Calculate simple interest
Find the interest and the final amount when interest never compounds: 1,250 at 4.8% a year for 2.5 years earns 150, for a final amount of 1,400. Free in your browser.
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Worked example
- Principal amount
- 1,250
- Annual rate
- 4.8
- Years
- 2.5
- Interest amount
- 150
- Final amount
- 1,400
This simple interest calculator finds both the interest earned and the final amount after a stated number of years. Enter the starting principal, an annual rate expressed as a percentage, and the length of time. The calculation applies the original principal throughout the full term, so interest does not compound or earn additional interest. It is useful for checking straightforward loans, savings projections, classroom exercises, and agreements that explicitly specify simple interest.
Enter the principal, annual rate, and time
Start with the principal, which is the original amount borrowed, invested, or deposited before any interest is added. Enter it as an ordinary nonnegative number in the currency or unit relevant to the agreement. Next, provide the annual rate as a percentage: enter 5 for five percent, not 0.05.
Finally, enter the duration in years. The years field accepts fractional values, so six months can be represented as 0.5 years when the underlying agreement uses that conversion. Keep all three inputs in a consistent context.
If the principal is in dollars, both the returned interest and final amount are in dollars; if it is in another currency, the same arithmetic applies without performing a currency conversion. The calculator rejects missing values, non-finite numbers, negative principal values, negative time, and especially a negative annual rate. A rate of zero is valid and produces no interest.
Review the rate convention in the source document before calculating, because monthly rates, decimal rates, and annual percentage rates are not interchangeable.
Understand the simple interest formula
Simple interest uses the formula interest = principal × annual rate × years, with the percentage rate divided by 100 before multiplication. For a principal of 1,000, a rate of 5%, and two years, the interest is 1,000 × 0.05 × 2, or 100. The final amount is then the original principal plus that interest, giving 1,100.
The defining feature is that every period uses the same original principal. Interest already earned never becomes part of the base for a later period. That makes the relationship linear: doubling the time doubles the interest, and doubling the rate also doubles it when the other inputs stay fixed.
Results use standard JavaScript numeric arithmetic and are not automatically rounded to cents. If a payment workflow requires a specific monetary rounding rule, apply the rule required by the contract or accounting system after receiving the result rather than assuming every currency and agreement uses two decimal places.
Know when simple interest is the right model
Use this calculation only when the product, note, or exercise explicitly applies simple interest to the original principal. It is common in educational problems and may appear in short-term lending, certain installment arrangements, or basic investment comparisons. It is not a substitute for compound interest, amortization, annual percentage yield, or a repayment schedule.
With compounding, previously earned interest joins the balance and can itself earn interest; with an amortized loan, payments reduce principal over time and each period may have a different interest charge. Fees, taxes, changing rates, day-count conventions, and irregular payment dates can also make a real agreement differ from this result. For partial years, confirm how the lender or institution converts days or months into years before entering a decimal duration.
The calculator deliberately performs one transparent formula and does not infer those business rules. That narrow behavior makes it useful as an independent check: compare its interest figure with a quoted simple-interest charge, document the three inputs, and investigate any difference instead of silently adjusting the calculation.
What you can do with it
Check a simple-interest loan quote
Compare the stated financing charge and total repayment amount with the values implied by the principal, annual percentage rate, and term.
Project non-compounding earnings
Estimate earnings when an account or agreement pays interest only on the original deposited amount.
Verify coursework and financial examples
Confirm the interest and final amount for a textbook problem without confusing a percentage rate with its decimal form.
FAQ
Can the time include part of a year?
Yes. The years input accepts fractional values such as 0.5, but you should use the year conversion specified by the relevant agreement.
Does this calculator compound the interest?
No. It always calculates interest from the original principal. Use a compound-interest calculation when earned interest becomes part of the balance.
How do I calculate simple interest?
Multiply the principal by the annual rate as a decimal and by the number of years. For 1,000 at 5% over two years that is 1,000 × 0.05 × 2, or 100 in interest, so the final amount is 1,100.
How should I enter the interest rate?
Enter the yearly rate as a percentage: 5 for five percent, not 0.05. Check your document first, because a monthly rate or a decimal rate is not the same thing and would give a very different result.
Is the result rounded to cents?
No. The interest and final amount are shown without automatic rounding. If your contract or accounting requires a specific rounding rule, apply it to the result afterwards rather than assuming two decimal places.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/calc2/simple-interest \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"principal":1000,"annual_rate":5,"years":2}'const res = await fetch("https://api.kit.forhosting.com/calc2/simple-interest", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"principal": 1000,
"annual_rate": 5,
"years": 2
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/calc2/simple-interest",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"principal": 1000,
"annual_rate": 5,
"years": 2
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/calc2/simple-interest", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"principal":1000,"annual_rate":5,"years":2}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"principal":1000,"annual_rate":5,"years":2}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/calc2/simple-interest", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"principal": 1000,
"annual_rate": 5,
"years": 2
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "calc2.simple_interest",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Understand the simple interest formula
The returned interest is the gain or financing charge alone, while final_amount is the total principal plus interest.
Technical questions
What formula does the calculator use?
It calculates interest as principal multiplied by annual_rate divided by 100, multiplied by years. It then adds that interest to the principal to obtain final_amount.
Should I enter 5 or 0.05 for a 5% annual rate?
Enter 5. The annual_rate input is a percentage, and the calculator converts it to 0.05 inside the formula.
What happens if the annual rate is negative?
The request fails with an invalid_input error because this capability does not accept negative annual rates.
What does an API request cost?
Each API request costs $0.002. The browser calculation uses the same deterministic formula.