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How can I figure out how a stock's price would change after I buy shares?
Stock price is based on supply and demand. Unless the stock you are looking to buy usually has very low volume trading 100 shares isn't likely to have any effect on price. There are many companies that have millions or tens of millions of shares trade daily. For stocks like that 100 shares is barely a trivial percentage of the daily volume. For thinly traded stocks you can look at the bid and ask size but even that isn't likely to get you an exact answer. Unless you are trading large volumes your trade will have no effect on the price of shares.