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Does a bid and ask price exist for indices like the S&P500?
Bid and ask prices of stocks change not just daily, but continuously.  They are, as the names suggest, what price people are asking for to be willing to sell their stock, and how much people are bidding to be willing to buy it at that moment. Your equation is accurate in theory, but doesn't actually apply.  The bid and ask prices are indicators of the value of the stock, but the only think you care about as a trader are what you actually pay and sell it for.  So regardless of the bid/ask the equation is: Since you cannot buy an index directly (index, like indicator) it doesn't make sense to discuss how much people are bidding or asking for it.  Like JoeTaxpayer said, you can buy (and therefore bid/ask) for ETFs and funds that attempt to track the value of the S&P 500.