Historical volatility calculator from closing prices
How much does this stock really move? Paste its closing prices and see the number traders call historical volatility. Then put it next to the option chain's IV and see if options are cheap or rich.
How it works: each day's move is turned into a log return: the natural log of today's close ÷ yesterday's close. The calculator takes the standard deviation of those returns (the sample kind, dividing by one less than the count). That is how much a normal day swings. To make it yearly, it multiplies by the square root of the prices in a year: √252 for daily stock closes. That yearly number is historical volatility (HV). Implied volatility (IV) is the same kind of number, but it comes from option prices instead of past prices. 21 closes give 20 returns, about one month of trading.
HV is the past. IV is the price tag.
Historical volatility tells you how much the stock really moved. Implied volatility tells you how much movement option buyers are paying for right now. When IV sits well above HV, options are priced for bigger swings than the stock has made lately. That's when sellers collect the most. But IV is often high for a reason: earnings, a lawsuit, a big news day coming. The calm past doesn't protect you from that. In the wheel strategy, a big gap is a reason to ask why before you sell. More in implied vs historical volatility.
Pick the right window
Use about one month of closes (21 prices) to see how the stock moves now. Use a year (253 prices) to see its normal range. Short windows jump around. One wild day can lift a 20-day HV for weeks, then it drops off the list and HV falls all at once. Make sure the list has no gaps and no stock split inside it. A split looks like a giant one-day crash. If you see one, fix the prices first with the stock split calculator. To turn a volatility number into a dollar range, try the expected move calculator.
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