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About this tool
Convert between concurrent users, requests per second and response time using Little's Law, with think-time support.
This calculator converts between concurrent users, requests per second and average response time using Little's Law (L = λ × W), the queueing-theory identity that holds for any stable system. It is built for performance engineers and backend developers sizing load tests, connection pools and worker counts, and it separates true in-flight concurrency from virtual users who spend part of each cycle in think time.
Open Concurrency vs RPS Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Give any two of users, RPS and response time and get the third — the same identity rearranged, never a guess.
Distinguishes requests in flight (RPS × response time) from virtual users needed (RPS × full cycle), which differ by orders of magnitude on real sites.
Little's Law needs no assumption about traffic shape — it holds for averages in any stable system, which is why every load-testing guide uses it.
Divide users by the time each user's full request cycle takes: RPS = users ÷ (response time + think time), both in seconds — Little's Law rearranged. 1,000 users with a 0.2 s response and 9.8 s of think time produce only 100 req/s, which is why user counts alone say little about server load.
Little's Law states L = λ × W: the average number of requests in a system equals arrival rate times average time in the system. Its power is that it is distribution-free — it holds for any stable, non-saturated system regardless of traffic pattern — so load-testing tools use it to size virtual-user counts for a target throughput.
No. 100 concurrent users generate 100 req/s only if each user completes exactly one request per second; with a 200 ms response time and no think time, 100 users would actually drive about 500 req/s, while with 5 seconds of think time they drive roughly 19 req/s. The conversion always depends on the per-user cycle time.
Think time is the pause a simulated user takes between requests, modelling reading a page or filling a form. It raises the number of virtual users needed for a given RPS without raising in-flight load — set it to 0 to stress raw concurrency, or to a realistic 3–10 seconds to mimic human browsing behaviour.
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