Simple Interest Calculator – Interest Rate Calculator. Most importantly, people borrow and lend money to start a business or buy something. When we borrow Free interest calculator to find the interest, final balance, and accumulation schedule using either a fixed starting principal and/or periodic contributions. Included are options for tax, compounding period, and inflation. Also explore hundreds of other calculators addressing investment, finance math, fitness, health, and many more. The Interest Rate Calculator determines real interest rates on loans with fixed terms and monthly payments. For example, it can calculate interest rates in situations where car dealers only provide monthly payment information and total price without including the actual rate on the car loan.