Put/call open interest ratio, explained
The put/call open-interest ratio divides total put open interest by total call open interest for one underlying and expiry, on one snapshot date.
What it measures
A ratio above 1 means more put contracts than call contracts are open at that snapshot; below 1 means more calls are open. It describes positioning as recorded in open interest, not a sentiment score and not a trade recommendation.
Reading a trend across snapshots
Compute the ratio for each snapshot date within the same expiry, then read the sequence of ratios in order. A rising ratio across snapshots means put open interest is growing faster than call open interest for that expiry; a falling ratio means the reverse.
Where the ratio breaks down
If call open interest is zero at a given snapshot, the ratio is undefined rather than infinite, and should be reported as undefined rather than a placeholder number. Ratios computed across different expiries, or across a change in contract multiplier, are not comparable to each other.
What it is not
The ratio is a count of open contracts, not a probability, not a directional signal, and not a substitute for reading the underlying open-interest table by strike.