Little's Law Calculator | Throughput Sizing

Little's Law calculator for concurrency and throughput: enter any two of arrival rate, latency and in-flight requests to size a pool. Free. Designed specifically for developers and professionals, this free online calculator runs entirely in your browser to process tasks quickly and securely. Enjoy unlimited usage with no data tracking and no registration required.

Also known as: littles law calculator · concurrency calculator online · throughput calculator · thread pool size calculator · connection pool sizing calculator · queue length calculator · requests per second to concurrency · capacity planning calculator

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System Parameters

Solve for

Throughput (requests per second)

Average latency (ms)

Concurrency (in-flight requests)

Target utilization (%)

Pools sized to exactly 100 percent queue on the first slow request. 70 to 80 percent is the usual target.

L = λ × W

Concurrency equals throughput times latency. The law holds for any stable system regardless of arrival distribution, which is why it works on thread pools, connection pools and queues alike.

Sizing Result

Concurrency, recommended pool size and a latency sensitivity table will appear here.

About Little's Law Calculator

Size a thread pool, connection pool or worker fleet from traffic numbers you already have. Little's Law states that the number of requests in flight equals arrival rate multiplied by the time each request takes, so given any two of throughput, latency and concurrency this calculator returns the third plus a pool size with headroom.

Key Features

Solves for concurrency, throughput or latency from the other two, using L = lambda times W
Converts a target utilization into a recommended pool size, because a pool sized to exactly 100 percent queues on the first slow request
Shows a latency sensitivity table so you can see what a p99 regression does to the concurrency you need
Reports headroom in slots, the buffer between the steady-state requirement and the pool you provision
Works for any stable system: HTTP worker pools, database connection pools, message consumers and thread pools
Copy or download the full sizing report for a capacity planning document or a design review

Use Cases

Size a Postgres connection pool for an API serving 500 requests per second at 120 ms in us-east-1
Work out how many Kubernetes replicas a service needs before a Black Friday traffic forecast
Check whether a thread pool of 200 can actually sustain the throughput a service level objective promises
Show a reviewer why a 45 ms latency regression forces a larger pool rather than a faster one
Convert a load test result into the steady-state concurrency a production fleet has to carry
Estimate the maximum requests per second a fixed pool can deliver before requests start queuing

Supported Formats

Throughput in requests per second

Latency in milliseconds

Concurrency as in-flight request count

Target utilization as a percentage

Related Keywords

littles law calculator

concurrency calculator online

throughput calculator

thread pool size calculator

connection pool sizing calculator

queue length calculator

requests per second to concurrency

capacity planning calculator

How many database connections does my service need?

Multiply your queries per second by the average query time in seconds. A service issuing 500 queries per second where each takes 20 ms needs 500 times 0.02, which is 10 connections in steady state. Divide by a target utilization of 80 or 70 percent to leave headroom, giving 13 or 15. This is almost always far smaller than the pool people configure, and an oversized pool is not free: every connection consumes memory on the database server and adds contention, which is why PgBouncer exists.

Calculated in your browser

Every figure is computed locally in JavaScript. Nothing about your traffic volumes, latency numbers or capacity plans is sent anywhere.

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Frequently asked questions

What is Little's Law Calculator used for?

Little's Law calculator for concurrency and throughput: enter any two of arrival rate, latency and in-flight requests to size a pool. Free.

Is Little's Law Calculator free to use?

Yes. Little's Law Calculator is completely free with no usage limits, and there is no paid tier required to use it.

Do I need an account to use Little's Law Calculator?

No. Little's Law Calculator works immediately in your browser with no signup, login, or email address required.

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