Vsize

Vsize, or virtual size, is a term used in several technical domains to measure size in a way that accounts for factors not captured by a simple byte count. In computing and blockchain contexts, it is often used to reflect the amount of resources associated with a unit of data, such as a process’s address space or a transaction’s fee responsibility.

In operating systems, virtual memory size (commonly abbreviated as VSZ or vsize) represents the total virtual address space that a process can reference. It includes the memory occupied by code, data, heap, stack, and memory-mapped files, as well as space that could be swapped out. VSZ is reported by process accounting tools such as ps and top. It does not indicate actual physical RAM usage; pages may be swapped out or shared with other processes. VSZ can exceed the sum of physical memory and swap space and is a rough upper bound on a program’s address space.

In cryptocurrency, vsize is used to calculate transaction fees under SegWit. It is defined as the non-witness portion of the transaction plus a quarter of the witness data: vsize = base_size + witness_size / 4. Equivalently, it is weight divided by four, where weight = base_size * 4 + witness_size. Fee rates are applied per vsize unit, so transactions with more witness data have a larger vsize and higher fees, but SegWit’s design reduces the added cost per unit of witness data compared with the old non-SegWit scheme.

Related concepts include weight (a measure used to compute vsize) and resident memory size (RSS), which reflects actual RAM usage rather than address space.