Convert hourly wage to salary, and other easy mental calculations

I’m surprised more people don’t know these quick, easy brain hacks!

To convert hourly wage to annual salary, multiply by 2 and add 3 zeros. ($15/hr = ~$30,000/year)
To convert annual salary to hourly wage, divide by 2 and remove 3 zeros. ($30,000/year = ~$15/hr)

To find the return of an investment in the US stock market over a period adjusted for inflation and including taxes:

  • For one year, add 3%. ($1000 one year later = ~$1030)
  • For three years, add 10% ($1000 three years later = ~$1100)
  • For five years, add 15% ($1000 five years later = ~$1150)
  • For ten years, add 30% ($1000 ten years later = ~$1300)

(Assuming 9% interest compounded yearly, a 30% tax rate, and a 3% inflation rate)

Got any more simple mental calculations or conversions?  Let me know in the comments!