RSI (Relative Strength Index) is a momentum oscillator that measures buying and selling pressure on a scale of 0–100. Above 70 signals overbought conditions; below 30 signals oversold. The 50-line crossover is the most actionable level — when RSI crosses above 50, buyers have taken control. APEX weights RSI at 18% of its composite score, cross-referenced with MACD, volume, and 5 other signals before a verdict is generated.
What Is RSI in Stocks? A Complete Guide
The Relative Strength Index is the most widely used momentum indicator in trading. Here is everything you need to know — from the formula to real trade setups.
What Is RSI?
RSI stands for Relative Strength Index. It's a number between 0 and 100 that tells you how aggressively a stock has been bought or sold over the past 14 trading days. That's really all it is. One number. High RSI means buyers have been winning. Low RSI means sellers have been winning.
J. Welles Wilder built it in 1978 and his original rules still hold: above 70 is overbought, below 30 is oversold. Traders have been using those levels for 45+ years — which tells you something. What didn't hold up is the assumption that those levels automatically mean buy or sell. They don't. That's where most beginners go wrong.
How RSI Is Calculated
RSI looks at the last 14 days and separates the up-days from the down-days. Then it compares them:
1. Calculate the average gain across up-days 2. Calculate the average loss across down-days 3. Divide them to get Relative Strength: RS = Avg Gain ÷ Avg Loss 4. Apply: RSI = 100 − [100 ÷ (1 + RS)]
A stock that's been climbing steadily will have high average gains vs low average losses — high RSI. A stock getting crushed will be the opposite. The 14-period default is standard because it smooths daily noise without making the indicator too slow to catch real momentum shifts. Most traders don't need to change it.
What RSI Levels Actually Mean
Above 70 — Overbought: The stock's been on a tear. It's risen faster than its 14-day average in both speed and size. This does not mean sell. NVDA ran with RSI above 70 for three months straight in 2023 while adding 80%. Overbought just means tighten your stop and watch for divergence — don't automatically bail.
Below 30 — Oversold: Sellers have been hammering it. But a falling stock can stay oversold for weeks in a real downtrend. Don't catch the falling knife. Wait for RSI to turn back up and close above 30 before treating it as a buy signal.
The 50 Crossover: Honestly the most underrated RSI level. When RSI crosses above 50, buyers are officially in control — momentum is shifting. A lot of institutional algorithms have triggers around 50. Watch for it.
RSI Divergence: The Most Powerful Signal
Here's where RSI gets genuinely useful. Divergence happens when price and RSI stop agreeing with each other — and that disagreement is usually the market telling you something is about to change.
Bearish divergence: price makes a new high, RSI makes a lower high. The rally is losing fuel even as the stock climbs. TSLA showed months of bearish RSI divergence before its -65% collapse in 2021.
Bullish divergence: price makes a new low, RSI makes a higher low. Sellers are exhausting themselves even as price drops. META printed multiple bullish divergences in late 2022 before going on to rally 300% in 2023.
Divergence tells you direction, not timing. It's a warning, not a trigger. Confirm it with MACD and volume before you act.
Common RSI Mistakes
Selling at 70 in an uptrend. This is the most expensive RSI mistake. Momentum stocks in a strong trend can stay above 70 for months. If you sold NVDA every time RSI hit 70 in 2023, you left +400% on the table. Overbought isn't a sell signal in a bull market.
Buying at 30 in a downtrend. Below 30 during a real downtrend isn't a setup — it's a symptom. The trend has to show signs of turning first. Waiting for the close back above 30 before entering saves a lot of pain.
Using RSI alone. One indicator isn't enough. RSI tells you momentum. It doesn't tell you if the business is broken, if earnings are coming, or if institutional money is selling. That's why APEX runs it alongside 12 other signals before generating a verdict.
How APEX Stock Intel Uses RSI
RSI carries 18% of APEX's composite score — second highest of any single signal. But it never gets acted on alone. APEX cross-references RSI with MACD direction, volume, and six other signals before a verdict is generated.
The MACD+RSI combination is the highest-conviction setup in the model. When both indicators agree on direction — both bullish or both bearish — the signal historically holds up much better than either alone. That's 20% of the composite score when they align. It's why APEX surfaces the combination as the top signal in most analyses.
APEX combines RSI with 7 other signals in under 30 seconds. Free to try.
Run a Free Analysis →