Calculation of Realized Profit/Loss

For long position: 

For example:
A trader buys 4,000 BTCUSD contracts at 5,000 USD, and it then rises to 5,100 USD,
Realized Profit = 4,000 x (1/5,000 – 1/5,100)
                      = 0.0156 BTC

For short position: 

For example:
A trader sells 4,000 BTCUSD contracts at 5,000 USD, and it then rises to 5,100 USD,
Realized Loss = 4,000 x (1/5,100 – 1/5,000)
                     = - 0.0156 BTC