$1,000 invested at 10% annual return compounded monthly grows to $7,328 in 20 years — $6,328 of that is compound interest. Adding $200 monthly contributions would grow the total to $159,202.

Key Numbers

• •Starting amount: $1,000
• •Value after 20 years at 10%: $7,328
• •Interest earned: $6,328
• •With $200/month added: $159,202
• •At 10%, money doubles roughly every 7.2 years (rule of 72)