For general shorting information such as the short interest ratio, which is the short interest divided by the average daily volume, you can usually go to any website that features a stock quotes service. For example, you can find this information on the Yahoo Finance website in Key Statistics under Share Statistics. The New York Stock Exchange (NYSE) also calculates its own short interest ratio for the entire exchange, which can be a useful metric for determining overall market sentiment.
If you are interested in acquiring more detailed information about a specific stock, such as specific numbers about volume, average daily share volume or days to cover, there are websites that you can visit that would provide such information free of charge. According to the NYSE, all of this data is acquired from broker-dealers as part of the exchange's regulations. However, the specific site you will need to visit will depend on the stock exchange in which the stock that you are seeking information for trades.
Seeking Shorted-Shares Info
For example, if the stock in question is traded on the Nasdaq, you would have to use Nasdaq Trader's Trading Data, where you can find Nasdaq's Monthly Short Interest Tool. If the stock that you are interested in is found on the NYSE, you can check out NYSE Data's NYSE Short Interest Tool.
Short interest is generally expressed as a percentage of the number of shorted shares divided by the total outstanding shares. A company with a 10% short interest, for example, might have 10 million short shares out of 100 million shares outstanding.
Day traders use short interest as a technical indicator. If there is a high short interest in a particular equity and a breakout occurs, traders could scramble to cover their shorts, creating a snowball effect that day traders use to compound their profits. (See also: Short Selling Tutorial and Short Interest: What It Tells Us.)