Momentum trading
Relative volume (RVOL) and premarket volume explained
By the PatternX team · Published
Short answer
Relative volume (RVOL) compares how much a stock has traded with how much it normally trades. The simple form divides today's volume by average daily volume; time-of-day versions compare against what normally trades by the same minute. Premarket volume is thin, so read RVOL alongside absolute share volume and dollar volume, never on its own.
What relative volume measures
Raw volume tells you how many shares changed hands, not whether that's unusual. A million shares is a quiet morning for a mega-cap and an extraordinary one for a stock that normally trades 300,000 a day. Relative volume fixes this by dividing current activity by a baseline for the same stock: a reading of 1 means about normal, 3 means about three times normal, and 0.5 means about half.
Momentum traders watch it because a big price move on unusual participation is generally treated as more meaningful than the same move on a trickle of shares, and many premarket scanners let you sort or filter by it. RVOL says nothing about direction. It only tells you that more people than usual are involved.
Two ways to calculate RVOL
Simple (full-day) RVOL
The simplest formula compares everything traded so far today with a full day's average:
The average covers a lookback window, often the last 20 or 30 sessions, and providers choose different windows. For example, suppose a stock averages 2,000,000 shares a day and has traded 300,000 shares by 08:30 ET. Its simple RVOL is 300,000 ÷ 2,000,000 = 0.15.
That looks low, but the comparison is lopsided: part of a premarket session is being measured against an entire day. Simple RVOL starts near zero every morning and climbs through the session, so 0.15 at 08:30 can be unusually high. You have to judge it against how much the stock normally trades before the open.
Time-of-day-adjusted RVOL
The adjusted version compares like with like:
For the same hypothetical stock, suppose it has traded an average of 40,000 shares by 08:30 over the last 20 sessions. Today's 300,000 shares give a time-adjusted RVOL of 300,000 ÷ 40,000 = 7.5.
| Method | Formula | Reading at 08:30 in the example | Strength | Weakness |
|---|---|---|---|---|
| Simple (full-day) | Volume so far ÷ average daily volume | 0.15 | Easy to compute; needs one average | Climbs all day, so early readings look small |
| Time-of-day adjusted | Volume so far ÷ average volume by this time | 7.5 | Compares the same part of the session | Needs intraday history; unstable when the baseline is tiny |
Same data, two very different numbers. Before you set a threshold or compare scanners, check which definition a tool uses, how long its lookback is, and whether its averages include extended-hours trading.
Why premarket volume is thin
Premarket trading runs from 04:00 to 09:30 ET, but much of the market sits it out. Many institutions wait for the regular session and its opening auction, fewer market makers quote, and some brokers open their extended-hours window well after 04:00. Orders are matched on electronic networks rather than in one central auction, and activity tends to cluster around news. The premarket glossary entry covers the session itself.
That has three practical consequences:
- A few trades move the totals. One 20,000-share block can double a stock's premarket volume at 06:00.
- Feeds see different amounts. A feed that starts late or covers a single venue undercounts. Alpaca's free IEX feed, for instance, covers premarket only from 08:00 ET.
- Volume is often front-loaded. A burst around a headline can leave a high RVOL long after trading has dried up.
Why dollar volume matters
Share counts mislead when prices differ. Dollar volume, shares traded × price, measures how much money actually changed hands, which is closer to what matters when you need to get in and out.
For example, Stock A trades at $2.00 and has printed 150,000 premarket shares: $300,000 of dollar volume. Stock B trades at $40.00 and has printed 60,000 shares: $2,400,000. Stock A has two and a half times the share volume but one-eighth of the money. A $5,000 order equals about 1.7% of everything Stock A traded all morning and about 0.2% of Stock B's, so your own order is far more likely to push Stock A's price.
Dollar volume is still a record of what already traded. The spread and the size quoted right now decide what you'll pay, which is why liquidity needs its own check; premarket liquidity and bid-ask spreads covers that side.
Pitfalls that distort RVOL
Early-session spikes
Time-of-day RVOL is most fragile when its baseline is tiny. For example, if a stock has averaged 500 shares by 04:15, a single 10,000-share print produces an RVOL of 20. The arithmetic is right and the signal is close to meaningless: one participant, one trade. The same thing happens in the first minutes after a headline, before a real two-sided market has formed. Requiring a minimum share count and a minimum dollar volume alongside RVOL screens out most of these readings.
Low-float stocks
Float is the number of shares actually available to trade. In a low-float stock, a modest wave of buying can turn over a large part of the float, so RVOL reaches extreme multiples and price can move violently on small orders. Those moves can reverse just as fast, volatility halts are more likely, and spikes are sometimes followed by share offerings. A huge RVOL here says trading is frantic, not that the exit will be easy. Float and market cap are different measures: a market-cap floor removes the smallest companies but doesn't measure how many shares are free to trade (see why market cap matters).
Distorted baselines
An average that includes a recent news day is inflated, which makes today's RVOL look smaller than it is. A newly listed stock has little history to average. And readings from two providers with different lookbacks or session coverage can't be compared directly.
How Momentum Lab handles volume
Momentum Lab, PatternX's premarket momentum tool for US stocks on the Elite plan, applies volume rules as filters before any entry. The defaults:
| Filter | Default | Notes |
|---|---|---|
| Minimum premarket volume | 50,000 shares | 100,000 for Momentum Reversal |
| Minimum dollar volume | $250,000 | Premarket shares × price during premarket |
| Minimum relative volume | Off | Today's volume ÷ average daily volume |
Three details matter if you change them:
- RVOL is the simple, full-day form. It isn't time-of-day adjusted. If you switch on a minimum for a premarket strategy, set it against a full day's average: a premarket reading of 0.3 already means 30% of a normal day's volume has traded before the open.
- RVOL needs IBKR data. The average daily volume comes from Interactive Brokers. With other data sources RVOL is unknown, and an unknown value can't satisfy an enabled minimum, so an RVOL minimum would skip every candidate.
- Missing data fails closed. The same rule covers premarket and dollar volume: unknown volume is never treated as enough volume.
Relative volume is also one of the weighted factors in Momentum Lab's optional AI filter, a fixed formula that is off by default. Every default is listed in the Momentum Lab guide; for the methodology behind the filters, see how Momentum Lab works.
A quick checklist
- Which RVOL definition does your tool use, and over what lookback?
- How many shares, and how many dollars, have actually traded?
- How big is your order compared with the morning's dollar volume?
- Did the volume come from many trades or from one or two prints?
- What do the spread and quoted size look like right now?
Volume describes participation, not direction. A heavy-volume move can fail as easily as a light one, so treat RVOL as context for a decision rather than the decision itself.
Frequently asked questions
What is a good relative volume for a premarket stock?
There's no universal number, because it depends on the definition. A time-of-day RVOL of 3 or more says the stock is far busier than usual for that hour, while a full-day RVOL of 0.2 before the open can be just as notable. Treat either as context and confirm it with absolute share and dollar volume.
Why does my scanner show a different RVOL from my broker?
They probably use different formulas or inputs: full-day versus time-of-day averages, a 10-day versus a 30-day lookback, or averages that include or exclude extended hours. Feeds that start at different times or cover different venues also count different volume.
Is high relative volume a buy signal?
No. It shows unusual participation, which can come from buyers or sellers, and heavy-volume moves can reverse. It's a reason to look closer, not a reason to trade.
Does Momentum Lab use time-of-day-adjusted RVOL?
No. Its RVOL is today's volume divided by average daily volume, the filter is off by default, and the average comes from IBKR, so the filter only works with IBKR data. Its default volume gates are a 50,000-share premarket minimum (100,000 for Momentum Reversal) and $250,000 of dollar volume.
Why use dollar volume instead of share volume?
Share volume isn't comparable across price levels: 100,000 shares of a $1 stock is $100,000, while 100,000 shares of a $50 stock is $5 million. Dollar volume shows how much money is actually trading, which is closer to how easily an order of your size can be absorbed.