Ulimit File Descriptors Calculator

Work out ulimit file descriptors instantly with clear inputs, formula shown and shareable results.

About Ulimit File Descriptors Calculator

Each connection consumes at least one descriptor, and a proxy needs two — one client-side and one upstream. Dividing by worker count gives the per-process requirement, and a 50 percent margin covers bursts and lingering sockets in TIME_WAIT. For systemd services the limit must be set with LimitNOFILE in the unit file; the default of 1024 is the classic cause of accept failures under load.

Formula

Descriptor budget

per worker = connections x descriptors per connection / workers + other files; recommended = per worker x 1.5; fs.file-max = recommended x workers x 1.2

Worked example

With the default values below, Ulimit File Descriptors Calculator returns Recommended per-process limit of 8,250. Change any input above and every figure here is recalculated in your browser.

Inputs used

Concurrent connectionsminimum 1: 20,000
Descriptors per connectionminimum 1: 2
Other open files and socketsminimum 0: 500
Worker processes sharing the loadminimum 1: 8

Results

Recommended per-process limit: 8,250
Descriptors each worker needs: 5,500
Recommended fs.file-max: 79,200
Configuration note: Set LimitNOFILE in the systemd unit; shell ulimit settings do not apply to services

Step by step

1Values used
2Descriptor budget
3Recommended per-process limit= 8,250
4Descriptors each worker needs= 5,500
5Recommended fs.file-max= 79,200
6Configuration note= Set LimitNOFILE in the systemd unit; shell ulimit settings do not apply to services

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is my service still limited to 1024 descriptors?

Because ulimit in a login shell does not affect systemd-managed services. Set LimitNOFILE in the unit or a drop-in override and reload the daemon.

Is there a cost to a very high limit?

The limit itself is cheap; each actually-open descriptor costs kernel memory. Setting a high ceiling is safe, but monitor actual usage.