While OI PCR is calculated by dividing put OI by call OI, Nifty volume PCR is calculated by dividing put trading volume by call trading volume. It is easy to get confused between them, so l lets differentiate.
To differentiate between volume and open interest, volume shows total number of transactions for a specific period, whereas, open interest shows the total number of unsettled contracts for a specific period of time.
Also, volume increases every time a transaction takes place(i.e. trade position is opened or closed), whereas open interest increases when new contracts are opened and decreases when the old ones are settled.
Again, a high PCR would suggest bearish sentiments in the market whereas a low PCR indicates bullish sentiments in the market. Nifty Volume PCR can be calculated manually or you can get the ratio at Nifty Trader.