The percentage of an investment portfolio that is held by a single asset. Portfolio weight can be determined by a number of factors including cost, units, sectors, regions, and types of securities. The calculation for portfolio weight by value is computed by dividing the value of a single asset by the value of the entire portfolio. For example, Stock A has a value of $400 and the total portfolio value is $2,000. The portfolio weight of stock A is 20%. (400 / 2,000 = .20 or 20%)