The float is extremely important when it comes to day trading stocks.
The float is the number of shares available for trading at any given time in a stock. Knowing the float is one of the most important factors to be aware of before making any trading decision. You should never make a trade without knowing this number. Why? because the size of the float can have a huge impact on a stocks potential to make big and volatile moves. Supply and demand is a huge factor in terms of generating price action. Unfortunately, sometimes websites showing this information can be inaccurate. This is because a whale may have taken a recent position to eat up some of the float and it has not yet been made public. There are many pump and dump artists who like to play this game by eating up a stocks float and then pumping the stock. So, while a website like finviz.com is showing the float as 5 million (extremely small) it can actually be more like 1 million. Often times, small floats can be accompanied by wide spreads which can cause large price swings. For traders not used to this kind of movement, it can be quite stressful. Most of the time, stocks with low very small floats will be on the Nasdaq exchange and in the $1-$10 range.
Related to your trading:
Tiny floats under about 20 mill shares can make huge moves and be very dangerous to short. Floats this size should be considered the "danger zone." If you are trading big size and the trade goes against you, you can be in deep trouble quickly. Margin calls and forced buy-ins commonly occur on these types of stocks. Especially when you are short and there happens to be a high short interest. Biotech stocks are a great example as they tend to have low floats and can move hundreds of % on something like positive phase 2 news. Likewise, stocks can have a big drop just as fast. With a tiny float it only takes a small amount of demand with that limited supply to cause the stock to move a great deal. Many traders simply cannot handle the emotion involved with floats of this nature and therefore avoid them completely. Its up to you to know your risk tolerance and decide if you can handle this type of price action. Bottom line is big money can be made or lost playing with these stocks, so be sure you know what you are doing before taking a position.