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Free Simple Interest Calculator

Calculate simple interest earned using I = P × r × t and compare it side-by-side with compound interest to see exactly how much more compounding earns.

This free simple interest calculator uses the formula A = P(1 + r × t) and compares the result against monthly compound interest so you can see the true advantage of compounding — instantly, no sign-up required.

What Is Simple Interest?

Simple interest is calculated only on the original principal — the interest earned each period is always the same flat dollar amount. A $10,000 investment at 5% earns exactly $500 per year, every year. After 10 years, total interest = $5,000.

In contrast, compound interest earns interest on interest. That same $10,000 at 5% compounded monthly earns $6,289 over 10 years — $1,289 more than simple interest, and the gap widens dramatically over 20 or 30 years.

Simple interest is most common in short-term loans (auto loans, personal loans) and some fixed deposits. For long-term savings and investments, always choose accounts that offer compound interest. Use our compound interest calculator to see exactly how much more you earn.

How to Use This Calculator

  1. 1

    Enter the Principal

    The starting amount — how much you are investing or lending. This is the base on which all simple interest is calculated. With simple interest, the base never changes, unlike compound interest.

  2. 2

    Set the Annual Interest Rate

    Enter the stated annual interest rate. Simple interest is always stated as an annual rate. Car loans and personal loans typically run 5–15%; savings accounts 3–5%.

  3. 3

    Enter the Time Period

    How many years? The longer the period, the more the compound interest advantage grows. Compare 5, 10, and 20 years to see the compounding gap widen.

The Simple Interest Formula

A = P × (1 + r × t)

Example: $10,000 at 5% for 5 years: A = 10,000 × (1 + 0.05 × 5) = 10,000 × 1.25 = $12,500

Compare: Same inputs with compound interest (monthly) → A = 10,000 × (1 + 0.05/12)^60 = $12,834 — $334 more just from compounding.

Simple vs Compound Interest — Real Examples

Same principal, same rate — different growth.

Short-Term: $10,000 at 5% for 2 Years

Simple interest: $10,000 × 0.05 × 2 = $1,000 interest, total $11,000. Compound (monthly): $11,050 — only $50 more. Over short periods, the difference is negligible. This is why simple interest is common for short-term loans.

Medium-Term: $10,000 at 7% for 10 Years

Simple interest: $10,000 + $7,000 = $17,000. Compound (monthly): $20,097 — $3,097 more. Over a decade, the compound advantage becomes substantial and represents real money for retirement planning.

Long-Term: $10,000 at 7% for 30 Years

Simple interest: $10,000 + $21,000 = $31,000. Compound (monthly): $76,123 — a difference of $45,123. At 30 years, compounding generates 2.5× more money than simple interest. This is the most powerful argument for starting to invest early.

Frequently Asked Questions

What is simple interest?
Simple interest is calculated only on the original principal — $10,000 at 5% for 3 years earns $1,500 in interest (I = P × r × t = 10000 × 0.05 × 3). Unlike compound interest, the interest earned each year never itself earns interest, so the growth is linear, not exponential.
What is the simple interest formula?
The simple interest formula is I = P × r × t, where I = interest earned, P = principal (starting amount), r = annual rate as a decimal, and t = time in years. Final amount = P + I = P(1 + r × t). For $10,000 at 5% for 5 years: I = 10000 × 0.05 × 5 = $2,500, total = $12,500.
What is the difference between simple and compound interest?
$10,000 at 5% for 10 years: simple interest earns $5,000 (final = $15,000), compound interest earns $6,289 (final = $16,289) — a $1,289 difference. The gap grows exponentially over longer periods. At 30 years the compound advantage is $27,939. Compound interest is always superior for savings and investments.
When is simple interest used?
Simple interest is used for short-term loans (car loans, personal loans, some student loans), Treasury Bills, and some savings certificates. Most mortgages, credit cards, and savings accounts use compound interest. For any long-term savings or investment goal, always seek accounts that compound interest.
How much interest does $10,000 earn at 5% for 5 years (simple)?
$10,000 at 5% simple interest for 5 years earns exactly $2,500 in interest — $500 per year, linear growth. With compound interest at the same rate, you earn $2,763 — an extra $263. Over 20 years the difference grows to $7,649. This is the true cost of choosing simple over compound interest.
Is simple interest good for borrowers?
Yes — simple interest loans are cheaper for borrowers than compound interest loans when paid down quickly. A simple interest auto loan at 5% on $20,000 over 5 years accrues $5,000 in interest total. Early payoff saves money on simple interest loans because interest is only on the remaining principal each day.

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