Simple Interest Calculator | I=PRT
Calculate simple interest using I = P × R × T formula.
Solve for:
What Is the Simple Interest Calculator | I=PRT?
The Simple Interest Calculator solves for any of the four variables, interest (I), principal (P), annual rate (r), or time (t), using the formula I = P × r × t. Supports years, months, and days. A year-by-year breakdown shows how interest accumulates linearly over the loan or investment term.
- ›Simple interest grows linearly: equal interest each period (unlike compound interest which grows exponentially)
- ›Solve for any variable: given three, find the fourth
- ›Time units: enter years, months, or days, converted automatically
- ›Total amount A = Principal + Interest = P(1 + rt)
Formula
Simple Interest Formulas
Interest
I = P × r × t
Principal
P = I / (r × t)
Rate
r = I / (P × t)
Time
t = I / (P × r)
Total
A = P + I = P(1 + rt)
Units
r in decimal, t in years
How to Use
- 1Select what to solve for: Interest, Principal, Rate, or Time
- 2Enter the three known values in their fields
- 3Select the time unit (years, months, or days)
- 4Press Enter or click Calculate Simple Interest
- 5View the result, interest bar, and year-by-year breakdown
- 6Use preset scenarios (car loan, personal loan, etc.) as starting points
Example Calculation
Car loan: P = $15,000, r = 6% per year, t = 3 years
I = 15,000 × 0.06 × 3
I = $2,700 total interest
A = $15,000 + $2,700 = $17,700
Monthly payment ≈ $17,700 ÷ 36 = $491.67
Simple vs. Compound Interest
Simple interest on $10,000 at 5% for 10 years = $5,000 interest. Compound interest (annual) on the same = $6,288.95. The difference of $1,288.95 represents the "interest on interest" that compounds but simple interest never charges.
Understanding Simple Interest | I=PRT
Interest Rate vs. Term Reference
| Principal | Rate | Term | Interest | Total |
|---|---|---|---|---|
| $5,000 | 5% | 2 years | $500 | $5,500 |
| $10,000 | 6% | 3 years | $1,800 | $11,800 |
| $15,000 | 7% | 5 years | $5,250 | $20,250 |
| $20,000 | 4% | 4 years | $3,200 | $23,200 |
| $25,000 | 8% | 3 years | $6,000 | $31,000 |
| $50,000 | 5% | 10 years | $25,000 | $75,000 |
Frequently Asked Questions
What is simple interest and when is it used?
Simple interest is the straightforward way to charge or earn interest: a fixed percentage of the original amount, multiplied by time. It never grows exponentially.
- ›Car loans: most use simple interest on the unpaid principal balance
- ›Personal loans with fixed terms: interest is a flat percentage
- ›Treasury bills and some government bonds: quoted on simple interest basis
- ›Contrast: savings accounts and mortgages typically use compound interest
How do you convert annual rate to monthly or daily?
The formula I = P × r × t requires r and t in consistent units. The standard approach is to express r as an annual decimal rate and t in years, then convert as needed.
- ›Annual rate 6%, 3 years: I = P × 0.06 × 3
- ›Annual rate 6%, 36 months: I = P × 0.06 × (36/12) = P × 0.06 × 3
- ›Annual rate 6%, 180 days: I = P × 0.06 × (180/365) = P × 0.02959
- ›This calculator divides months by 12 and days by 365 automatically
What is the difference between simple and compound interest?
The gap between simple and compound interest widens with time and rate. For short periods or low rates, the difference is small. For long investments, compound interest significantly outperforms simple interest.
- ›$10,000 at 8% for 1 year: simple = $800, compound = $800 (same for 1 year)
- ›$10,000 at 8% for 5 years: simple = $4,000, compound ≈ $4,693
- ›$10,000 at 8% for 20 years: simple = $16,000, compound ≈ $36,610
- ›Rule of 72: compound doubles in 72÷8 = 9 years; simple takes 12.5 years to double
How do you solve for the interest rate?
Solving for rate is useful when a lender quotes a total payback amount without clearly stating the interest rate, a common practice in some short-term loans.
- ›r = I / (P × t), divide interest by principal times time
- ›Example: $450 interest on $5,000 over 1.5 years → r = 450/(5000×1.5) = 6%
- ›Watch for fees added to the loan that inflate the effective rate
- ›APR (Annual Percentage Rate) includes fees; the nominal rate may be lower
How do you solve for principal from interest?
This is useful in reverse-engineering loans, if you know the total interest charged and the rate and term, you can back-calculate what the original principal must have been.
- ›P = I / (r × t), divide interest earned by rate times time
- ›Example: $360 at 6% for 2 years → P = 360/(0.06×2) = $3,000
- ›Useful for checking if a quoted loan principal matches the interest charged
- ›Also used in forensic accounting to detect misreported loan amounts
What is the effective annual rate for a simple interest loan?
When payments reduce principal over time (as in amortizing loans), the effective cost is actually close to twice the stated rate, because you have, on average, only half the principal outstanding.
- ›Flat-rate loan at 10% for 2 years on $10,000: total interest = $2,000
- ›Effective APR ≈ 19.5% (because average outstanding balance ≈ $5,000)
- ›Declining balance (mortgage style) at 10% costs much less in interest
- ›Always compare loans on APR, not nominal rate, for a fair comparison
Is simple interest good for borrowers or lenders?
The answer depends on context, whether you are the borrower or lender, and whether you are focused on debt or savings.
- ›Borrower advantage: paying early reduces interest (no compounding penalty)
- ›Lender disadvantage: doesn't earn interest on interest in savings context
- ›For investments: compound interest accounts are strictly better over time
- ›For short-term loans (under 1 year): difference between simple and compound is small