PHP-FPM Pool Calculator
Calculate correct pm.max_children and spare-server values for your PHP-FPM pool from RAM and worker memory, with a ready-to-paste www.conf snippet
MB — check via ps (see below)
Results
- pm.max_children = —
- Maximum worker processes that can run at once
- pm.start_servers = —
- Workers spawned when FPM starts
- pm.min_spare_servers = —
- Minimum idle workers kept ready
- pm.max_spare_servers = —
- Maximum idle workers before FPM kills some
www.conf
Most people guess at PHP-FPM pool sizes or copy values from a random blog post. The result is either wasted RAM or an out-of-memory crash under load. This calculator takes three real numbers and gives you a correct www.conf snippet.
How it works
The pool size is bounded by how much RAM you have and how much each worker actually uses:
pm.max_children = floor((RAM × (1 − buffer/100)) / worker_memory)The safety buffer leaves headroom for the OS, the web server, and traffic spikes. The spare-server values scale off max_children.
Choosing a pm mode
The calculator outputs pm = dynamic, the right default for most sites. The mode decides which of the directives below actually apply:
| Mode | Behavior | Directives used | Best for |
|---|---|---|---|
static | Fixed number of workers, always running | max_children only | Predictable high traffic; max throughput, no spawn latency |
dynamic | Pool grows and shrinks between spare bounds | max_children, start_servers, min/max_spare_servers | Most sites — balances RAM use and responsiveness |
ondemand | No idle workers; spawn on request, reaped after a timeout | max_children, process_idle_timeout | Low-traffic or many small pools; saves RAM at the cost of cold-start latency |
max_children matters in every mode — it’s the OOM safeguard. The spare-server values only take effect under dynamic.
What each value does
pm.max_children— the hard cap on concurrent worker processes. This is the number that prevents OOM kills. Set it too high and a traffic spike eats all your RAM.pm.start_servers— how many workers spawn when FPM starts. A quarter of the maximum is a sane warm pool.pm.min_spare_servers— the minimum idle workers FPM keeps ready to absorb sudden requests without spawn latency.pm.max_spare_servers— the maximum idle workers FPM tolerates before reaping them to free memory.
Finding your real worker memory
Don’t guess the per-worker figure. Measure it on a running server:
ps --no-headers -o rss -C php-fpm | awk '{sum+=$1} END {print sum/NR/1024 " MB"}'This averages the resident set size of every php-fpm process. For a typical Shopware 6 shop, 60 MB per worker is a realistic starting point — heavier setups with many plugins can hit 100 MB or more.