Calculate simple interest and total amount instantly using the SI = (P×R×T) / 100 formula. Free online simple interest calculator for loans, deposits, and study.
The Tooloogle Simple Interest Calculator computes the interest earned (or owed) and the final total amount on any principal at any rate over any time period using the classic formula SI = (P × R × T) / 100. Enter principal, annual rate, and time in years; see the simple interest amount and the total (principal plus interest) immediately. Useful for short-term personal loans, fixed deposits with simple-interest terms, exam preparation in commerce and arithmetic, basic financial planning, and verifying the math on lending offers. Calculation runs in your browser; no signup, no upload.
Simple interest is the most basic form of interest: it's charged or earned only on the original principal, never on accumulated interest. Contrast this with compound interest, where interest itself earns interest in subsequent periods. Simple interest is straightforward, predictable, and used in many short-term lending contexts (some auto loans, short-term personal loans, certain fixed-deposit schemes), in education for teaching the basic time-value-of-money concept, and in some legal contexts (e.g. statutory interest on overdue payments). The formula is SI = (Principal × Rate × Time) / 100, where rate is in percent per annum and time is in years. The total amount you'll have (or owe) at the end is principal plus interest: A = P + SI.
Live calculation — interest and total update as you change any input.
Sensible defaults — principal 1,000, rate 5%, time 1 year are pre-filled so you see a working example without having to enter values first.
Decimal-friendly — rate accepts fractional percentages (e.g. 6.75%); time accepts half-year or quarter-year increments (e.g. 1.5 years for 18 months).
Two-decimal precision — results are rounded to two decimal places.
Wide range — principal accepts any non-negative value; rate is clamped 0–100% to prevent typos; time accepts any non-negative value.
Browser-only — calculation runs locally in JavaScript. No data is uploaded.
Enter the principal amount (the original sum borrowed or invested) in the "Principal Amount" field.
Enter the annual interest rate as a percentage (e.g. 5 for 5% per annum, 7.25 for 7.25%).
Enter the time period in years (e.g. 3 for three years, 1.5 for eighteen months, 0.25 for a quarter).
Read the two results: simple interest (the interest amount earned or owed) and total amount (principal plus interest).
Adjust any input to model different scenarios — what would I earn at 7% instead of 5%? what if I left the deposit in for 5 years instead of 2?
Borrowers verifying the interest cost on a personal loan or auto loan that uses simple interest (some do, especially for very short terms). Lenders quoting the total payback amount to a borrower. Investors evaluating fixed deposits or savings products that use simple interest (less common today than compound but still found in certain markets). Students preparing for commerce, accounting, banking, or competitive exams where simple-interest problems are standard fare. Tutors teaching the time-value-of-money concept — simple interest is the gateway to compound interest, bonds, annuities, and present-value math. Lawyers computing statutory interest on overdue payments where the law specifies simple interest. Family members calculating informal loan terms (a friend lending money with a flat annual rate). Real-estate agents computing earnest-money interest. Anyone wanting to sanity-check a lending offer's headline numbers.
The formula SI = (P × R × T) / 100 can be rearranged to solve for any of the four quantities given the other three:
Find Interest: SI = (P × R × T) / 100 — the standard form.
Find Principal: P = (SI × 100) / (R × T) — useful when you know the interest you'd like to earn and need to know how much to deposit.
Find Rate: R = (SI × 100) / (P × T) — useful when comparing investment options against a target interest amount.
Find Time: T = (SI × 100) / (P × R) — useful when you know the interest you need and the available rate, and want to know how long it will take.
The Tooloogle calculator solves the standard form (find interest given P, R, T). For the other rearrangements, plug your known values into the formula above and solve algebraically — or use the calculator iteratively (try different values until the interest matches your target).
Example 1: Short-term loan. You borrow 5,000 at 8% per annum simple interest for 2 years. Interest = (5000 × 8 × 2) / 100 = 800. Total payback = 5,800.
Example 2: Fixed deposit. You deposit 10,000 in a simple-interest fixed deposit at 6.5% for 3 years. Interest = (10000 × 6.5 × 3) / 100 = 1,950. Total amount at maturity = 11,950.
Example 3: Quarter-year period. 2,000 lent at 12% per annum for 3 months (= 0.25 years). Interest = (2000 × 12 × 0.25) / 100 = 60. Total = 2,060. Note: the rate must always be expressed per annum even when the time is in months — convert months to fractional years (months / 12) before plugging in.
Simple interest grows linearly: at 10% per annum, a principal of 1,000 becomes 1,100 after one year, 1,200 after two, 1,300 after three. Compound interest grows exponentially: at 10% per annum compounded annually, the same 1,000 becomes 1,100 after one year, 1,210 after two (1,100 + 10% of 1,100), 1,331 after three (1,210 + 10% of 1,210). The gap between the two grows over time and with compounding frequency. For long time horizons (decades), the difference is enormous — this is why most savings, investment, and mortgage products use compound interest. Simple interest is mostly limited to short-term lending (where the difference is small enough not to matter), legal contexts (statutory simplicity), and education (it's easier to teach first).
Always confirm whether a lending or saving product uses simple or compound interest before agreeing to the terms — on multi-year products the difference can be substantial. When the time period is in months, convert to fractional years (e.g. 18 months = 1.5 years) before plugging into the formula. When the rate is quoted per month rather than per annum (rare but it happens, especially with unscrupulous short-term lenders), multiply by 12 to convert to per annum before using the formula — or use a different formula that takes the rate in the units it's quoted in. For very short periods (days), express time as days/365 (or days/360 for some banking conventions). Use this calculator alongside the Tooloogle Compound Interest Calculator to compare the two on the same principal — you'll quickly see why compound interest dominates real-world finance.
Calculation runs entirely in your browser using a single arithmetic formula. No principal, rate, or time value is uploaded; the financial data you're modeling stays private. Verify with DevTools that no requests fire as you change inputs.
Tooloogle's calculator is focused: principal, rate, time, two results. No signup, no ads in the math, no upsell to a "pro finance toolkit". The formula and worked examples on this page double as a learning resource — useful for students preparing for exams or anyone who hasn't touched the formula since school. Pair the calculator with the Tooloogle Compound Interest Calculator for side-by-side comparison, the Percentage Calculator for any percentage question, and the Days Between Two Dates calculator for converting non-standard time periods into fractional years. Bookmark the page next to your banking app for quick verification of any simple-interest offer.
How to Use Simple Interest Calculator - Calculate SI Online with Formula
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Calculate simple interest and total amount instantly using the SI = (P×R×T) / 100 formula. Free online simple interest calculator for loans, deposits, and study.
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