Sector PB is the average price-to-book value ratio of a particular sector. Sector PB ratio is calculated as market capitalisation of the sector divided by gross book value of the sector. 

Market capitalisation of the sector is calculated as the sum total of the outstanding equity shares multiplied by the close price for each company of the sector. Similarly, the gross book value is the sum total of book value of each sector company.

The PB ratio is a measure to know how expensive the stock is when compared to scrips within the same sector. Sector PB ratio can be used as an effective comparison benchmark for this purpose.