$100,000 invested at 7% annual return compounded monthly grows to $403,874 in 20 years — $303,874 of that is compound interest. Adding $200 monthly contributions would grow the total to $508,059.

Key Numbers

• •Starting amount: $100,000
• •Value after 20 years at 7%: $403,874
• •Interest earned: $303,874
• •With $200/month added: $508,059
• •At 7%, money doubles roughly every 10.3 years (rule of 72)