$100,000 invested at 7% annual return compounded monthly grows to $200,966 in 10 years — $100,966 of that is compound interest. Adding $200 monthly contributions would grow the total to $235,583.

Key Numbers

• •Starting amount: $100,000
• •Value after 10 years at 7%: $200,966
• •Interest earned: $100,966
• •With $200/month added: $235,583
• •At 7%, money doubles roughly every 10.3 years (rule of 72)