Virtual transaction size and Transaction weight are not consensus-critical, but are used in bitcoin-core IsStandard() policy, and v0.13.0+, RPCs calls return virtual transaction size instead of serialized transaction size. Although non-normative, BIP 141 should include definitions for these concepts.
https://github.com/bitcoin/bitcoin/pull/8747 is a PR to fix the RPC help text to refer to virtual transaction size instead of transaction size.
ACK
131 | + 132 | +==== Transaction size calculations ==== 133 | + 134 | +''Transaction weight'' is defined as ''Base transaction size'' * 3 + ''Total transaction size'' (ie. the same method as calculating ''Block weight'' from ''Base size'' and ''Total size''). 135 | + 136 | +''Virtual transaction size'' is defined as ''Transaction weight'' / 4 (rounded up to nearest integer).
"Rounded up" and "nearest integer" seem to contradict?
"rounded up to the next integer"?
That makes sense, at least.
133 | + 134 | +''Transaction weight'' is defined as ''Base transaction size'' * 3 + ''Total transaction size'' (ie. the same method as calculating ''Block weight'' from ''Base size'' and ''Total size''). 135 | + 136 | +''Virtual transaction size'' is defined as ''Transaction weight'' / 4 (rounded up to nearest integer). 137 | + 138 | +''Base transaction size'' is the transaction size in bytes with the original transaction serialization without any witness-related data.
Prefer to phrase this in terms that will make sense post-activation. In other words, the size of the transaction serialised with the witness data stripped.