How to use this interest rate calculator
This single tool covers three common “what is my rate?” questions. Use the tabs above the form to switch modes — you stay on one page.
Loan mode
- Enter the loan amount and optional setup fees (added to principal).
- Choose a secondary figure: monthly payment, total interest over the term, or total amount repaid.
- Enter the loan term in months or years.
- Click Calculate loan interest rate % — the solver uses monthly compounding and the Newton-Raphson method for an accurate annual rate.
Example: $10,000 borrowed, $1,000 monthly payment, 20 months → about 93.06% annual rate with $10,000 total interest. For full amortization schedules, use the Loan Calculator.
Interest % Savings mode
- Enter your initial balance.
- Choose whether you know the end balance, total interest earned, or a known interest rate % (to project growth).
- Set a time period (years and months) or a date range.
- Click Calculate savings rate % for nominal rate, effective APY, and a principal vs interest breakdown.
Example: $10,000 growing to $20,000 in 5 years with monthly compounding → nominal rate about 13.94% and APY about 14.87%. To project future balances forward, try the Savings Calculator or Regular Investment Calculator.
Interest % Simple mode
- Enter a from amount (starting balance) and to amount (ending balance).
- Click Calculate simple interest rate % for the total simple return percentage and interest dollars.
Example: $10,000 to $20,000 → 100% simple interest ($10,000 earned). No compounding or time period is applied — for timed simple interest, see the Simple Interest Calculator.
What is my interest rate?
An interest rate is the percentage charged on borrowed money or earned on savings and investments. On loans it is a cost of borrowing; on deposits it is your return. This calculator works backwards from amounts you already know to reveal the implied rate.
Compound vs simple interest
Compound interest (savings and most loans here) earns or charges interest on prior interest. Simple interest applies only to the original principal. Savings mode reports both a nominal annual rate and an effective rate (APY) that includes compounding.
Nominal vs effective interest rate
Effective annual rate (APY / AER) from nominal rate
The nominal rate is the quoted annual figure before compounding effects. The effective annual rate (APY or AER) is what you actually earn or pay after interest compounds within the year. A 5% nominal rate compounded monthly produces about 5.12% APY.
Nominal rate from effective rate; n = compounding periods per year
Loan rate solving
Loan mode treats your payment schedule as a standard amortizing loan with monthly compounding. The solver iterates until the payment formula matches your inputs — the same approach used by professional loan analysis tools.
Examples and use cases
Worked example
Savings mode: $10,000 grows to $20,000 in 5 years with monthly compounding:
- Nominal annual rate ≈ 13.94%
- Effective APY ≈ 14.87%
- Interest earned = $10,000
Real-world use cases
- Payday loan audit: A borrower enters the loan amount, payment, and term to discover the true annual rate behind a short-term offer.
- Savings goal check: Someone compares two year-end account balances to see what rate they actually earned vs what the bank advertised.
- Investment return snapshot: An investor enters start and end portfolio values in simple mode for a quick total return percentage before diving into IRR.
Related calculators
- Loan Calculator — payments, amortization, and extra payments
- Savings Calculator — forward projections with deposits and withdrawals
- Regular Investment Calculator — investment growth with contribution options
- Simple Interest Calculator — interest over a defined period