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, first 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.

The option "transfer the full account balance, reap the sender" does not appear - what should I do?

If you do not see this option, then your account either holds exactly 1 DOT, or the transferrable balance of your account is below 1 DOT. 

To fix this, send a small amount of DOT to this account to increase the transferrable balance so that the transaction fee can be deducted without the transferrable balance dropping below 1 DOT. Transaction fees are very low on Polkadot, so increasing the balance to e.g. 1.005 DOT should be sufficient. You will then see the "reap the sender" option appear and you will be able to send your full balance out. 

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