The Habesha Finance 187 rule calculator is simple to use. Multiply a monthly expense you have by 187. The amount calculated is what you could potentially earn in 10 years if you invest that same amount monthly in a low cost S&P 500 Index Fund. I’ve also included an estimate of a 40 year return at 8% (maybe call that the “3357 rule” or nah?). Don’t forget the average return of all 40 year periods of S&P 500 is about 11.8%. The 8% lets us account for inflation and is generally a safe assumption. I hope this help you ask the question, “What will I have if I invest this monthly payment?”