🧮 PERFTRACTOR
👥 Virtual Users Calculator (Little's Law)
Calculate the number of virtual users needed to hit your target load.
👥 Virtual Users Calculator (Little's Law)
Calculate the number of virtual users needed to hit your target load.
virtual users required
The Formula
N = Z × (R + T)
- N = Number of virtual users
- Z = Transactions per second (target throughput)
- R = Average response time in seconds
- T = Think time in seconds
How it works
Little's Law is the fundamental queueing relationship used to size a load test: the number of concurrent users (N) equals throughput (Z) multiplied by the total time each user spends in the system: response time (R) plus think time (T).
If your target is 10 transactions per second, each transaction takes 2 seconds, and users think for 3 seconds between actions, you need 10 × (2 + 3) = 50 virtual users.
Undersizing VU count is the most common reason a load test fails to reach its target throughput: the system is never given enough concurrent work to saturate it.
Frequently Asked Questions
What is Little's Law in performance testing?
Little's Law states that the number of concurrent users N equals throughput Z (transactions per second) multiplied by the time each user spends in the system: N = Z × (R + T), where R is average response time and T is think time. It is the standard way to size virtual user counts for load tests in JMeter, k6, Gatling, or LoadRunner.
How many virtual users do I need for my load test?
Multiply your target transactions per second by the sum of average response time and think time (both in seconds). For example, 20 TPS with 1.5s response time and 4s think time requires 20 × 5.5 = 110 virtual users.
Why does my load test not reach target throughput?
The most common cause is too few virtual users for the response time and think time you configured. Recalculate N with Little's Law and check that your load generators are not CPU- or memory-saturated, which artificially slows VUs.