If you want to send your total balance out of your account, be aware that your account will then be deactivated. Only accounts with a balance higher than the Existential Deposit, are active on the Polkadot network. You can learn more about this here.

On the SEND screen, enter the full amount you want to send.

Then disable the "keep alive" check. This will make a new option appear below: "transfer the full account balance, reap the sender". 

When you switch that option on, the transaction costs will automatically be deducted from the total amount, as you can see in the image above. Also, your account will be reaped, as it will fall below the Existential Deposit amount.  

You can reactivate it anytime by depositing a minimum of 1 DOT.

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