What percent of a company are you buying when you purchase stock?

I just noticed this question. Shares have been reduced since most answer here. Today there are 4.28B shares, but a 4 to 1 split is coming, so it will soon be 17.12B . And a market cap approaching $2T.

Note: 1 share would then be 1/171.2M % = 0.00000000584112%

Your question has already been answered, you divide the amount of shares you own * 100% by the total amount of shares. However, I feel it is somewhat misleading to talk about owning a percentage of the company by owning shares. Strictly speaking, shares do not entitle you to a part of the company but instead give you a proportional amount of votes at shareholder meetings (assuming no funky share classes).

What this means is that someone who owns 30% of a company's shares can't just grab 30% of the company's assets (factories, offices and whatever) and say that they are entitled to own this. What they actually own is 30% of the voting rights in this company, this means that they control 30% of all available votes when the company calls a vote on corporate actions, choosing a new director etc. which is how shareholders exert their influence on a company.

As you can see at https://www.google.com/finance?q=NASDAQ%3AAAPL the number of apple shares at this very moment is 5.25B, so if you have 1 share you own 1 / 5.25B of the company.