Up Volume and Down Volume are the two raw building blocks of volume breadth — the total share volume transacted in the day's advancing issues (Up Volume) versus the total in the day's declining issues (Down Volume), summed across an exchange or index. Where the advance-decline line counts how many stocks rose or fell, up/down volume measures how much capital moved with them — a conviction and participation gauge rather than a headcount. The core tension: a market can rise on thin, narrow up-volume (suspect) or fall on overwhelming down-volume (capitulation), and the relationship between the two is where the signal lives. The best-known named application is Lowry's "90% Days" — extreme single-session volume lopsidedness used to flag panic lows and the buying thrusts that confirm bottoms.

How it's calculated / formed

Each session, every issue in the universe (classically NYSE-listed common stocks) is bucketed by its close versus the prior close:

• Up Volume (Upside Volume) = sum of the day's volume for all issues that closed higher.
• Down Volume (Downside Volume) = sum of the day's volume for all issues that closed lower. Unchanged issues are excluded.

From these, several derived measures are built:

• Up/Down Volume Ratio = Up Volume ÷ Down Volume. A 50-day ratio above ~1.0 favors accumulation; common smoothing windows are 10-day and 50-day (per StockCharts/Decker).
• Net Advancing Volume = Up Volume − Down Volume — the input to the cumulative AD Volume Line and the McClellan Volume Oscillator/Summation Index.
• AD Volume Percent = (Up Volume − Down Volume) ÷ (Up Volume + Down Volume), bounded −100% to +100% (StockCharts ChartSchool).

Because absolute volume has grown enormously over decades, raw Up/Down Volume is not comparable across eras — ratio or percentage normalization is essential for any historical comparison (more so than for advance-decline counts, since volume is unbounded while issue counts are capped). Data is exchange-specific; NYSE up/down volume is the traditional series, and mixing it with composite or Nasdaq feeds corrupts the read.

How to read it

• Confirmation / thrust: A rally session where Up Volume swamps Down Volume (AD Volume Percent > +70%, per StockCharts) shows broad buying conviction; the reverse below −70% on a down day shows heavy selling pressure.
• Divergence: Price making new highs while the AD Volume Line fails to confirm (rising index, flat/falling net up-volume) warns that fewer dollars back the advance — a classic breadth-internals warning.
• Distribution days (IBD/O'Neil): A distribution day is a major index closing down ≥0.2% on higher volume than the prior session — read as institutional selling. A cluster of roughly five or six within a few weeks is O'Neil's signal to turn defensive. This is the down-volume-on-decline read at the index level.
• TRIN linkage: Up/down volume also feeds the Arms Index (TRIN) = (advancing issues ÷ declining issues) ÷ (Up Volume ÷ Down Volume); TRIN < 1 is broadly bullish. (See sibling node TRIN / Arms Index.)

Lowry's "90% Days" (the named signal)

Lowry's Research — a breadth-research firm dating to 1938 — built its bottom-detection framework on volume lopsidedness, popularized by Paul Desmond's 2002 Charles H. Dow Award paper, Identifying Bear Market Bottoms and New Bull Markets.

• 90% Downside Day: Downside Volume ≥ 90% of (Upside + Downside Volume) AND Points Lost ≥ 90% of (Points Gained + Points Lost). Read as panic/capitulation. Per Desmond, these tend to occur several times within a major decline, often spaced ~30 trading days apart, so a single one is not a bottom signal — it marks danger and exhaustion, not the turn.
• 90% Upside Day: the symmetric thrust — Upside Volume and Points Gained each ≥ 90% of their respective totals. Read as intense, broad buying.
• The bottom signal: Lowry's looks for two or more 90% Downside Days, followed within a short window by a 90% Upside Day (or, occasionally, two back-to-back 80% Upside Days). Desmond stressed that requiring this follow-through thrust — not a lone capitulation day — is what separates durable bottoms from bear-market bounces. Back-to-back 90% Upside Days are rare and have clustered near the start of major rallies.

Adoption, debate & evidence

Volume breadth is a long-standing institutional and technician tool (StockCharts, Lowry's, IBD all build on it). The honest read on the 90%-day thrust signals:

• They have meaningful qualitative support for flagging durable bottoms and come from Lowry's — a credible, long-tenured breadth-research firm with a documented (Dow Award) methodology. The logic — capitulation washes out sellers, a volume thrust confirms buyers stepping in — is sound and widely respected.
• But single days are noisy, and confirmation is everything. Desmond's own 1960-onward dataset records roughly 129 90%-downside and 67 90%-upside days — far too many to each mark a turn. Reviews of the record (e.g. New Low Observer) note the first 90% downside day in a decline typically arrives only after ~half the drop is already done, and that the signal "correctly identified nine bear-market bottoms and missed three" over the period while generating a number of extraneous signals. So: useful as one corroborating internal, not a standalone timing trigger, and the precise hit/miss counts vary by who tabulates them — attribute these figures to Lowry's/Desmond rather than treating them as settled fact.

Strengths & limitations

• Strengths: measures conviction (dollars), not just breadth (count); volume thrusts are hard to fake; capitulation/thrust framing is regime-aware; complements price and AD-line analysis.
• Limitations: highly exchange/data-dependent (NYSE vs Nasdaq vs composite gives different readings; ETF and dual-listed volume muddies the modern NYSE series); raw figures non-comparable across eras without normalization; single-day extremes are common and individually unreliable; the 90%-day framework needs clustered downside days plus an upside thrust to mean anything — the #1 misuse is treating one 90% downside day as a buy signal.

Sources

• StockCharts ChartSchool — Advance-Decline Volume Percent and Advance-Decline Volume Line (formulas, ±70% thresholds, Net Advancing Volume).
• StockCharts Articles (Dancing/Decker) — Up Volume / Down Volume Indicators (Ch. 8, CGMBI): construction, ratio normalization caution.
• Paul Desmond, Lowry's Research — Identifying Bear Market Bottoms and New Bull Markets (2002 Charles H. Dow Award): 90% Day definitions and signal methodology.
• New Low Observer (2014) — Review: Lowry's 90% Downside Days: Desmond dataset counts and hit/miss record (attributed, qualified).
• Proactive Advisor Magazine — Keys to identifying major/minor equity market bottoms: 90% upside/downside follow-through framing.
• Investor's Business Daily / William O'Neil — distribution-day definition (≥0.2% decline on higher volume; 5–6 cluster).
• Corporate Finance Institute / Fidelity — Arms Index (TRIN) formula and up/down-volume linkage.