Money left to compound doesn't grow in a straight line — each period's gain joins the pile and earns its own gain next time. CompoundingInterest on interest. A fixed percentage applied to a balance that already includes past gains, so growth accelerates — the curve bends upward rather than rising in equal steps. turns a steady rate into a curve that bends upward — and top it up with a regular contribution and the balance climbs faster still. The Rule of 72 tells you how long the rate alone takes to double. Run it backwards — shrinking future money back to today — and you've got net present value.
Your €10 000 plus €500 every month — €70 000 paid in all told — grows to €105 197, of which €35 197 is interest.
The faint dashed line is the money you've put in — your starting sum plus every deposit. The bright curve is the balance, and the shaded wedge between them is interest: €35 197 here, widening as the balance compounds on itself and on your deposits.
| Rate | years to double |
|---|---|
| 1% | 70rule of 72: 72 |
| 2% | 35rule of 72: 36 |
| 4% | 18rule of 72: 18 |
| 8% | 9rule of 72: 9 |
| 12% | 6.1rule of 72: 6 |
| 16% | 4.7rule of 72: 4.5 |
| 24% | 3.2rule of 72: 3 |
| 36% | 2.3rule of 72: 2 |
The Rule of 72 is a shortcut for the doubling time, and the bar is how far it misses the exact figure. It's nearly perfect around 8% and drifts at the extremes — short bars are a faithful estimate, long ones an optimistic one. Click a rate to load it above.