Stock vs S&P 500 calculator: did you beat the index?

Your brokerage app says a stock you bought is up 60%. That sounds like a win, but it doesn't answer the question that matters: would the same money have done better in a plain S&P 500 index fund? The calculator above answers it for one purchase. Type the ticker, the day you bought and the amount, and it shows what that money is worth today in the stock and what it would be worth in the S&P 500 had you bought the index on the same day.

In short

• The comparison uses the same amount on the same day in both, so the only difference is what you picked.
• Both sides include reinvested dividends (adjusted closing prices), so a high-dividend stock isn't punished.
• The S&P 500 side is the SPY ETF, the oldest and largest fund tracking the index.
• A stock that is "up" can still have lost to the index — that gap is the cost of picking it.

How to use the calculator

1. Stock or ETF — the symbol as Yahoo Finance writes it: AAPL, NVDA, BRK-B. Stocks from other markets work with their exchange suffix, for example SAP.DE (Xetra) or CDR.WA (Warsaw).
2. Purchase date — the day you bought. On a weekend or holiday, the calculator uses the next trading session.
3. Amount — what you paid in total, in US dollars, euros or Polish zloty. A foreign stock is converted at each day's exchange rate, so the currency's moves count the way they did in your account.

You get both values today, both returns, the dollar gap between them and a chart of the two paths. The link in your address bar keeps your inputs, so you can share the result.

What the numbers mean

The calculator divides your amount by the stock's adjusted close on the purchase day to get the number of "units" you bought, then multiplies by the adjusted close on every day since. It does the same for SPY. Adjusted closes assume every dividend was reinvested on its ex-date and remove the effect of stock splits, so a 10-for-1 split doesn't look like a 90% crash.

In formula form, for each day t:

• value in the stock = amount × (stock adjusted close on t ÷ stock adjusted close on the purchase day)
• value in the index = amount × (SPY adjusted close on t ÷ SPY adjusted close on the purchase day)

When the stock trades in another currency than your amount, each side is also multiplied by the change in the exchange rate between the two dates.

A worked example

Say you put $10,000 into a stock that is now up 60%: it's worth $16,000. Over the same stretch the S&P 500, with dividends, rose 80%: the same $10,000 would be $18,000. You made $6,000, and you still trailed the index by $2,000. That $2,000 is what the stock pick cost you compared with doing nothing clever.

It can go the other way just as easily:

Amount	Stock return	S&P 500 return	Stock today	S&P 500 today	Gap vs index
$10,000	+60%	+80%	$16,000	$18,000	−$2,000
$10,000	+120%	+50%	$22,000	$15,000	+$7,000
$10,000	−10%	+30%	$9,000	$13,000	−$4,000

In the second row the pick added $7,000 over the passive choice; in the third, the loss on the stock is only part of the story, because the index money would have grown meanwhile. The point isn't that one answer is always right; it's that "up 60%" on its own tells you nothing about whether the decision was good.

Why compare with the S&P 500 at all

The S&P 500 is the yardstick professional managers are measured against, and most of them miss it. S&P Dow Jones Indices publishes the SPIVA scorecard twice a year, and over long horizons the large majority of actively managed US large-cap funds trail the index. Individual stocks are even more lopsided: Hendrik Bessembinder's study of US stocks since 1926 found that most of them returned less over their lifetime than one-month Treasury bills, and a small minority of companies accounted for all of the market's net wealth creation. An index fund owns those few big winners automatically. Picking single stocks means betting that you'll own them too.

That's why a fair test of any purchase is not "did it go up?" but "did it beat the simplest alternative I had on that day?"

Common mistakes

• Comparing price change to price change. A price chart leaves dividends out. Over ten years, reinvested dividends have added a large part of the S&P 500's total return, so comparing a stock's price with the index price understates what the index paid. The calculator uses total return on both sides.
• Using today's exchange rate. If you bought a US stock with zloty or euros, the dollar's move is part of your result. Converting both sides at each day's rate keeps the comparison honest.
• Judging one purchase as the whole portfolio. One winning pick doesn't mean the strategy beats the index, and one loser doesn't mean it fails. You only know when you compare everything you bought, on the dates you bought it.
• Forgetting new money. If you added to the position later, a single-purchase comparison doesn't capture it. Each purchase needs its own comparison — which is exactly what a portfolio tracker does.

From one purchase to your whole portfolio

The calculator handles one purchase. Your account has many, made on different days, plus sales and dividends. The button under the result adds this purchase to a portfolio you can try without an account; there you can add the rest by hand or upload a broker export, and GetHossa runs the same comparison for every deposit: each dollar you invested is "bought" into the S&P 500 on the same day, so you see whether your choices beat the index overall. That is the portfolio vs S&P 500 comparison. Create a free account only if you want to keep it. To turn a total return into a yearly rate, use the CAGR calculator.

Data and limits

Prices are daily closes from Yahoo Finance, the index side is SPY's adjusted close, and the latest value is the last completed session. The comparison ignores taxes, fees and the fund's expense ratio (0.0945% a year for SPY), which slightly flatters the index side against a stock you hold with no costs. Symbols that changed or were delisted may not be available.

The calculator shows what happened in the past; it doesn't predict anything, and nothing here is investment advice.