Bitcoin transaction fees are calculated based on the byte size of the transaction. They are given in "satoshi per byte" (a satoshi representing the smallest amount within bitcoin, i.e. 1 satoshi = 0.00000001 bitcoin). Transaction fees fluctuate depending on the network load. You can check your fees by going to https://bitcoinfees.21.co/ for the bitcoin network.
Look at the value displayed to the left of the first green bar from the top. This value indicates the number of satoshi per byte to be included in the transaction, if we want it to go through without delay.
The weight of your transaction is displayed when you carry out the transaction.
You can check the amount of FEE offered by your wallet by simply multiplying the number of satoshi per byte by the byte weight of the transaction.