Output

An output is a package of bitcoins created in bitcoin transaction.

You can create multiple outputs in a transaction, where each output contains an amount of bitcoin and a lock on it. A future transaction can then spend these outputs (as inputs) by unlocking them, and create new outputs with new locks on them.

So outputs are the building blocks for sending and spending money in bitcoin.

A transaction can only spend previously unspent outputs, and once an output has been spent it cannot be spent again.

Deposit Boxes

It's easiest to think of an output as a safe deposit box.

When you make a bitcoin transaction, you're selecting an existing deposit box, opening it up, and moving its contents in to new deposit boxes. The locks you place on these deposit boxes prevent anyone else from opening them up (other than the person you want to "send" the bitcoins to).

So you can think of the blockchain as one big vault of safe deposit boxes. Or as we call them in Bitcoin, outputs.

Change

When you spend an output in a transaction you have to use up the entire amount of the output; you can't just open up an output and only spend some of the bitcoins contained inside it, leaving the rest behind locked up in the original output.

So if the output contains more bitcoins than you'd like to actually send to someone, you need to create an extra change output to send the remaining bitcoins back to yourself.

Most transactions make use of a change output.

There's no specific output that you have to use for change. You could construct the transaction so that the first output contains your change (and the second output is the payment you're making), or even have multiple change outputs. There's no set rule, and you're free to create however many outputs in any order you like.

Nonetheless, the change input is typically the last output in a transaction.

Any leftover amount of bitcoins that do not get locked up in outputs will be classed as the transaction fee and will be collected by a miner. So if you forget to add a change output to your transaction, you will lose some bitcoins.

Structure

What does a transaction output do?

An output inside a raw transaction is just an amount (an integer) along with a locking script:

This structure repeats for every output.

So on a technical level, outputs are pretty straightforward.

Amount

The amount field is 8 bytes in size, so it can hold the values between 0 (0x0000000000000000) and 18446744073709551615 (0xffffffffffffffff). The field is also in little-endian, so inside raw transaction data the bytes appear to be backwards.

The amount is also in satoshis (1 satoshi = 0.00000001 BTC).

That maximum value is equivalent to 184,467,440,737.09551615 BTC, which is more BTC than is in existence.

The amount field of an output can be zero if you want.

The total value of the outputs in a transaction cannot exceed the total value of the inputs.

ScriptPubKey Size

This field contains the size of the upcoming locking script.

It's also a compact size field, so whilst it's usually 1-byte, it can expand if it's indicating a very large upcoming locking script. These are the compact size fields of the most common locking scripts:

ScriptPubKey

The ScriptPubKey field contains the lock you want to place on the output.

There are various different types of locks you can place on an output, but the most common type is to lock the output to someone else's public key (i.e. address). These types of locks are referred to as P2PKH and P2WPKH.

It's technically possible for the ScriptPubKey field to be empty. However, this would mean there would be no lock on the output, so anyone would be able to spend it.

Examples

What does a transaction output look like?

1 Output

01000000011fde77dd3bc6640714747f158e88260efe7f40236ee0446aca1a0808aec2caab010000006b483045022100f2c45824f970775daa1264f7b0779ee5df86359fab9ef47a0fee85ec97d46c0502201e1efe643c6c2bc56cced92a25c74158b01a5bfd06197a77851795a59fe5d80c0121034fb95ed3287f94561aa26df5ad19bb24762808ba278f7b26d1c85848f86a7ec4ffffffff0100e1f505000000001976a914299da5537e8b65bf45c70a9ece75988ebfca86b588ac00000000

{
  "version": "01000000",
  "inputcount": "01",
  "inputs": [
    {
      "txid": "1fde77dd3bc6640714747f158e88260efe7f40236ee0446aca1a0808aec2caab",
      "vout": "01000000",
      "scriptsigsize": "6b",
      "scriptsig": "483045022100f2c45824f970775daa1264f7b0779ee5df86359fab9ef47a0fee85ec97d46c0502201e1efe643c6c2bc56cced92a25c74158b01a5bfd06197a77851795a59fe5d80c0121034fb95ed3287f94561aa26df5ad19bb24762808ba278f7b26d1c85848f86a7ec4",
      "sequence": "ffffffff"
    }
  ],
  "outputcount": "01",
  "outputs": [
    {
      "amount": "00e1f50500000000",
      "scriptpubkeysize": "19",
      "scriptpubkey": "76a914299da5537e8b65bf45c70a9ece75988ebfca86b588ac"
    }
  ],
  "locktime": "00000000"
}

This is a very basic transaction that only contains one output:

The amount field is 00e1f50500000000, and if we convert that from little-endian to big-endian we get 0000000005f5e100, and if we convert that from hexadecimal to decimal we get 100000000.

This output also had a classic P2PKH locking script on it, which is always 25 bytes in size. We can identify this as a P2PKH script because they have a fixed pattern.

Furthermore, if we check the previous transaction we can see that the only input had a value of 100050000 sats. So the leftover 50000 was counted as the transaction fee.

The inputs in a raw transaction do not tell you their value. So to work out the value of an input, you have to go back to the previous transaction that created the output and get the amount from there.

These one-output transactions are quite rare (outside of coinbase transactions), as they're basically moving all of the bitcoins from one address to another.

2 Outputs

01000000011cc6d0c72e9b8311c675ce9157c385ee939568965b9609d1ced52b7bdf356e15040000006b483045022100bcbe87cdc8aaf78e89c373a1528abd9fec9f45e78bc699e13b569a4c854b719d02206ffdeb0af05b61e7c81d4b8648339a3edbb6b23598233e7f6ab2aa5415816f2301210277f43598c17f8163d33167b79793722dda1115bbc68e03a0f354c8b162bd1d80ffffffff02e2b3190000000000160014af267b26ef0ebd333b80dac1546f7db5d048d0274814de0c000000001976a914ff1a978dd905cf676b2ee8870ba47728294e609588ac00000000

{
  "version": "01000000",
  "inputcount": "01",
  "inputs": [
    {
      "txid": "1cc6d0c72e9b8311c675ce9157c385ee939568965b9609d1ced52b7bdf356e15",
      "vout": "04000000",
      "scriptsigsize": "6b",
      "scriptsig": "483045022100bcbe87cdc8aaf78e89c373a1528abd9fec9f45e78bc699e13b569a4c854b719d02206ffdeb0af05b61e7c81d4b8648339a3edbb6b23598233e7f6ab2aa5415816f2301210277f43598c17f8163d33167b79793722dda1115bbc68e03a0f354c8b162bd1d80",
      "sequence": "ffffffff"
    }
  ],
  "outputcount": "02",
  "outputs": [
    {
      "amount": "e2b3190000000000",
      "scriptpubkeysize": "16",
      "scriptpubkey": "0014af267b26ef0ebd333b80dac1546f7db5d048d027"
    },
    {
      "amount": "4814de0c00000000",
      "scriptpubkeysize": "19",
      "scriptpubkey": "76a914ff1a978dd905cf676b2ee8870ba47728294e609588ac"
    }
  ],
  "locktime": "00000000"
}

This is a more typical transaction. It contains 2 outputs:

So presumably the input (217582852 sats) was split in two: the first output being the payment to someone else, and the second output being the change being locked back to the original owner.

We can't be certain that the second output was the change however, as there's no designated way to set the change output on a transaction. But typically most wallets set the change output as the last output in a transaction.

3 Outputs

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

{
  "version": "01000000",
  "inputcount": "02",
  "inputs": [
    {
      "txid": "ec517e7b4c707c5e3afaf693aacbbd4ebe7d0d5c645381258c72e0e183b929d5",
      "vout": "00000000",
      "scriptsigsize": "8b",
      "scriptsig": "483045022100afcc65e5ed4284de7229320007b3938d443721cf4c3a8be221e7d872c637e550022070e9fd53679633d5421024952960fa47403c91170a68aeb9193f19c8959b7f39014104f4c33b57af42019e38a756163d20f08da3d7e55b6810ed3c3d5355bff563573bf06c1900d65cea45583599e5ad966565695198239345fd04ea1a4ad3b13ad8d0",
      "sequence": "ffffffff"
    },
    {
      "txid": "02d90d933c1d2f7a2283b9404de6a85c9aef9f4cbc96a212214c99c631a66e74",
      "vout": "01000000",
      "scriptsigsize": "8b",
      "scriptsig": "483045022100c01185dc7196b0de04fba00a8e964e8ad98cd2a4516784c7f5312c887c45b27d0220700b2a9f8c4b845e7bfc703927a85c1ce0f52d10266996304ea1295ab0fca40f0141049a3d95bc9711a3fe30f98790d179caea0ef7ba5a683c3555a566687659e21b3c2c83a7db8267353cf2ddeb8f521d8c1d4f6dc8bc5fcc4e1534623a9d6c0675c5",
      "sequence": "ffffffff"
    }
  ],
  "outputcount": "03",
  "outputs": [
    {
      "amount": "801a060000000000",
      "scriptpubkeysize": "19",
      "scriptpubkey": "76a914c0ea92934c59cf1bfa8db64deab8faac2b108ee588ac"
    },
    {
      "amount": "20a1070000000000",
      "scriptpubkeysize": "19",
      "scriptpubkey": "76a914b58e94278b9dc5b3ca3d8337876bd747bb365e5388ac"
    },
    {
      "amount": "7090000000000000",
      "scriptpubkeysize": "19",
      "scriptpubkey": "76a914b3c61b2cd26075419cb0302d1af241b8c7db62ed88ac"
    }
  ],
  "locktime": "00000000"
}

This is a more complex transaction with 2 inputs and 3 outputs:

This transaction could have been sending bitcoins to two different people at once, and then using the third output as change.

You can create as many outputs in a transaction as you like (as long as the transaction can fit inside a block), so you're not limited to only creating 1 or 2 outputs at a time. Therefore, you can send money to many different people in one transaction. Bitcoin transactions are very flexible.

The most outputs I've ever seen in a transaction is 13,107: dd9f6bbf80ab36b722ca95d93268667a3ea6938288e0d4cf0e7d2e28a7a91ab3

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

{
  "version": "01000000",
  "inputcount": "01",
  "inputs": [
    {
      "txid": "e44e51e602eb117e8fa18e0e970ef530a399eaabefd69c5f89dbfdb0569a7cc7",
      "vout": "01000000",
      "scriptsigsize": "fd1b01",
      "scriptsig": "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",
      "sequence": "ffffffff"
    }
  ],
  "outputcount": "02",
  "outputs": [
    {
      "amount": "90ecf03200000000",
      "scriptpubkeysize": "19",
      "scriptpubkey": "76a914bcefb891e8ecd882fd9785e24258bad9b7b96e0788ac"
    },
    {
      "amount": "0000000000000000",
      "scriptpubkeysize": "17",
      "scriptpubkey": "a914465a5922d985b0ab3f155e638ec5630ad5fffa4487"
    }
  ],
  "locktime": "00000000"
}

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

{
  "version": "01000000",
  "inputcount": "02",
  "inputs": [
    {
      "txid": "a08d75950a62c4a7d3b2ce5d5d451f340d26e27ab88c8d1d6a4d1e875846035a",
      "vout": "00000000",
      "scriptsigsize": "fd6001",
      "scriptsig": "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",
      "sequence": "ffffffff"
    },
    {
      "txid": "4ba60a7b2dc4ba89f10bb7939a9b8a6d6735642e655ca173625594432a006acd",
      "vout": "01000000",
      "scriptsigsize": "fd5e01",
      "scriptsig": "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",
      "sequence": "ffffffff"
    }
  ],
  "outputcount": "01",
  "outputs": [
    {
      "amount": "0000000000000000",
      "scriptpubkeysize": "00",
      "scriptpubkey": ""
    }
  ],
  "locktime": "00000000"
}

Summary

This is Satoshi's explanation of why we use outputs in Bitcoin:

So yeah, they're just small digital safe deposit boxes.