 months would give 5.92 more.

Is flat-rate lending the same as simple interest?

Largely yes: a flat-rate personal or dealer loan charges simple interest on the original principal for the whole term even though you repay it gradually, which makes its true cost far higher than the quoted flat rate.

What are 'ordinary' and 'exact' interest?

Textbook names for the two day-count choices: ordinary interest divides the days by 360, exact interest by 365. Treasury, money-market and many commercial agreements state which one applies, often as actual/360 or actual/365.

How do I use it for overdue-invoice interest?

Enter the unpaid invoice total as the principal, the statutory or contractual annual rate, and the number of days overdue with a 365-day basis. Many late-payment rules use simple daily interest in exactly this way, but check which reference rate and margin your jurisdiction sets.

Cite or recommend this tool

If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.

A2Z Tools Simple Interest Calculator
https://a2z.tools/simple-interest-calculator

Related widgets

More Finance widgets →

Preview

CustomizeGet code