Bitcoin blocks are limited to 4,000,000 weight units, about 1.5 to 2.5 MB in practice. Learn why the limit exists and how it creates the fee market.
The rule
A block's total weight must be at most 4,000,000 weight units (WU). Weight counts non-witness bytes four times and witness bytes once. See Transaction size, weight and vbytes.
| If a block is… | Its size is about |
|---|---|
| All legacy transactions | up to 1 MB |
| Typical modern mix | 1.5 to 2.5 MB |
| Mostly witness data | close to 4 MB at the extreme |
Why limit it?
Every node must download, check and store every block. Bigger blocks would make running a node more expensive, so fewer people would do it, weakening What does decentralized mean in Bitcoin?.
Before and after SegWit
Until 2017 the limit was 1,000,000 bytes. SegWit replaced it with the weight limit as a soft fork: old nodes still see blocks under 1 MB (they don't see witness data), while upgraded nodes allow more data in total.
Why fees exist
Limited space plus demand means transactions compete. When blocks are full, the fee rate needed to get in rises.
How full are blocks?
Open the latest blocks on MyBitcoinExplorer: each tile shows how full the block is by weight. When the mempool holds more than one block's worth, nearly every block is close to the 4,000,000 WU limit.
The block size debate, briefly
From 2015 to 2017, people disagreed over raising the 1 MB limit. SegWit, a soft fork that changed how size is counted, activated in 2017. A group wanting much larger blocks split off as Bitcoin Cash.
Key takeaways
- The block limit is 4 million weight units.
- The limit keeps full nodes affordable to run.
- Scarce block space is what creates the fee market.
Common questions
Why not just make blocks bigger?
Bigger blocks raise the cost of running a node and of propagating blocks quickly, which tends to centralize the network. This was the core of the 2015–2017 block size debate.
Is 1 MB still the limit?
Not exactly. The 1 MB limit was replaced by the 4 million weight unit limit in 2017.
What is the maximum Bitcoin block size in MB?
There's no MB limit any more; the limit is 4,000,000 weight units, which allows up to about 4 MB in extreme cases.
Will the block size ever increase?
Any increase would need broad agreement among users, and most developers consider larger blocks a centralisation risk.