Can I buy and sell the same stock in one day?

You can. Buying a stock at 10 am and selling it at 2 pm is legal, and in a cash account you can do it as often as you like, provided you are using settled cash. What you cannot do in a cash account is spend the proceeds of an unsettled sale to buy something else and then sell that too, which brokers flag as a "good faith violation."

The rule people mean when they ask this is the pattern day trader rule. For years, FINRA required anyone who made four or more day trades in five business days in a margin account to keep at least $25,000 in the account, or be restricted. FINRA moved to overhaul that rule in 2025, so the specifics may differ from what older articles say. Your broker’s help page will have the current version.

The more important question is whether you should. Day trading is a competition against professional firms with faster data and lower costs, and the studies of retail day traders are grim: the large majority lose money over a year, and the fraction who beat the market consistently is tiny.

If you want to try it, do it with an amount you would spend on a hobby, keep a written log of every trade, and check after three months whether you actually made money after taxes. Most people who do that stop.