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Simple Interest Calculator

A free online simple interest calculator that computes interest between a From and To date, at a daily, weekly, monthly, or yearly rate, using the formula SI = P x R x T / 100. Includes a principal-vs-interest breakdown, a year-by-year growth table, and a simple-vs-compound interest comparison.

How the Simple Interest Calculator works

Enter your interest rate and the period it's quoted for (per day, week, month, or year), pick a From and To date, and an initial amount, then press Calculate. The calculator counts the number of days between your two dates — including both the From and To day — converts that into the number of rate periods elapsed, and applies the simple interest formula: SI = P × R × T ÷ 100.

How days convert into your rate's time period

A day counts as 1 period, a week as 7 days, an average month as 30.44 days, and an average year as 365.25 days — using average lengths (rather than exact calendar months or years) keeps the math consistent across leap years and different-length months. The total days between your two dates is divided by the period length to get the number of periods used in the formula.

Reading your results

The result panel shows the total interest as a percentage of your initial amount, the exact amount due to interest, and the final amount (initial amount plus interest). For date ranges spanning a full year or more, a year-by-year growth table is available, and a compound-interest comparison shows how much more you'd earn if the same amount compounded annually instead.

Common Mistakes to Avoid

Picking the wrong rate period

Solution:

If your rate was quoted "per month" but you select "Year" (or vice versa), the result will be off by an order of magnitude. Always match the period selector to how the rate was quoted.

Forgetting both dates are inclusive

Solution:

The From and To dates both count as full days, so a range from January 1 to January 31 is 31 days, not 30 — this adds one extra day of interest compared to an exclusive count.

Confusing simple interest with compound interest

Solution:

Simple interest is only ever calculated on the original principal — it doesn't grow faster over time the way compound interest does. Use the built-in comparison to see exactly how much more compound interest would earn on the same numbers.

Entering the rate as a decimal instead of a percentage

Solution:

Enter the rate the way it's usually quoted — e.g. 5 for 5%, not 0.05. The calculator already divides by 100 internally.

Choosing a From date that is after the To date

Solution:

The To date must be on or after the From date — the calculator won't produce a result until the range is valid.

Treating 'amount due to interest' as the full payoff amount

Solution:

That figure is the interest only — the final amount (what you actually owe or receive) is the initial amount plus the interest.

Frequently Asked Questions

Simple interest is SI = P × R × T ⁄ 100, where P is the principal, R is the rate for the chosen period, and T is the number of those periods elapsed. The final amount is the principal plus the interest.
Simple interest is calculated only on the original principal for the whole term. Compound interest is calculated on the principal plus any interest already earned, so it grows faster the longer the money is invested or borrowed.
The calculator counts every day from the From date through the To date, with both endpoints included (inclusive) — so a one-year range from January 1 to the following January 1 counts as 366 days, not 365.
It means both the start date and the end date are counted as full days of interest. This matches how interest periods are typically defined for statutory, legal, and loan-interest calculations.
Match whatever period your rate was quoted in. A savings account advertising "5% per year" should use Year; a short-term loan charging "2% per month" should use Month. Picking the wrong period will scale your result incorrectly.
The calculator shows a comparison against annual compound interest on the same principal, rate, and date range, so you can see the difference directly.
No — every calculation runs locally in your browser. Nothing you enter is uploaded, logged, or stored on any server.

Free Online Simple Interest Calculator

Calculate simple interest between any two dates at a daily, weekly, monthly, or yearly rate. Enter your rate, pick a From and To date, and an initial amount — the calculator counts the days between the two dates (inclusive) and works out the interest and final amount automatically.

What You Get

  • Interest and final amount calculated between any two calendar dates
  • Rate expressed per day, week, month, or year — whichever matches how your rate was quoted
  • Inclusive day counting, matching statutory and loan-interest conventions
  • A principal-vs-interest visual breakdown
  • A year-by-year growth table showing how interest accumulates over time
  • A quick comparison against what the same amount would earn under annual compound interest

Who Uses This Calculator

Anyone working out interest owed or earned over a specific date range — late-payment interest, short-term loans, fixed-deposit returns, or savings growth between two dates.