Find interest, amount, principal, rate or time with step-by-step working, convert rates and dates, compare with compound interest, and calculate simple discount and late-payment interest.
With simple interest the rate applies only to the original principal, never to interest already accrued. The formula is I = P × r × t, where P is the principal, r the rate per period (as a decimal) and t the number of periods, in the same unit as the rate. The amount is A = P × (1 + r × t). With 1,000 at 2% per month for 6 months: I = 1,000 × 0.02 × 6 = 120 and A = 1,120. Because each period adds the same amount, the chart is a straight line. Rearranging the same formula answers any unknown: P = I ÷ (r × t), r = I ÷ (P × t) and t = I ÷ (P × r); if you know the amount instead of the interest, use I = A − P. The Solve tab does this and shows the formula with the numbers substituted, in the form textbooks and exams use. With simple interest, rates are proportional: 12% per year is 1% per month and 12 ÷ 360 % per day. How days are counted still changes the result: on the commercial 30/360 basis 90 days is exactly 3 months, and 10,000 at 12% a year earns 300.00; on the exact 365-day basis it earns 295.89. The Units and dates tab shows the three bases side by side and counts the time between two dates. The Simple vs compound tab shows what changes once interest starts earning interest: the two regimes tie at t = 1, then compound curves upward while simple stays on a line. With 1,000 at 2% per month, after 36 months simple reaches 1,720.00 and compound 2,039.89. The Discount tab (true and bank discount) and the Late payment tab (a one-time fee plus pro rata daily interest) cover the most common real-world uses. Financial products such as cards and loans usually use compound interest; simple interest appears in exams, in agreements between people, in note discounting and in pro rata late interest.
Paste the code into your HTML and the tool shows up on your page, without J-Kit's navigation and ads. It still runs in the browser of whoever visits your site.
<iframe
src="https://jkit.tools/embed/en-US/simple-interest-calculator"
width="100%"
height="600"
style="border:0"
loading="lazy"
title="Simple Interest Calculator (with Compound Comparison)"
></iframe>These references help contextualize formulas, standards, APIs and limitations used on this page. They do not replace professional validation when a result has legal, financial, medical or operational impact.
I = P × r × t, where P is the principal, r the rate per period (as a decimal, 2% = 0.02) and t the number of periods, always in the same unit as the rate. The amount is A = P + I = P × (1 + r × t). Example: 1,000 at 2% per month for 6 months earns I = 1,000 × 0.02 × 6 = 120, and the amount is 1,120.