Updated June 20, 2026
Planning regular investing?
The Investment Calculator projects future value from monthly contributions and an expected market return.
The compound interest formula
A = P(1 + r/n)^(nt). Here A is the ending balance, P is the principal, r is the annual rate as a decimal, n is the number of times interest compounds per year, and t is the number of years. When you add a monthly deposit, each deposit becomes new principal that compounds for the remaining time, and the calculator sums the whole stream.
Simple vs. compound
Simple interest pays only on the original principal, so a $10,000 balance at 5 percent earns a flat $500 every year. Compound interest pays on the growing balance, so year two earns interest on $10,500, year three on more again. Over a decade that difference is the gap between a straight line and a curve that bends upward.
Use it, then sanity check it
The ending balance assumes a fixed rate for the whole term, which suits a CD or a fixed-rate savings account better than a stock portfolio whose return moves every year. For variable investments, run a few rates to bracket the outcome. For definitions of APY and how compounding is disclosed, see the investor education materials from the U.S. Securities and Exchange Commission at investor.gov.
This tool is informational and educational. It is not financial or investment advice.
A reformed credit analyst, Jessica Martinez turns dense financial paperwork into something you can actually use. She writes the explainers behind these calculators and checks every formula against a primary source before it ships.