The Rule of 72 Do you know how long it may take for your investments to double in value? The Rule of 72 is a quick way to figure it out. Related Content When Does Your Personal Car Become a Commercial Vehicle? At what point can you call a personal vehicle commercial? Roth 401(k) vs. Traditional 401(k) This calculator compares employee contributions to a Roth 401(k) and a traditional 401(k). Immediate vs. Deferred Annuities Looking forward to retirement? It's critical to understand the difference between immediate and deferred annuities.