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 of 1 DOT are active on the Polkadot network. You can learn more about this here.

Note: Not all wallets that support DOT have the technical capability to allow you to send out the existential deposit. Below you will learn how to do this in the Polkadot-JS UI

If your current wallet does not have this functionality, you can export your private key from there and import it to Polkadot-JS UI or contact the support of your wallet to see if there's a way to deactivate your account. Once imported, you can proceed as below. Note that exposing your private keys on a website comes with a security risk, therefore you should only do this if you do not plan on reactivating this account in the future. 

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 very close to it.

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 even 1.02 DOT should be sufficient, but to be safe it's recommended that you have at least 1.1 DOT. You will then see the "reap the sender" option appear and you will be able to send your full balance out. 

Another possibility is that your account has other types of balances, like locked or reserved. In that case the account can't be reaped, so you need to manually enter the amount you want to send. Also, the transferable balance can be below 1 DOT, but if it's too low the UI won't let you send out funds.