How interest on a fixed original principal adds up over time
Simple interest in this calculator grows in a straight line because every period's interest is based on the original principal, never on previously earned interest. It is a useful classroom and fixed-principal comparison model.
The visual divides your chosen term into five checkpoints and shows interest accrued by each one. Equal fractions of the term earn equal amounts because this model never changes the principal used in the calculation.
The five accrual checkpoints use the original principal throughout, making the straight-line growth visible. The final amount and earned interest are separate, with a clear warning about real loans that use declining balances.
How principal, annual rate and time in years set the interest
The three inputs multiply each other directly, so a change in any one moves interest in the same proportion. That straight-line behavior is what separates this model from compounding.
- Original principal
- The amount interest is always charged or earned on, even late in the term. Interest moves in exact proportion to it: 1,000 at 5% for 3 years earns 150, and 2,000 would earn 300. If the balance falls because a loan is being repaid, this fixed basis no longer applies; the Amortization Calculator follows a declining balance.
- Annual simple interest rate
- A yearly percentage typed as a whole number, such as 5 rather than 0.05. The difference is large: on 1,000 for 3 years, 5 gives 150 of interest but 0.05 gives just 1.50. A monthly or per-period rate must be converted to an annual figure first, or the result will be off by the same factor.
- Time in years
- Decimals are accepted, so six months is 0.5 and earns exactly half a year's interest, 25 in the example. Because time is linear here, 20 years at 5% only doubles 1,000 to 2,000, whereas annual compounding over the same period reaches 2,653.30. Convert months or days into years before entering them.
- Interest per full year
- The result also reports one year's interest, principal times the annual rate, which is 50 in the example. Multiplying it by the years entered reproduces the total, a quick way to check a quote that states yearly interest. It never changes across the term in this model.
Steps
- Enter the original principal in any one currency.
- Enter the stated annual percentage rate and a time in years; decimals such as 0.5 are allowed.
- Calculate to see interest earned, the final amount and how interest accrues across the term.
Reading tip: Read the final amount alongside interest earned. The chart shows accrued interest only; add the unchanged principal to any checkpoint if you need its total amount at that time.
When a fixed-principal interest figure is the right check
Learn the interest formula
Check a classroom example and see exactly how the annual rate and time multiply the original principal.
Compare fixed terms
Try different years at the same rate to see the straight-line change in interest under a fixed-principal assumption.
Inspect a quoted rate
Estimate the amount implied by a stated simple annual rate before checking a product's actual compounding and payment terms.
Separate principal from interest
See how much of the final modeled amount is the money entered and how much is earned interest.
How the calculation works
- P
- Original principal in one currency, held constant as the interest basis.
- r
- Annual simple interest rate entered as a percentage, not a decimal.
- t / x
- Total time / elapsed time in years; decimal years such as 0.5 are allowed.
- I / A
- Interest earned over the term / original principal plus that interest.
The chart's five checkpoints use the same fixed principal. It is a linear illustration, not a repayment or compounding schedule.
What every part means
- P
- Original principal in one currency, held constant as the interest basis.
- r
- Annual simple interest rate entered as a percentage, not a decimal.
- t / x
- Total time / elapsed time in years; decimal years such as 0.5 are allowed.
- I / A
- Interest earned over the term / original principal plus that interest.
Calculate it step by step
A 5% annual rate becomes 0.05 per year.
Use the same original principal P for every part of the term, including fractional years.
The final amount includes both your starting money and the interest calculated above.
Decimal arithmetic keeps the unrounded intermediate result. For the sample inputs, 1,000 × 0.05 × 3 = 150 interest and 1,000 + 150 = 1,150 final amount. Only display values are rounded to two decimal places.
See how Calculation Guide tests examples and handles calculation limits.
See it in practice
- Original principal
- 1000 currency units
- Annual simple interest rate
- 5 %
- Time
- 3 years
P = 1,000, r = 5%, t = 3 years: I = 1,000 × (5 ÷ 100) × 3 = 150; A = 1,000 + 150 = 1,150.
- Interest earned150
- Starting principal1,000
Interpretation & limitations
How to read your result
At a fixed rate, doubling the time doubles interest in this model, while the original principal remains unchanged. The interest chart is linear; it does not show actual payments, deposits or changing balances.
Where this estimate stops
No compounding, additions, withdrawals, fees or taxes are included. A decimal year is treated as a fraction of a year, without a day-count calendar. Real simple-interest installment loans often charge interest on the unpaid balance, which falls as principal is repaid; this fixed-principal result is not that loan schedule.
Frequently asked questions
01Does this interest compound?
No. Interest is always calculated from the original principal. Use Compound Interest when earned interest is added to the balance.
02Can I use this for a loan quote?
Only if its terms use a fixed original principal with this exact time convention. Many simple-interest loans calculate charges on the declining unpaid balance, so their payment schedule differs.
03What happens at a 0% rate or zero time?
Earned interest is zero and the final amount equals the original principal.
04Can I enter six months?
Yes. Enter 0.5 years. The calculation treats it as exactly half of an annual period, without counting specific calendar days.
05Why is the interest graph a straight staircase?
Each equal fraction of time adds the same interest because the calculation basis is the unchanged original principal.
06Why can compound interest give a larger result?
Compounding adds earned interest to the balance, which can then earn more interest. At positive rates and multiple periods, this can exceed fixed-principal simple interest.
07How do I enter a term measured in days?
Divide the days by the year basis your agreement uses and enter the result as years. On 1,000 at 5%, 90 days on a 365-day basis is about 0.246575 years and earns 12.33, while a 360-day basis gives exactly 0.25 years and 12.50. Keep enough decimal places; rounding 0.246575 to 0.25 quietly switches to the 360-day answer.