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Lesson 11 / 21 · Advanced

SegWit and transaction weight

Connect a structural change to practical fee accounting.

14 MIN WITH PRACTICEREAD → TRY → REFLECTNO WALLET NEEDED

By the end, you’ll be able to…

  • Explain witness separation at a high level.
  • Calculate virtual size from weight.
Your learning map

Three questions to carry into this lesson.

01Explain witness separation at a high level.
02Calculate virtual size from weight.
Use these goals to guide your reading. Try the paper exercise, then explain the result in your own words.

Witness data has a distinct role

Segregated Witness, described in BIP 141, separates witness information used to satisfy spending conditions from the traditional transaction serialization used for the transaction identifier. This addresses important transaction-malleability concerns for the relevant forms and introduces a weight-based way to constrain block resources. It does not mean that signatures disappear or stop being validated.

Weight and virtual bytes are related

Transaction weight combines base size and total size according to the specified formula. Virtual size is weight divided by four, rounded up. Wallet fee estimates commonly use virtual bytes, so a comparison based only on raw file length can be misleading. The output and input types influence the data that needs to be represented.

Explain benefits without universal promises

A SegWit label does not guarantee that every transaction is cheaper than every older-format transaction. Input counts, script structure and fee rates still matter. Likewise, a protocol improvement does not make an unknown wallet implementation safe. When reviewing a technical claim, separate the deployed consensus feature from product support and a particular transaction’s measured characteristics.

For a practical estimator, label whether its input is bytes, weight units or virtual bytes before applying a fee rate. A correct multiplication with the wrong unit still produces a wrong fee.

Your turn / A paper experiment

Practice on paper

A hypothetical transaction weighs 561 weight units. What virtual size should be used before multiplying by a fee rate?

I’ve tried it — show the worked answer

Divide by four to get 140.25 and round up: 141 virtual bytes. At an illustrative 3 sats/vbyte, the corresponding fee calculation is 423 sats.

Want to explore with buttons and instant feedback? Try the practice lab ↗

Think it through

Make a choice. Discover why.

Choose an answer and check the explanation. You can retry as often as you like. These are practice questions, not a test of mastery; answers are not saved or sent.

1. Does witness separation mean signatures are no longer checked?
  • Yes
  • No
Read the explanation

No. Witness data remains part of validation.

2. Should virtual size round down when weight is not divisible by four?
  • Yes
  • No
Read the explanation

No. BIP 141 specifies rounding up.

One idea to take with you

Use the protocol’s resource units when comparing fees.

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