The Loss You Have to Survive, Not the Average
Two funds both averaged 9% a year over a decade. On paper, identical. But one of them dropped 55% at its worst moment and the other never fell more than 18%. If you owned the first fund and panic-sold at the bottom, that 9% average never reached your account. Maximum drawdown measures the number that actually tests your nerve: the deepest peak-to-trough decline your portfolio suffered before recovering.
The calculation tracks your portfolio value through every period, marks each new high, and measures how far the value fell from that high before reaching a new peak. The largest of all those declines is your maximum drawdown. It is expressed as a percentage of the peak value, because a $40,000 loss means something very different on a $100,000 account than on a $1,000,000 one.
Here is why the math is brutal. A 50% drawdown does not require a 50% gain to recover. It requires a 100% gain. If $100,000 falls to $50,000, you need to double your money just to get back to even. A 20% drawdown needs a 25% gain to recover. A 33% drawdown needs roughly 50%. The deeper the hole, the steeper the climb, and that asymmetry is the single most important reason to care about drawdown rather than just average return.
This tool also reports two timing measures most people overlook. Drawdown duration is how long the decline lasted from peak to trough. Recovery time is how long it took to climb back to the old high. The 2007 to 2009 decline saw the S&P 500 fall around 55% from peak to trough, and it took roughly four years to fully recover. An investor who needed that money in 2010 did not have it.
Drawdown matters most because it is where investors actually quit. Behavioral research consistently finds that the typical investor underperforms the very funds they own, largely by buying after rallies and selling during deep drawdowns. The average return assumes you hold the whole time. Maximum drawdown is the moment that assumption gets tested, when the paper loss is large enough that staying invested stops feeling rational and starts feeling reckless. A portfolio you abandon at the bottom of a 50% drawdown never delivers its long-run average, no matter how good that average looked on the brochure.
Average return tells you the destination. Maximum drawdown tells you the worst stretch of road you have to drive to get there. One number sells the trip. The other tells you whether you will still be in the car when it arrives.