Free calculator
Simple interest calculator
Enter a principal, a nominal annual interest rate in percent, and a time as whole years plus months (0–11). We compute simple interest I = P × (R/100) × t and ending balance A = P + I on the same principal for the whole period—no compounding and no mid-period deposits. Illustration only, not a loan offer or tax advice.
When to use this calculator
Quick pencil checks when interest should not compound on prior interest—same use cases many textbook and interview problems assume.
- Estimate interest earned or owed on a single balance when the product or problem states simple interest and an annual rate.
- Compare mentally with compound growth using our compound interest tool when you need interest on interest or contributions.
- Copy the Sheets/Excel cells to match P, R%, and t in your workbook—keep currencies and units aligned with your model.
We keep one transparent path: fixed principal, constant nominal annual rate, and time expressed as years + months converted to fractional years.
Interest and balance
I = P × (R/100) × t and A = P + I, also written A = P (1 + (R/100) t). P is principal, R is the annual percent, t is years.
Years and months → t
We set t = wholeYears + wholeMonths / 12 with months in 0–11. The rate is always treated as per calendar year for that t—not a hidden daily or bank convention.
Not modeled on purpose
Compound interest, recurring deposits or withdrawals, solve-for unknowns, date-range day counts, EMI / level payment loans, and daily simple interest on a declining balance are out of scope for this page—see linked tools and FAQ.
Use the numbers as a teaching and spreadsheet alignment aid, not a substitute for issuer disclosures or professional advice.
When interest should compound or you add contributions, use the compound interest calculator instead.
For a fixed-rate loan with a constant payment and full schedule, use the amortization schedule calculator instead.
Google Sheets & Excel
Treat principal, annual rate %, and term in years as three cells. Simple interest is principal × (rate/100) × years; ending is principal + interest.
=A2*(B2/100)*C2A2 = principal; B2 = annual rate as a percent (e.g. 5); C2 = t in years (e.g. 1.5 for eighteen months). Result is currency in the same unit as A2.
=A2+D2D2 holds the interest from the previous cell (or use =A2*(1+(B2/100)*C2) in one step).
Frequently asked questions
What does this simple interest calculator do?
It estimates simple interest and ending balance from principal, nominal annual rate %, and time as years plus months. Interest applies only to the original principal for the whole period—no compounding.
What is the simple interest formula?
I = P × (R/100) × t, where P is principal, R is the annual rate in percent, and t is time in years. Ending balance is A = P + I, same as A = P (1 + (R/100) t).
Why must rate and time use the same “per year” story?
The annual % is defined per year. We convert your months into fractions of a year so R and t stay consistent. If you need a monthly simple rate on purpose, convert explicitly (see next question)—we do not silently switch units.
How is this different from compound interest?
Compound interest applies the rate to an updated balance (principal plus prior interest). Simple interest uses the original P every period. For growth with compounding, use the compound interest calculator on this site.
Is this the same as a “simple loan” payment calculator?
No. Many search results labeled “simple loan” are payment calculators with a level monthly payment and an amortizing balance. This page is not that—it is interest on a fixed principal only. For payments and a schedule, use the amortization schedule tool.
What about daily simple interest on loans?
Some lenders accrue simple interest daily on the outstanding balance, which declines as you pay principal. That is a different model from a single P for the whole horizon here. Treat this page as textbook I = Prt on one balance.
Can I type a monthly rate instead?
This tool expects an annual percent. If you truly have a simple monthly rate r_m and n months, you can set t = n/12 and R = 12 × r_m only when the problem defines it that way—otherwise you may double-count. When in doubt, keep annual R and t in years.
How does this relate to the rule of 72?
The rule of 72 is a doubling heuristic for compound growth. This page is linear simple interest—use the rule of 72 calculator when you want a quick compound doubling check alongside an exact line.
How is this different from the CD calculator?
The CD tool models compounding with an APY and a compounding frequency on a lump sum. Simple interest here does not reinvest interest into the balance. Pick the tool that matches the product or problem wording.
How do I match this in Google Sheets or Excel?
Use the copy cards: principal, annual %, and t in years in three cells, then =A2*(B2/100)*C2 for interest. In localized Excel, argument separators may be ;—use Formulas → Insert function to match your language pack.
Is this investment, tax, or lending advice?
No. It is a free educational calculator and not a recommendation to borrow, save, or invest, and not legal or tax advice for your situation.