I built a free tool that runs the mix you own today through every market since 1999
Most brokerage apps show you what you made. Very few show how much the exact mix you hold right now could lose.
I study health science and business finance at USC, and I built PortfolioAutopsy to show that number. You export your positions from your broker, paste the file in, and it applies today's weights to daily market data going back to January 1999. The report shows how that mix would have held up through the dot-com crash, 2008, March 2020 and 2022.
The site ships with a sample portfolio so you can try it without your own file. It holds VTI, a Schwab S&P 500 index fund, Apple, Nvidia, a dividend ETF and about 14% cash.
Run it and the first thing you see is a yellow warning. A -49% twelve-month stretch is hiding in this mix, from November 2007 to November 2008.
The rest of the report goes further. The deepest drawdown is -53.5%. Of every one-year window since 1999, 16.1% lost money. Stretch the horizon to five years and the share falls to zero.
Then it shows the levers you control. Move 10% of the portfolio to cash and the worst drawdown improves to -49.3%. Move 30% and it improves to -40.1%, and the backcast return drops from 19.3% a year to 13.4%. You get to see the trade before you make it.
None of this predicts the next crash. It shows the size of the hole your current mix has fallen into before, so you can decide whether you could sit through it again.
I didn't want anyone's holdings on a server, so the whole app is one HTML file of about 600KB. Daily returns for 35 common tickers sit inside the page as 16-bit integers in basis points. Your browser decodes them and does all of the math. Close the tab and nothing about your portfolio exists anywhere else.
That choice has costs. The ticker list is fixed, so the report flags any holding outside it as not analyzed. Returns are rounded to about one basis point a day. Newer funds also enter the math only after they started trading, which skews the earliest years for portfolios that hold them. I'm working on a better way to handle that.
The second tab runs the same report as of a past date, using only the data available that day. The October 2007 replay shows what the warning said at the market peak, followed by what happened over the next 18 months. One replay covers a year where the warning missed, because a risk gauge that never misses is one that someone tuned after the fact.
PortfolioAutopsy is free and in public beta at portfolioautopsy.pages.dev. The demo button loads the sample portfolio. If a number looks wrong to you, email me at coopermb@usc.edu and tell me what you'd expect to see.
This is educational analysis of historical data, not investment advice.