How much is it from Northfields to Westbourne Park on the tube using an oyster card?

my friends and i are going to the notting hill carnival but i need to know how much it will cost on oyster as you have to change at hammersmith does that mean you will be charged for 2 journeys even tho hammersmith H&C line and pic/district line are cled as the same station. if anyone knows please let me know!