One possibility is to measure the value of derivatives outstanding on a typical date. However, since futures, an important component of this industry, are resettled each day and reset to zero value, such a measure would obviously not work.
Open interest is the number of derivatives contracts outstanding.
In the futures and options markets, open interest is the number of contracts currently held by buyers. Of course, since there are as many buyers as sellers, we could alternatively define open interest as the number of contracts currently sold. But this measure does not adjust for substantial differences among contract specifications in the number of units of the underlying asset, or for substantial differences in the price of the asset underlying each unit. Finally, multiplying this notional value by the open interest, gives the total value of the assets underlying the contract. The attached table gives estimates of the size of the international market for derivatives measured in terms of the value of the assets underlying derivatives.