Simple Interest Calculator I = Prt
Calculate interest, principal, rate or time using the simple interest formula. Instant, step-by-step results.
| Variable | Value |
|---|
Interest earned on $1,000 at various rates and time periods:
| Principal | Rate | 1 Year | 3 Years | 5 Years | 10 Years |
|---|---|---|---|---|---|
| $1,000 | 3% | $30 | $90 | $150 | $300 |
| $1,000 | 5% | $50 | $150 | $250 | $500 |
| $1,000 | 7% | $70 | $210 | $350 | $700 |
| $5,000 | 5% | $250 | $750 | $1,250 | $2,500 |
| $10,000 | 5% | $500 | $1,500 | $2,500 | $5,000 |
What Is Simple Interest?
Simple interest is a method of calculating interest where the amount is always based on the original principal. Unlike compound interest, simple interest does not add earned interest back to the principal before the next calculation. This makes simple interest predictable and easy to calculate by hand or with a formula.
Simple interest is commonly used for short-term loans, car loans, savings bonds, and certain savings accounts. It is one of the foundational concepts in personal finance and banking.
The Simple Interest Formula
The formula for simple interest is I = Prt, where each variable represents a specific value:
- I = Interest earned or paid (in dollars or currency)
- P = Principal, the starting or original amount
- r = Annual interest rate expressed as a decimal (divide percentage by 100)
- t = Time in years (convert months by dividing by 12, days by dividing by 365)
To find total amount (principal plus interest), use the formula A = P + I, or equivalently A = P(1 + rt).
How to Calculate Simple Interest
Calculating simple interest requires three known values. Here is a step-by-step example:
- Identify the principal: $2,000
- Convert the rate to a decimal: 6% becomes 0.06
- Convert time to years: 18 months becomes 1.5 years
- Apply the formula: I = 2000 × 0.06 × 1.5 = $180
The total amount owed or received is $2,000 + $180 = $2,180.
Rearranging the Formula
You can rearrange I = Prt to solve for any unknown variable:
- Principal: P = I / (rt)
- Rate: r = I / (Pt)
- Time: t = I / (Pr)
Simple vs Compound Interest
Simple interest and compound interest differ in how interest accumulates over time. Simple interest applies only to the original principal throughout the entire term. Compound interest applies to the growing balance, which includes previously earned interest added to the principal at each compounding period.
For short-term loans or investments, the difference between the two methods is small. Over long periods, compound interest grows significantly faster. Borrowers generally prefer simple interest loans, while investors typically benefit more from compound interest accounts.
When Is Simple Interest Used?
Simple interest appears in several real-world financial situations:
- Auto loans and personal loans with a fixed term
- US Treasury bills and some government savings bonds
- Installment loans where principal reduces with each payment
- Short-term business loans and bridge financing
- Everyday interest calculations in school and academic settings
Frequently Asked Questions
The simple interest formula is I = Prt, where I is interest, P is principal, r is the annual interest rate as a decimal, and t is time in years.
To calculate simple interest, multiply the principal by the annual rate as a decimal and by the time in years. For example, $1,000 at 5% for 3 years: I = 1000 × 0.05 × 3 = $150.
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus accumulated interest from previous periods, so it grows faster over time.
Rearrange the formula to P = I / (rt). Divide the interest amount by the product of the rate as a decimal and time in years to find the original principal.
In the formula I = Prt, P stands for principal (the starting amount), r stands for the annual interest rate as a decimal, and t stands for time measured in years.