How often does a stock price change and where is this defined?

Let's suppose I want to collect stock prices for let's say AAPL on NASDAQ.

1. Where do I find information on how often an official stock price is calculated? Is it every millisecond, every second, or not that often?

2. Does the public have the opportunity to get every officially calculated price, or do we only get some price calculations like every minute or so?

• Every time the stock is traded, the price changes/updates. Your quote provider may only provide price updates every x seconds, and those quotes may be delayed or in real-time -- it depends on your source. Commented Jun 16, 2015 at 20:50
• Yes, I see. But in order to be traded the stock has to have a price at any given "moment". But how often that "moment" arises must be defined in some sort of specification I suppose. Commented Jun 16, 2015 at 21:01
• There is no moment, there is only the last price where two parties agreed to exchange. If the stock doesn't trade for 3 hours, it will be the last trade price from 3 hours ago. No calculation needed. Or semantics. Maybe this 'moment' you refer is the last moment two parties agreed on a price and made an official exchange, whenever that may have been. The next price will be whatever the next two parties agree upon and so forth. Commented Jun 16, 2015 at 21:02
• Ok so the price changes every time a trade has occurred, sure. Now I suppose for AAPL that happens almost every 500 microseconds or so, does NASDAQ present the public this course change, I doubt that. I'm looking for specifications here. Commented Jun 16, 2015 at 21:06
• There is no course. There are only individual agreements to buy and sell for a given price, with NASDAQ reporting the most recent sale. Commented Jun 16, 2015 at 21:42

Stocks prices are determined whenever a buyer and seller agree to trade at a given price. The company (you use AAPL as an example) doesn't set its own stock price. Rather, the investors set the price every time it trades. There's no "official" price -- just the last trade.

Likewise, you can offer to trade a stock at whatever price you want: that's the definition of a limit order. You might not find a willing buyer or seller at that price, but you can certainly open an order.

Stock quotes that you get from your broker or a finance web site reflect the price as last traded. These quotes are updated throughout the trading day and the frequency and delay varies amongst quote providers.

Like Knuckle-Dragger suggests in the comments, there are ways to get real-time quotes.

It's often more helpful to think in terms of bid/ask instead of "official price". See this question for details.

• Or, for those of us thinking long-term, to just say "buy up to \$1000 worth of XYZCO" and take however many shares that gets you. Commented Jun 17, 2015 at 3:29
• @keshlam Since that would likely not be a round lot of shares, won't the commission be larger than for a round lot? Commented Jun 17, 2015 at 10:37
• Per share? Sure, slightly, just as buying one more share would always lower the per-share cost if you're being charged per transaction. That's a reason to think in terms of larger transactions (in terms of number of shares) generally, which reduces that cost and reduces this difference. Or to work with mutual funds, where one can buy and sell fractional shares and not pay per-transaction fees at all. Commented Jun 17, 2015 at 13:24