Arms Index (TRIN)TRIN
The Arms Index, or TRIN, is a market breadth indicator that divides the advance-decline ratio by the advance-decline volume ratio. Readings near 1 are balanced; high readings show heavy volume in falling stocks and are often read as short-term oversold.
Is the market's drop broad and heavy, or narrow and light? The Arms Index answers by combining how many stocks rose or fell with how much volume went into each group. Developed by Richard Arms in 1967, TRIN is a short-term breadth oscillator traders use to spot overbought and oversold extremes. This entry explains the formula, how to read it and its quirks.
- Category
- Indicators and Tools
- Reading time
- 3 min
- Also called
- Arms index, Short-term trading index, TRIN indicator
- Updated
TRIN in plain words
The Arms Index is a breadth indicator created by Richard W. Arms in 1967, calculated by dividing the advance-decline ratio by the advance-decline volume ratio.1
It acts as an oscillator, so it is often used to spot short-term overbought and oversold conditions.1
Like the advance-decline line, it is built from the counts of advancing and declining stocks.2
How to calculate TRIN, step by step
Count advancers and decliners
Number of stocks up and down.
AD ratio
Advances ÷ declines.
Sum volumes
Volume in rising and falling stocks.
AD volume ratio
Up volume ÷ down volume.
Divide
AD ratio ÷ AD volume ratio.
Smooth if needed
A moving average reduces noise. See Moving Average.
Worked examples
Example 1 — Balanced day
TRIN 1.01,500 advancers, 1,500 decliners; 2bn shares up, 2bn down.
| AD ratio | 1.0 |
| AD volume ratio | 1.0 |
| TRIN | 1.0 ÷ 1.0 = 1.0 |
TRIN divides the advance-decline ratio by the advance-decline volume ratio, so a balanced day reads about 1.1
Example 2 — Heavy selling
Oversold signal1,000 advancers, 2,000 decliners; up volume 1bn, down volume 6bn.
| AD ratio | 1,000 ÷ 2,000 = 0.5 |
| AD volume ratio | 1 ÷ 6 ≈ 0.167 |
| TRIN | 0.5 ÷ 0.167 ≈ 3.0 |
On an unsmoothed TRIN, surges above 3 have been treated as oversold, which traders may use to look for bounces in an uptrend.1
Example 3 — Strong buying
Near overbought2,200 advancers, 800 decliners; up volume 5bn, down volume 1bn.
| AD ratio | 2.75 |
| AD volume ratio | 5.0 |
| TRIN | 2.75 ÷ 5 = 0.55 |
Dips below 0.50 on an unsmoothed TRIN are often considered overbought, so 0.55 is close to that zone.1
Reading TRIN
| Aspect | Raw TRIN | Smoothed TRIN (moving average) |
|---|---|---|
| Oversold | Above about 3 | Above about 1.20 |
| Overbought | Below about 0.50 | Below about 0.80 |
| Noise | High | Lower |
| Best use | Intraday extremes | Swing signals |
How traders use TRIN
Oversold bounces
Buying extremes in uptrends.
Intraday bias
Rising TRIN = selling pressure.
Breadth confirmation
Pair with breadth. See Market Breadth.
Combining tools
With TICK and McClellan. See TICK Index.
Advantages and disadvantages
Advantages
- Combines breadth and volume.
- Flags short-term extremes.
- Works on many indexes.
Disadvantages
- Inverted, so it confuses beginners.
- Very noisy unsmoothed.
- Thresholds vary by index and period.
Going deeper
Read as far as you need. Each level builds on the one before it.
Read it inversely
Beginner
High TRIN readings reflect heavy volume in falling stocks; low readings reflect heavy volume in rising stocks.1
Trade with the trend
Intermediate
When an index is above its rising 100-day moving average, oversold TRIN readings are preferred for bullish signals in line with the bigger trend.1
Mathematical quirks
Advanced
Because TRIN is a ratio of ratios, very different breadth and volume combinations can give the same reading, so many traders track the components too. See McClellan Oscillator.
Common mistakes
Reading it like RSI
High TRIN is bearish pressure, not strength.
Fixed thresholds everywhere
Adjust for index and smoothing.
Trading against the trend
Prefer signals with the bigger trend.
Using it alone
Confirm with price. See Divergence.
Check your understanding
Test yourself before moving on. Each answer comes with an explanation.
Key points to remember
- TRIN = AD ratio ÷ AD volume ratio.
- Around 1 is balanced.
- High readings show selling pressure; low readings show buying pressure.
- Used to spot short-term extremes.
- Smoothing changes the thresholds.
Frequently asked questions
What does TRIN stand for?
Is TRIN bullish or bearish when high?
Which indexes can TRIN use?
Further learning
- Market Breadth
The bigger picture.
- Put-Call Ratio
Another sentiment gauge.
- McClellan Oscillator
Breadth momentum.
Sources
This article is written in our own words. The sources below support the specific facts, rules and figures marked with a number in the text. Links open the original publisher’s page.
- StockCharts ChartSchool. Arms Index (TRIN) (opens in a new tab) Checked .
- StockCharts ChartSchool. Advance-Decline Line (opens in a new tab) Checked .
Educational content, not financial advice. Examples use hypothetical prices and simplified costs; actual fees, margin rules and regulations vary by broker, market and country. We are not affiliated with any publisher linked above. Last reviewed . Report an error or request a source credit.