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Simple Interest Calculator

Computes interest that accrues only on the original principal (no compounding) over a term in years, and the resulting total amount.

When to use

The agreement states simple, non-compounding interest, e.g. short-term personal loans, some bonds and statutory interest on late payments, or you want a quick estimate.

Do not use when: Interest is reinvested or charged on accrued interest (use compound-interest), or you need the level payment of an amortizing loan (use loan-payment).

Formula

interest = principal × (annual_rate_percent / 100) × years; total_amount = principal + interest

Inputs

Parameter	Type	Unit	Required	Description
`principal`	number		yes	Amount lent, borrowed or deposited. Range: ≥ 0, ≤ 1000000000000
`annual_rate_percent`	number	%	yes	Simple annual rate in percent; 5 means 5 %. Range: ≥ 0, ≤ 1000
`years`	number	years	yes	Term in years; fractional values allowed (6 months = 0.5, 90 days = 90/365 ≈ 0.2466). Range: ≥ 0, ≤ 200

Outputs

Output	Type	Description
`interest`	number	Total simple interest over the term.
`total_amount`	number	Principal plus interest.
`interest_per_year`	number	principal × rate, the constant yearly interest.
`interest_per_month`	number	interest_per_year / 12.

Example

1,000 at 5 % for 3 years: {"principal":1000,"annual_rate_percent":5,"years":3} → {"interest":150,"total_amount":1150,"interest_per_year":50,"interest_per_month":4.17}

25,000 at 4.5 % for 2.5 years: {"principal":25000,"annual_rate_percent":4.5,"years":2.5} → {"interest":2812.5,"total_amount":27812.5}

GET https://tttkmbb.com/api/v1/calculate/simple-interest?principal=1000&annual_rate_percent=5&years=3

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/simple-interest (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/simple-interest · Markdown: https://tttkmbb.com/finance/simple-interest.md · JSON definition: https://tttkmbb.com/finance/simple-interest.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="simple-interest"
OpenAPI operationId: calculate_simple_interest
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Investopedia – Simple Interest: Who Benefits, With Formula and Example (reference)
Wikipedia – Interest (simple interest) (reference)

FAQ

How do I enter a term in months or days?

Convert to years: months / 12, days / 365 (or / 360 if the contract uses a 360-day banker's year). Example: 18 months = 1.5 years.

How does simple interest compare with compound interest?

Simple interest grows linearly; compound interest grows exponentially because earned interest also earns interest. Over one period at annual compounding they are equal; over longer terms compounding yields more.

Related calculators

Compound Interest Calculator — Interest on interest at a chosen compounding frequency.
Loan Payment Calculator — Level monthly payment on an amortizing loan.