How to calculate the expected move of a stock

Most traders think of "expected move" as a number that lives on an options chain, something for derivatives traders and not much use to everyone else. That is a mistake. Understanding how far a stock could realistically move over any given period is one of the most practical inputs into position sizing, stop placement, and deciding whether a price move is normal noise or a genuine signal. The calculation is accessible to anyone willing to spend five minutes with a formula.

The standard formula and what it actually tells you

The textbook method uses implied volatility (IV) to estimate a one standard deviation range over a chosen time window:

Expected Move = Stock Price × Implied Volatility × √(Days / 365)

Walk through a concrete example. Take a £200 stock with 40% implied volatility, and you want the expected move over 30 days:

• √(30/365) = 0.2867
• £200 × 0.40 × 0.2867 = £22.94
• Expected range: £177.06 to £222.94

What that means in plain terms: the options market is pricing a roughly 68% probability that the stock stays within that £44.88 band over the next 30 days. It is not a target. It is not a prediction of direction. It is a probabilistic band derived from what the market is collectively willing to pay for uncertainty.

For a single-day expected move, substitute Days = 1:

£200 × 0.40 × √(1/365) = £200 × 0.40 × 0.0523 = £4.18

Notice that volatility scales with the square root of time, not linearly. The 30-day expected move is not 30 times the 1-day move. It is roughly 5.5 times larger. This is why short-term traders and long-term investors need to think about range very differently, even when looking at the same stock.

The faster shortcut: the ATM straddle

If you have access to an options chain, there is a quicker way that does not require any maths. Find the at-the-money (ATM) call and put for the expiry date you care about, then add their prices together. That sum approximates the expected move for that window.

Example: ATM call at £6.50, ATM put at £5.80. Add them together and you get £12.30. Some traders apply a 0.85 multiplier to get a tighter one-standard-deviation estimate, giving roughly £10.45. The logic is that straddle prices include a small premium above the pure statistical range, so adjusting down brings you closer to a clean 68% band.

This shortcut works well for liquid, large-cap stocks with active options markets. It becomes unreliable for thinly traded names where the options spread is wide and bid-ask noise distorts the straddle price.

What "one standard deviation" really means in practice

A 68% probability of staying within range also means a 32% probability of moving outside it. That is roughly one in every three trading periods. In a month of 22 trading days, you would statistically expect the stock to breach its one standard deviation range on about 7 of them. That is not an edge case. It is routine.

Real market returns make this worse. Stock prices have fat tails: extreme moves happen more frequently than a clean normal distribution would predict. The 2020 COVID crash, the 2021 meme stock squeezes, and the 2022 rate shock all produced moves several standard deviations beyond what the formula would have suggested was plausible. This is not a reason to abandon the calculation. It is a reason to treat the output as a baseline, not a boundary, and to stress-test positions against moves that are two or three times the expected range.

Implied volatility versus historical volatility: which to use

IV is forward-looking. It reflects what options buyers and sellers collectively believe about future uncertainty, which means it rises ahead of known events like earnings or regulatory decisions and collapses sharply once those events pass. That collapse is IV crush, and it is the mechanism that destroys the value of long options positions held through binary events.

Historical volatility (HV) is backward-looking. It measures what the stock actually did, expressed as an annualised standard deviation of daily returns. When IV sits significantly above HV, the market is pricing in an event premium. When IV sits below HV, the market expects calmer behaviour ahead than what recently occurred. Neither condition is permanent, and the divergence between the two is often itself a useful signal.

For practical range estimation, using IV gives you the market's current best guess. Using HV grounds you in what the stock has actually produced. Both perspectives are valuable. IV tells you what the crowd is paying for; HV tells you what actually showed up.

A condition-based alternative when options data is not available

The IV formula requires liquid options, which rules it out for small-caps, many international stocks, and most crypto assets. Even for large-caps, IV can be distorted by event premiums that inflate the range beyond what historical behaviour would support.

Price Drift takes a different approach: it derives expected ranges from how the stock has actually behaved under its current price conditions, classifying each ticker as narrow, normal, increasing, or extreme based on recent historical patterns. The floor and ceiling you see are not options-derived estimates. They are drawn from real historical movement in similar conditions, which means they reflect what the stock actually did rather than what the market was willing to pay to hedge against. For traders who do not use options or who want a second reference point that is independent of options sentiment, this is a useful complement to IV-based calculations.

How to apply expected move in your trading

The most practical uses are risk calibration, not prediction. Use the expected move to ask three questions before entering any position: is your stop placed inside the expected range (if yes, it is almost designed to be hit by normal noise); is your position size realistic given the daily range the stock is capable of; and when the stock moves, is that move within the range you expected or has it gone beyond, which would suggest something structural has changed.

A move that exceeds the expected range early in the session often means volatility is repricing in real time, either because new information has arrived or because a regime shift is underway. Both situations call for reassessing your position, not waiting passively for the range to reassert itself.

Practical takeaway

The IV-based formula tells you what the market is pricing for uncertainty. Historical-behaviour ranges tell you what the stock has actually produced in similar conditions. Both are calibration tools, not guarantees. The traders who use them well size their positions and place their stops as though the tails will show up, because eventually they always do.

Related reading

For the concepts behind why ranges expand and contract over time, start here: What is stock volatility and why does it matter?