RSI Indicator: How to Read It and Trade It

The RSI indicator — the Relative Strength Index — answers one question: over the last stretch of bars, how lopsided has the buying been against the selling? It compresses that into a single number between 0 and 100, and it is the most-used momentum gauge in trading for a reason. One glance tells you whether a move has been one-directional enough to be worth doubting.

It does not tell you when that move ends. Most of what goes wrong with RSI comes from treating a reading as a timer. This guide gives you the exact formula the engine runs, how to read the extremes, and — with real examples — the situations where the indicator will hand you a loss.

What RSI measures

RSI is a ratio dressed up as a scale. Over the lookback window it separates every close-to-close change into gains and losses, averages each side, and asks which side dominated.

All gains and no losses drives the number toward 100. All losses and no gains pushes it toward 0. A market where up days and down days balance out sits near 50. So the reading is not about price level at all — it is about the character of the recent path. A coin can make a new high with RSI falling, because the way it got there was weaker than the way it got to the last one.

That is the whole intuition — everything after it is an argument about where to draw the thresholds and what to do when price crosses them.

How RSI is calculated: the exact formula

Most explanations stop at "it's a momentum oscillator". Here is what actually runs.

Take the change from each close to the next. Positive changes are gains, negative ones become losses as positive numbers, and flat bars count as zero on both sides. Seed each average with a plain arithmetic average of the opening fourteen. From there, every new bar folds in with Wilder smoothing — the previous average weighted 13/14 against the new value at 1/14 — which is why RSI reacts smoothly instead of jumping.

How Vike computes it. batch_rsi in vike-indicators seeds avg_gain and avg_loss with a simple mean of the first 14 bars, then folds each later bar as avg = (avg × 13 + value) ÷ 14, and returns 100 − 100 ÷ (1 + avg_gain ÷ avg_loss). When avg_loss is zero the result is exactly 100. The first 14 bars have no value at all — RSI needs history before it means anything.

Two consequences fall out of that formula and both matter in practice. Because the averages are smoothed rather than reset, a single violent bar cannot slam RSI from 50 to 20 — it takes a run of one-sided bars. And because the denominator is an average of losses, a market that simply stops falling will lift RSI even with no buying at all.

Reading overbought and oversold

The conventional lines are 70 and 30. Above 70 the recent path has been overwhelmingly up; below 30, overwhelmingly down.

Here is where nearly everyone goes wrong. Those are not sell and buy signals. An RSI of 75 says buyers have been in complete control — which, in a real trend, is a reason the move continues rather than a reason to fade it. The mistake I made early was treating 30 as a buy button, and strong trends punish that fast: they routinely pin RSI above 70 for weeks, and every reading is a fresh invitation to lose money shorting strength.

Treat the extremes as a condition, not a trigger. Oversold means the fuel for further downside is thinning; it does not mean the bottom is in. The signal is what happens next — RSI turning back through the line, price confirming, a hammer or shooting star printing at the same moment. Not before.

How to trade RSI

Four rules turn RSI from a decoration into something that survives contact with a real market.

Trade the exit from the extreme, not the entry into it. Wait for RSI to cross back above 30 on a long, or back below 70 on a short. That turn is your evidence the one-sided pressure actually eased.

Demand price confirmation. A momentum reading is not a price event. Let price close through a level — the prior bar's high on a long, its low on a short — before committing.

Respect the trend above the oscillator. In a strong downtrend, oversold readings are noise; the useful RSI signals there are the overbought bounces to sell into. I skip counter-trend RSI trades entirely now, and it removed most of my losses with this indicator.

Watch for divergence, carefully. Price making a lower low while RSI makes a higher low says selling pressure is fading. It is the most respected RSI signal and also the most abused: divergence can persist for a long time before anything happens, so it is context, not a trigger.

Where RSI fails

Here is the honest part, and the failure below is a real one the engine flagged.

RSI fails hardest in strong trends. It will read oversold repeatedly all the way down a bear market, and every one of those readings looks like an opportunity. Oversold can always get more oversold — the scale is bounded at 0, but price is not.

Three other failure modes recur. Low-liquidity assets distort the averages, because a handful of outsized bars carry far more weight than they deserve inside a fourteen-bar window. The warm-up is thin too. And any trader who reads 30 and 70 as instructions rather than descriptions gets run over eventually by a trend that simply keeps going.

Every version of the mistake has the same shape. RSI describes what has happened; it carries no information about what happens next except by inference, and that inference gets weaker the stronger the trend.

RSI with candlestick patterns

This is where the indicator earns its place. RSI and price patterns fail in different ways, so together they filter each other.

A candlestick reversal at an RSI extreme is a far better setup than either signal alone. The candle says buyers or sellers were rejected at one specific price on one specific bar, which is a much narrower and more testable claim than the oscillator ever makes. The oscillator says the market was stretched thin exactly then. Two independent reasons — one about a single bar's structure, one about the fourteen bars behind it.

A doji printing with RSI at 28 and turning up is worth attention. The same doji at RSI 52 is noise.

Frequently asked questions

What is a good RSI setting? Fourteen periods is the default and what nearly every chart shows, which matters because widely-watched levels attract reactions. Shorter settings react faster and produce far more false signals.

Is RSI above 70 a sell signal? No. It reports that buying has dominated lately, which in a trending market argues for continuation. The tradeable event is RSI crossing back below 70 with price confirming.

What is RSI divergence? Price making a new extreme while RSI does not. A lower low in price against a higher low in RSI suggests fading selling pressure. Treat it as context — divergence can run for weeks before price responds.

Why does RSI have no value on the first bars? It needs fourteen bars of closes before either average exists, so Vike deliberately returns no value at all for those bars rather than handing you a misleading partial number that looks real.

This is educational material, not financial advice. Indicators describe probabilities, not certainties, and every real trade carries real risk — size positions so a wrong read costs little.

See the candlestick patterns that pair with RSI