Moving Averages Explained: SMA vs. EMA
Two of the most common lines on any stock chart, SMA and EMA look similar but react to new prices very differently. Here's the distinction.
What a moving average is doing, mechanically
A moving average takes a stock's closing prices over a set number of recent periods — 50 days, 20 weeks, whatever window you choose — and plots their average as a single line that updates as new data comes in. The "moving" part just means the window slides forward one period at a time, dropping the oldest price and adding the newest.
The simple moving average: equal weight for every day
A simple moving average (SMA) is the plain arithmetic average: add up the closing prices for the last 50 days, divide by 50. Every one of those 50 days counts exactly the same in the calculation, whether it happened yesterday or seven weeks ago. That equal weighting is what makes an SMA smooth — a single unusual day gets diluted across the whole window and doesn't swing the average much on its own.
The exponential moving average: recent days count more
An exponential moving average (EMA) also averages recent prices, but applies a weighting formula that gives more importance to the most recent data and progressively less to older data. The practical effect is that an EMA responds to new price changes noticeably faster than an SMA covering the same number of periods — a sharp move in the last few days shows up in the EMA line almost immediately, while it takes longer to meaningfully shift a same-length SMA.
- An EMA reacts faster to new information, which can help identify a trend change sooner.
- An SMA reacts more slowly, which can help filter out short-term noise and avoid reacting to moves that don't hold up.
- Both are typically available on any charting platform, often as an option you can toggle when adding a moving average.
The tradeoff, in practice
Faster isn't automatically better. An EMA's quicker response also makes it more prone to reacting to short-lived price spikes that later reverse — sometimes called "whipsaws" — generating a signal that looks meaningful in the moment but doesn't hold up. An SMA's slower, smoother response reduces false signals but can mean recognizing a genuine trend change later than an EMA would have.
An EMA tells you about a trend sooner. An SMA tells you with a bit more confidence that the trend is real. Most traders end up choosing one tradeoff or the other, not both.
How they're commonly used together
Rather than picking just one, many traders plot two moving averages of different lengths on the same chart — for example, a 50-day and a 200-day SMA — and watch for "crossovers," when the shorter-term average crosses above or below the longer-term one. A shorter average crossing above a longer one is often read as a bullish signal; the reverse is often read as bearish. This combines the smoothing benefit of moving averages with a concrete, rules-based signal for when the trend may be shifting.
This article is educational and general in nature. It isn’t personalized investment, tax, or legal advice — always weigh your own circumstances, or talk to a licensed professional, before making financial decisions.