Block Weight

Anna sends Bitcoin, but the next block does not include her payment. That alone does not mean the money has disappeared. An unconfirmed transaction may be waiting in a Mempool, an individual node’s local waiting area. A node is software that checks transactions and blocks and communicates with other participants in the network. A miner selects transactions for a block with limited capacity and can choose to make a smaller block. Fees provide an incentive, not an instruction to include a particular payment. Block Weight explains one constraint on that choice, not every reason for a delay. [1] [7]

The upper limit is 4 000 000 WU, including that exact value. WU stands for weight unit. A Full Node, which independently checks Bitcoin’s rules, rejects a block above this limit. High fees cannot rescue it, nor can valid Proof of Work, the computational proof required to mine a block. Finding that proof and constructing a valid block are both necessary. A miner’s wishes alone therefore cannot determine capacity. [1] [3] [4]

A byte is a standard unit of data size. SegWit separated some of the data proving permission to spend, including signatures. This data is called witness. A byte of base data contributes 4 WU, while an additional byte in the full representation with witness contributes 1 WU. The latter also includes the marker and flag fields of that representation. This preserves the base transaction structure while making room for verification data, without counting every byte equally. [1] [2]

A signature lets a sender prove the right to spend particular coins. A short program called a script specifies the spending conditions. Witness contributing less weight does not allow a validating node to ignore the applicable signatures or script conditions. The data also remains in the full serialization. This is a capacity-accounting rule; it does not mean processor work, network traffic or storage requirements fall in the same proportion. [1] [4]

Serialization means writing a block as the precise sequence of bytes computers exchange. Let B be its size without witness and T its full serialized size. BIP 141 uses W = 3 × B + T. Base data is already included in T, so it is counted four times overall. Data added by witness serialization is counted once. The formula applies to the entire block, not just ordinary payments inside it. [1] [8]

Imagine sizes B = 500000 and T = 2500000 bytes. The result is W = 4000000 WU. A different mix can have B = 750000 and T = 1750000 bytes, again giving W = 4000000 WU. These are illustrative calculations, not measured blocks. Both use the same weight, but the second requires fewer transmitted bytes. “A full block is always 4 MB” is therefore wrong. The number of transactions in a full block is not fixed either. [1]

A block is not merely a collection of user payments. It has an 80-byte header containing, among other things, a reference to the previous block and a summary of its transactions. Next comes the transaction count, encoded with CompactSize, a variable-length integer encoding. The first transaction is the Coinbase Transaction, the special transaction for the miner’s reward; this name does not refer to an exchange. All these parts count. Adding the weights of the other transactions does not yet give the whole block’s weight. [5] [7] [8]

A wallet is an application or device for receiving and sending bitcoin. It usually uses a transaction’s virtual size, expressed in vByte units. This is the weight of the whole transaction divided by 4 and rounded upward. It helps compare fees for the capacity used. Adding these rounded transaction sizes does not give the exact block weight: the unit differs, each transaction has been rounded, and block overhead is missing. A large monetary payment does not necessarily require a large data transaction. [1] [8]

Weight alone does not describe all the work a node performs. There is also a limit of 80 000 weighted cost units for signature verification under the applicable rules. This is not a simple count of every signature in the block. Tapscript, the scripting language introduced with Taproot, has a separate budget for each input, the part of a transaction spending previously received coins; its signature operations do not count toward that older block-wide limit. BIP 342 describes this different constraint. Meeting the weight limit therefore does not guarantee that arbitrarily demanding verification is allowed. [1] [3] [9]

A block explorer is an application for looking up transactions and blocks on a particular network. In Bitcoin Core v29.0, getblock returns size, strippedsize and weight among other fields: full byte size, size without witness and weight. You may also encounter chainwork. It describes the chain’s accumulated computational work, not its data weight. For an everyday user, the useful distinction is capacity versus price: Block Weight limits block space, while a payment’s actual fee depends on its size and chosen rate. None of these figures alone promises a confirmation time. [6]