A block explorer may show a Bitcoin block’s size in bytes and its weight in weight units. Those numbers can differ substantially without either being wrong. They describe related measurements under rules that count parts of the data differently.
BIP 141, the Segregated Witness consensus specification, defines block weight as three times the base size plus the total size. Base size excludes witness data, while total size includes it. Because base data is also present within total size, it effectively contributes four weight units per byte; witness data contributes one.
Work through a small hypothetical block
Suppose an illustrative block has a base size of 600,000 bytes and a total size of 1,200,000 bytes. Its weight would be:
3 × 600,000 + 1,200,000 = 3,000,000 weight units
This is a calculation example, not a claim about a particular mined block. It shows why multiplying total bytes by four does not give the right answer when witness data is included.
The same specification sets the block-weight limit at 4,000,000 units. That does not mean every valid block contains exactly one fixed number of serialized bytes. Its composition matters.
Read the unit beside the number
When comparing explorers, record the block hash and the field names. Check whether “size” means total serialized size, stripped size or another displayed measure. A rounded figure in megabytes is not directly interchangeable with a figure in weight units.
Also keep a block measurement separate from transaction-level fee estimates. A wallet’s fee-rate display may use virtual bytes, and its terminology should be checked before comparing it with an explorer’s block summary. A large-looking number without its unit tells you little.
For a personal reference note, save one explorer page and reproduce the weight calculation using the fields it actually exposes. If the necessary base-size figure is absent, do not invent it from the total. The useful habit is to trace each number back to its definition before drawing a conclusion about block capacity.