# Block weight and the limit

Why a block's capacity is counted in weight units, the 4,000,000 WU consensus limit, and the 400,000 WU limit on a standard transaction.

URL: https://blockspace.wiki/basics/block-weight-and-the-limit/

A block's capacity is not counted in bytes. It is counted in [weight units](https://blockspace.wiki/glossary/weight-unit/) (WU), and consensus caps a [block](https://blockspace.wiki/glossary/block/) at **4,000,000 WU**. This page is the short version; the formulas and worked transactions are on [weight units and vbytes](/concepts/weight-units-and-vbytes/).

## How weight is counted

block.space's data-types reference, citing BIP 141, gives the rule:

```
weight = stripped_size × 3 + total_size
```

Here `total_size` is the whole transaction and `stripped_size` is the transaction without its [witness](https://blockspace.wiki/glossary/witness/). In plain terms:

- every byte outside the witness costs **4 WU**;
- every byte inside the witness costs **1 WU**.

A legacy transaction, with no witness, weighs exactly four times its size.

## What the limit allows

Because base and witness bytes weigh differently, the same 4,000,000 WU can hold different numbers of bytes. Blockspace Value Theory's worked example (§3.6) gives a block's capacity as 1,000,000 vB, and BVT §3.1 notes that a block made almost entirely of witness data approaches 4 MB, and works it through: 4 MB × 144 blocks a day × 365 days is about 210 GB a year. BVT calls that a ceiling, "not a schedule": the most that could be used if every block were filled to the limit with the most discounted data, which "has never been sustained". *BVT is a living document and an untested thesis.*

## A smaller limit for relay

Consensus decides what is valid. Relay policy decides what a node passes on. The reference gives the policy limit: a [standard transaction](https://blockspace.wiki/glossary/standard-transaction/) weighs at most **400,000 WU**, about 100,000 vB. A bigger transaction is still valid, but it has to be sent to a miner directly. blockspacelabs.com lists both figures for Blockspace Holdings' own node: max standard tx 400,000 WU, max block weight 4,000,000 WU (read 2026-10-20 09:07 UTC). See [relay policy](/concepts/relay-policy/).

## Is the limit fixed?

BVT §3.3 argues it is "a social variable, not a constant": the 4M weight limit is where the community currently believes the [decentralization constraint](https://blockspace.wiki/glossary/decentralization-constraint/) binds, "revisable in either direction". blockspacelabs.com lists the block weight limit and the witness discount among the consensus levers in its descriptive study "Blockspace expansion policy", which, in its words, "takes no side".

## What the chain actually used

The census has counted 777.07 GB of [raw bytes](https://blockspace.wiki/glossary/raw-bytes/) over 972,319 blocks (as of block 972,318, 2026-10-20 12:15:32 UTC). How that is sized in fee terms is on [vbytes](/basics/vbytes/).

Sources:

- block.space docs: data types · https://block.space/docs/datatypes
- block.space docs: Blockspace Value Theory · https://block.space/docs/bvt
- The blockspacelabs.com page (index.html) · https://blockspacelabs.com
- Blockspace Labs facts (data/facts.json) · https://blockspacelabs.com
- block.space meter and summary APIs · https://block.space/api/summary

Live figures: census as of block 972,318, 2026-10-20 12:15:32 UTC

Updated: 2026-10-20