The Rule of 72 - Estimate How Fast Your Money Grows: Tired of funding calculators that appear like rocket science? The Rule of 72 is your secret weapon for estimating doubling time ... atOptions = { 'key' : 'a8dfc29b76f8e0e43144f40c1f6e4e79', 'format' : 'iframe', 'height' : 90, 'width' : 728, 'params' : {} }; document.write('');