Locktime

Post-dating a bitcoin transaction

Diagram showing how the locktime field can be used to prevent a transaction from being mined until a specific block height or time in the future.

The locktime field allows you to prevent a transaction from being mined until after a specific block height or time.

A transaction with a locktime in the future will not be accepted or relayed by nodes either, so you have to store it locally until the locktime you have set on the transaction has passed.

In other words, setting a locktime on a transaction is the equivalent of post-dating a cheque.

An image of a post-dated cheque representing a transaction with a locktime of a specific block height.

Usage

How does locktime work?

The locktime field is 4 bytes in size and can hold values between 0 (0x00000000) and 4294967295 (0xffffffff).

You can set a specific block height or time by using different ranges of value:

Locktime Description
<=499999999 Transaction cannot be mined until after a specific height.
>=500000000 Transaction cannot be mined until after a specific time.

This is also known as "absolute locktime", as you're setting a specific height or time in the future. It's also possible to set a relative locktime on a transaction if you prefer.

For the locktime field to be enabled, at least one of the sequence values on the inputs must be set to 0xfffffffe or below. If all of the inputs' sequence values are set to the maximum value of 0xffffffff, the transaction is considered "final" and the locktime feature is disabled.

Height

0 to 499999999

Diagram showing the locktime field being used to set a specific block height.

By setting the locktime between 0 (0x00000000) and 499999999 (0x1dcd64ff) you can specify that the transaction can only be mined in to the blockchain after a specific height.

This is a more than suitable range, as the blockchain is not expected to reach a height of 499,999,999 for another .

Time

500000000 to 4294967295

Diagram showing the locktime field being used to set a specific block time.

By setting the locktime between 500000000 (0x1dcd6500) and the maximum value of 4294967295 (0xfffffff) you can specify that the transaction can only be mined in to the blockchain after a specific time.

This time value is in Unix Time:

tool-6623ee1c4db65
Tool Icon

Unix Time

Convert the number of seconds since 1 Jan 1970, 00:00:00 to a date.

0d
0 secs

The actual time restriction is then based on the time field inside a block header. The time set inside a block is controlled by the miner, and whilst it's usually pretty close to the current time, it can sometimes be an hour or two out.

This range of values allows you to set a locktime between and .

Examples

Here are a couple of simple examples of the locktime in action:

And for good measure, here are a couple of examples where the locktime field was used, but it wasn't actually functioning because none of the sequence values were set to below the maximum value of 0xffffffff:

Raw Transaction

The locktime field is always the last 4 bytes of a transaction:

020000000001019a40d4b676ad05adea5a0aa1d093a5c16f298c4b7e31d70fd157e262e86d08900100000000feffffff0228e227000000000017a914ae72b0ccd1a65ec89a7be021e47eccc60a440bb3874dcbf91600000000160014553be08a6faa63e4038b4627996af76637522c500247304402200d9f0f85b355a29f4c14b3ee16257d92fc67429b47a15c1658d8b6737425a1c7022000d0c64c2004b0c1c47968b94c9d24820d1df20c13d37a66d20f62b8feca67b00121021853828191c3e1a4fa29c685e5e1da4710dd1dc35c9b85d6bab23c0e00109f45500e0c00

Transaction: f168381d64b32d7b03b3f0b82cadba72e815351686e3bff2b8b5ab92f65a58bf

The locktime field in a raw transaction is in little-endian. So in the example above, if we reverse the byte order of 500e0c00 we get 000c0e50, and if we convert that from hexadecimal to decimal we get 790096.

So this transaction set the locktime as a block height of 790,096 (and was mined in to the block after that).

Little-Endian

Resources