Interest Calculator

How Interest Calculator Works

For simple interest, the calculator uses I = P × r × t, where P is the principal, r is the annual rate, and t is the time in years — interest accrues only on the original principal, never on previously earned interest. Time entered in months or days is first converted to years (months ÷ 12, days ÷ 365) before the formula runs.

For compound interest, it uses A = P(1 + r/n)^(n×t), where n is how many times per year interest compounds — annually, semi-annually, quarterly, monthly, or daily. Total interest earned is simply the future value minus the principal.

The effective annual rate (EAR) shows what the nominal rate actually works out to once compounding is applied: EAR = (1 + r/n)^n − 1, and it rises as compounding gets more frequent. Doubling time — how long it takes the principal to double — is derived from the same rate and compounding frequency using the Rule of 72-style logarithmic formula.

Who Uses Interest Calculator and Why

• Comparing how much more a savings account earns under compound interest versus simple interest for the same rate and time period.
• Finding the effective annual rate of an account so you can compare it fairly against another account with a different compounding frequency.
• Estimating how many years it would take a lump sum to double at a given rate and compounding schedule.
• Converting a rate quoted in months or days into an equivalent annual-rate calculation for consistent comparison.

Mistakes to Avoid

• Assuming simple and compound interest give roughly the same result over a short period and not checking the actual gap — the example shows compounding monthly for 5 years already produces about $333 more than simple interest at the same nominal rate.
• Comparing two accounts' nominal rates directly when they compound at different frequencies — the effective annual rate (EAR), not the nominal rate, is what makes accounts genuinely comparable.
• Entering time in months or days without realizing the calculator converts it to years for the simple-interest formula (months ÷ 12, days ÷ 365) — a rough date entry can introduce small rounding differences versus an exact day count.

Tips for Best Results

• Always compare the effective annual rate (EAR), not the nominal rate, when deciding between two accounts that compound at different frequencies.
• If you're testing a doubling-time question ("how long until this doubles?"), the built-in doubling-time result saves you from working the compound interest formula backward by hand.

Fixing Common Problems

My compound interest result looks only slightly higher than simple interest. — This is expected for shorter time periods or lower rates, since compounding needs time to meaningfully outpace simple interest — the gap widens the longer the money stays invested.

Terms Explained

Effective Annual Rate (EAR): The true annual return once compounding frequency is factored in, always equal to or higher than the nominal rate.

Doubling time: How many years it takes a principal to double at a given rate and compounding frequency, assuming no withdrawals.