📦 SegWit Segregated Witness
A 2017 Bitcoin upgrade that moves the signature ("witness") data into its own section of each transaction. That frees up room in every block and fixes a long-standing bug.
🚚 The simple version — a truck and a trailer
Every Bitcoin transaction has two parts: what it does (which coins move where) and the proof it's valid (the digital signatures). Picture a delivery truck where the bulky signed receipts take up cargo space meant for packages. SegWit moves those receipts into a light trailer towed behind the truck. The receipts still travel along to prove each delivery, but the truck (the 1 MB block) now carries more actual packages.
⚖️ How the new limit works — weight, not megabytes
SegWit replaced the old 1 MB block-size cap with a 4,000,000 weight-unit limit. The trick is how the two kinds of data are counted.
| Data type | Cost | What it is |
|---|---|---|
| 📤 Regular data | 4 weight units / byte | What the transaction does |
| ✍️ Witness data | 1 weight unit / byte | The signatures — a 75% discount |
📊 That discount lets more transactions fit per block. In practice effective capacity rose to roughly 1.7–2x, with full blocks holding about 1.8 MB of typical data.
🐛 The bug it fixed — transaction malleability
Before SegWit, the transaction ID (TXID) was calculated using the signatures too. Someone could tweak a signature, change the TXID, and confuse software that was tracking the transaction — a flaw called transaction malleability. SegWit computes the TXID from only the non-witness data, so the signature can no longer be tweaked to change it. That fix is exactly what made the Lightning Network and other Layer-2 systems reliable.
🔁 Why nothing broke — a soft fork
SegWit shipped as a backward-compatible soft fork (BIP 141). Non-upgraded nodes keep working; they simply receive the witness data stripped out. No new coin or separate chain was created. It locked in on 9 August 2017 and was enforced about two weeks later, around block 481,824. If you want to compare, see how a hard fork differs.
🏷️ Where beginners meet SegWit — addresses
You mostly run into SegWit through wallet address formats. Picking a SegWit address usually means lower fees, because witness data is discounted.
- 3️⃣ Nested SegWit — P2SH addresses that start with
3; older and widely supported - 🔤 Native SegWit — bech32 addresses that start with
bc1q; usually the lowest fees - 🌿 Taproot — bech32m addresses that start with
bc1p; Taproot builds on the SegWit v0 foundations
❓ FAQ
- Did SegWit raise Bitcoin's block size to 4 MB?
- No. The new cap is 4,000,000 weight units, not 4 MB of bytes. Because real transactions mix regular data and witness data, a full block usually lands around 1.8 MB of actual data, not 4 MB.
- Did SegWit create a new coin or split the chain?
- No. SegWit was a backward-compatible soft fork, so non-upgraded nodes kept working and no new coin or separate chain came out of it. Bitcoin Cash was a separate hard fork driven by the surrounding scaling debate, not by SegWit itself.
- Do I need to do anything to use SegWit?
- Mostly you just pick a SegWit address in your wallet. Native SegWit addresses start with 'bc1q' and usually pay the lowest fees because witness data is discounted. Older addresses still work, they just cost a little more.