$100,000 invested at 7% annual return compounded monthly grows to $811,650 in 30 years — $711,650 of that is compound interest. Adding $200 monthly contributions would grow the total to $1,055,644.

Key Numbers

• •Starting amount: $100,000
• •Value after 30 years at 7%: $811,650
• •Interest earned: $711,650
• •With $200/month added: $1,055,644
• •At 7%, money doubles roughly every 10.3 years (rule of 72)