Load Generator Calculator

What is the Load Generator Calculator?

The Load Generator Calculator estimates how many virtual users a single load generator (load injector) machine can run before it runs out of memory. A load generator that is short on memory becomes the bottleneck and skews your results, so sizing it properly before a big test is essential.

How it works

The first virtual user typically costs more memory than subsequent ones (protocol initialization overhead). Subtracting OS overhead from total RAM, then subtracting the first VUser and dividing the remainder by the per-VUser cost, gives the maximum VUsers a single machine can host.

Formula to calculate VUsers per load generator

Where:

• Total RAM — RAM installed on one load generator machine, in MB.
• OS Usage — RAM used by the operating system and background processes, in MB.
• First VUser Memory — Memory used by the first virtual user, in MB.
• Additional VUser Memory — Memory used by each additional virtual user, in MB.
• ⌊ ⌋ / ⌈ ⌉ — Round down / round up to the nearest whole number.

Example of load generator calculation

You are sizing a 16 GB load generator for an HTTP script.

Inputs

• First VUser Memory = 50 MB
• Additional VUser Memory = 5 MB
• Total RAM = 16384 MB (16 GB)
• OS Usage = 2048 MB (2 GB)

Step-by-step

1. Available RAM for VUsers = 16384 − 2048 = 14336 MB
2. Max VUsers per LG = (14336 − 50) ÷ 5 + 1 = 2858.2
3. Rounded Max VUsers per LG = ⌊2858.2 ÷ 500⌋ × 500 = 2500
4. Required Load Generators = ⌈2500 ÷ 2858.2⌉ = 1

Common mistakes to avoid

• Underestimating OS and JVM/agent overhead, leaving less headroom for VUsers than expected.

• Sizing for average memory per VUser instead of measuring the first VUser separately — protocol setup often makes it disproportionately larger.

• Not leaving a safety margin below the calculated maximum, since real machines rarely sustain 100% of theoretical capacity under load.