Simple interest
- When to use
- Short-term loans, basic interest examples, classroom math.
- Formula
- I = P × r × t
- Notes
- Interest is calculated only on the original principal, not on previously earned interest. Final amount A = P + I.
A short calculator guide.
A quick reference for every calculator on the site — when to reach for it, what each input means, and the formula behind the numbers.
Compound interest
- When to use
- Most savings and investment products — anywhere interest “earns interest”.
- Formula
- A = P × (1 + r/n)^(n·t)
- Notes
- r is the nominal annual rate, n is how many times per year interest compounds. Higher n produces a slightly higher final amount.
Loan / mortgage
- When to use
- Auto loans, student loans, personal loans and home mortgages.
- Formula
- M = P × [r·(1+r)^n] / [(1+r)^n − 1]
- Notes
- r is the monthly rate (annual rate ÷ 12), n is the number of months. Extra monthly payments shorten the term and reduce total interest.
Savings
- When to use
- Savings accounts, CDs and other deposit products with monthly contributions.
- Formula
- Future value of recurring deposits with periodic compounding.
- Notes
- Each month your contribution is added before interest is applied for the period — making time-in-the-market the key driver of returns.
Investment
- When to use
- Brokerage accounts, retirement portfolios, long-term planning.
- Formula
- Future value with optional inflation adjustment.
- Notes
- You can model an expected annual return and (optionally) an inflation rate. The inflation-adjusted final figure shows the value in today’s dollars.