Simple Interest Calculator
Calculate simple interest and total amount from principal, rate, and time—no compounding. Runs locally in your browser. Free online tool — no sign-up.
Trust summary Engine tested · Specification checked · v1.3.0
- Input interpretation
- Enter values to calculate.
- Result
- —
- Model
- Simple interest and end balance, or principal / rate / term from the other variables.
- Scope
- Constant simple rate; no compounding
- Verification
- Engine tested · Specification checked · v1.3.0
- Named expert review
- Optional · Not performed
- Specification basis
- Federal Reserve Bank education / standard banking arithmetic — simple interest I = P·r·t
- SEC Investor.gov — contrasting simple vs compound interest for learners
Formulas
Core equations used by this calculator.
How to use
Choose what to solve
Find balance (default), or solve for principal, rate, or term.
Enter the known values
Principal, annual rate %, and term — or end balance when solving backwards.
Pick term units if needed
Years, months, or days. Read interest and balance in the result.
Example calculations
Common configurations with formula and result.
Standard balance
P = 20,000 · 3%/yr · 10 yr
Five-year loan
P = 10,000 · 5%/yr · 5 yr
Nine months
P = 10,000 · 4%/yr · 9 mo
Find rate
A = 26,800 · P = 22,000 · 4 yr
Find principal
A = 26,000 · 3%/yr · 10 yr
Sample schedule — P = 20,000 at 3%/yr (simple)
Common values at a glance.
| Year | Interest that year | Balance |
|---|---|---|
| 1 | 600 | 20,600 |
| 2 | 600 | 21,200 |
| 5 | 600 | 23,000 |
| 10 | 600 | 26,000 |
Simple Interest calculator specification
Version 1.3.0 · Engine tested
- Engine tested 4 published cases
- Named expert review Not performed
- Calculation version 1.3.0
- Definition
- Simple interest is calculated only on the original principal — not on accumulated interest. Interest I = P × r × t and end balance A = P(1 + rt), with r as a decimal annual rate and t in years. This calculator finds balance, principal, rate, or term from the other inputs.
- What it calculates
- Simple interest and end balance, or principal / rate / term from the other variables.
- Inputs
- Depending on mode: principal P, end balance A, rate %/yr, term (years/months/days)
- Outputs
- End balance A and interest I, or P, r, or t
- Formula
I=P·r·t; A=P(1+rt); P=A/(1+rt); r=(1/t)(A/P−1); t=(1/r)(A/P−1)- Assumptions
- Constant simple rate; no compounding
- No fees, taxes, or extra deposits/withdrawals
- Units
- Currency units consistent for P and A
- Rate % per year
- Time → years (months/12, days/365)
- Boundary conditions
- P=0 or r=0 blocks some reverse solves
- Negative inputs clamped to 0
- Example
- P=20000, 3%/yr, 10 yr → I=6000, A=26000
- Validation cases
4 published on this page
- 20000, 3%, 10 yr → I=6000, A=26000
- 10000, 5%, 5 yr → I=2500, A=12500
- 10000, 4%, 9 months → I=300, A=10300
- A=26800, P=22000, 4 yr → rate → 5.45%/yr
- Specification basis
- Federal Reserve Bank education / standard banking arithmetic — simple interest I = P·r·t
- SEC Investor.gov — contrasting simple vs compound interest for learners
- Calculation version
- 1.3.0
Background
Interpretation and common distinctions.
Calculate simple interest on a loan or deposit — interest on the original principal only.
I = P × r × t A = P(1 + rt) = P + I
Default example: P = 20,000 · 3%/yr · 10 years → I = 6,000 · A = 26,000.
Modes
| Tab | Solves | Needs |
|---|---|---|
| Find balance | A, I | P, rate, term |
| Find principal | P | A, rate, term |
| Find rate | r | A, P, term |
| Find term | t | A, P, rate |
Term units: years, months (÷12), or days (÷365).
Simple vs compound
| Simple | Compound | |
|---|---|---|
| Interest base | Original principal only | Principal + accrued interest |
| Yearly interest | Constant (P×r) | Grows as balance grows |
| Typical use | Short-term loans, some coupons | Savings, credit cards, most loans |
Same 10,000 at 5% for 5 years: simple → 12,500 total; monthly compound → about 12,834. Use compound interest when interest compounds.
Related tools
Other calculators in this family: Compound Interest, Percentage Calculator .
Frequently asked questions
Key distinctions behind the calculation.
What is simple interest?
Interest charged or earned only on the original principal. Previously earned interest does not itself earn interest. Formula: I = P × r × t.
What formula does this calculator use?
Interest I = P × r × t and end balance A = P(1 + rt), where r is the annual rate as a decimal and t is time in years. You can also solve for P, r, or t.
How is simple interest different from compound interest?
Simple interest always uses the starting principal. Compound interest adds earned interest to the balance each period, so you pay or earn interest on interest. Over long terms, compounding usually costs more as a borrower and earns more as an investor.
How do I enter months or days?
Select Months or Days under Term. Months convert as ÷12; days as ÷365. Or enter years as a fraction (6 months → 0.5).
When is simple interest used?
Often on short-term loans, some auto loans, and certain bonds or coupons. Most savings accounts and credit cards use compound interest instead.
Is the rate monthly or yearly?
Enter an annual percent rate. If you only have a monthly rate and want I = P×r×n with n months, convert carefully — this tool’s rate field is % per year with t in years (or months/days converted to years).
Can I solve for the interest rate?
Yes — use Find rate with end balance A, principal P, and term t: r = (1/t)(A/P − 1).
Is this financial advice?
No. Results are mathematical illustrations with a fixed rate and no fees or taxes. Confirm terms with your lender or bank.