CParkinson3 Ratio- Financial Metric Explained
What Is the Parkinson3 Ratio?
The Parkinson3 ratio (often written as P3 or the Parkinson volatility ratio) is a volatility measurement that uses the high and low prices of a security to estimate how much it moves over a given period. Michael Parkinson introduced it in a 1980 paper, arguing that the traditional high-low range captures price movement more efficiently than closing-price-based methods.
Most traders know about standard deviation as a volatility tool. The problem? It ignores everything between open and close. The high and low prices tell a different, more complete story about a stock's actual trading range.
The Parkinson3 ratio is that story, quantified.
The Formula
Here is the Parkinson3 ratio formula:
P3 = (âN Ă ln(H/L)) / (4 Ă âln(2))
Where:
- H = Highest high over the period
- L = Lowest low over the period
- N = Number of trading periods
- ln = Natural logarithm
Most charting platforms simplify this into a single volatility number you can plot directly on a chart without doing the math yourself.
Parkinson3 vs. Other Volatility Measures
You have options when measuring volatility. Here is how P3 stacks up against the alternatives:
| Measure | Data Used | Efficiency | Best For |
|---|---|---|---|
| Parkinson3 Ratio | High & Low only | High (uses full daily range) | Capturing intraday swings |
| Standard Deviation | Close prices | Lower (ignores range) | Long-term portfolio risk |
| Garman-Klass | O, H, L, C | Higher than P3 | More precise daily volatility |
| ATR (Average True Range) | H, L, C | Moderate | Stop-loss placement |
The P3 ratio is roughly 5 to 8 times more efficient than standard deviation at estimating volatility from the same data. That is not a small advantage when you are trying to spot regime changes quickly.
How to Calculate the Parkinson3 Ratio â Step by Step
Let us walk through a practical example using 5 trading days of a stock:
Step 1: Gather Your Data
- Day 1: High $105, Low $98
- Day 2: High $108, Low $100
- Day 3: High $107, Low $99
- Day 4: High $112, Low $103
- Day 5: High $110, Low $102
Step 2: Find the Highest High and Lowest Low
H = $112 (Day 4)
L = $98 (Day 1)
Step 3: Apply the Formula
ln(112/98) = ln(1.143) = 0.133
â5 = 2.236
P3 = (2.236 Ă 0.133) / (4 Ă 0.833)
P3 = 0.297 / 3.332
P3 = 0.089 (or 8.9%)
That 8.9% represents the annualized volatility estimate based purely on the high-low range over those 5 days.
How Traders Actually Use It
The P3 ratio is not just an academic exercise. Traders use it in a few concrete ways:
Volatility Regime Detection
When P3 spikes, it means the high-low range is widening. This often signals increased uncertainty or the start of a trending move. When it compresses, markets are coiling â a breakout is coming.
Comparing Securities
You can run P3 across different stocks to see which one is genuinely more volatile. A stock trading at $50 with a P3 of 15% is doing something different than a $200 stock with the same percentage. The ratio normalizes across price levels.
Options Pricing Context
Higher P3 values mean wider expected price swings, which translates to higher option premiums. Knowing when volatility is underpriced relative to the actual range gives you an edge before earnings or events.
Limitations You Need to Know
No metric is perfect. The Parkinson3 ratio has real constraints:
- It ignores gaps. If a stock opens 5% higher than the previous close, P3 does not capture that jump. It only measures what happened within each trading day.
- Smooth data works best. In thinly traded or manipulated markets, high-low ranges can be distorted by spread widening rather than genuine price movement.
- Not a standalone signal. P3 tells you volatility is changing. It does not tell you direction. Use it alongside price action analysis.
Getting Started
If you want to start using the Parkinson3 ratio today:
- Pick your platform. TradingView has P3 as a built-in indicator. Thinkorswim and MetaTrader may require custom scripting.
- Set your period. 14 to 21 periods is standard for daily charts. Shorter periods catch faster regime changes; longer periods smooth out noise.
- Watch the baseline. Compare current P3 to its 6-month average. A reading above 1.5x the average is a meaningful signal.
- Combine with volume. P3 spikes alongside unusual volume confirm the move is real.
Is P3 Worth Your Attention?
If you are measuring volatility with nothing but closing prices, you are leaving information on the table. The Parkinson3 ratio takes 30 seconds to add to a chart and gives you a clearer picture of what the market is actually doing.
It is not a replacement for standard deviation or ATR. It is a complement â one that catches intraday extremes those other tools miss. Use it to confirm, not to lead.