How is PostgreSQL max_connections estimated?

The tool uses min(floor(memory_bytes / 9531392), 5000) where memory_bytes = GiB * 1024^3.

How are MySQL and MariaDB max_connections estimated?

MySQL uses floor(memory_bytes / 12582880). MariaDB 10.5+ uses min(floor(memory_bytes / 25165760), 12000), while MariaDB 10.4 uses floor(memory_bytes / 12582880).

What does safety headroom mean?

Headroom reserves a percentage of connections for bursts and operational safety before computing usable connections.

Does this replace load testing?

No. It helps planning and review, but real production behavior must still be validated with load tests and monitoring.

RDS Connection Limit Estimator

Calculate connection safety for PostgreSQL, MySQL, and MariaDB deployments. All logic runs client-side; share scenarios via URL.

Learn more about what drives RDS connection limits

Inputs

Scenario presets

Apply a preset to populate common pool sizing scenarios.

Engine

Instance class

Instance memory (GiB)

Custom max_connectionsEnable override

If disabled, AWS default engine formula is used.

Pods/instances count

Pool size per pod

Reserved connections

Safety headroom % (0-50)

Learn how this works

Why this service exists

This estimator helps prevent "too many connections" incidents when horizontal scaling increases app pods and each pod opens its own DB connection pool.

How calculations work

PostgreSQL: min(floor(memory_bytes / 9531392), 5000)
MySQL: floor(memory_bytes / 12582880)
MariaDB 10.4: floor(memory_bytes / 12582880)
MariaDB 10.5+: min(floor(memory_bytes / 25165760), 12000)
Usable connections: floor(max_connections * (1 - headroom)) - reserved
Total needed connections: pods × pool size

AWS DBInstanceClassMemory differs from raw GiB due to reserved memory; values are estimates.

What each input means

Engine: selects formula family and behavior.
MariaDB version: selects 10.4 or 10.5+ default formula.
Instance class / memory: defines memory baseline for estimation.
Custom max_connections: override defaults with your parameter-group value.
Pods/instances count: number of application replicas.
Pool size per pod: max DB connections one pod can open. For PHP apps, this often relates to PHP-FPM pm.max_children and per-request DB usage.
Reserved connections: kept for admin/migrations/operational access.
Safety headroom: percentage intentionally left unused for burst safety.

Example

pods = 25
pool size per pod = 40
total needed = 25 × 40 = 1000
usable connections = 600
status = DANGER

Suggested fixes:
- reduce pool size per pod
- limit autoscaling upper bound
- increase DB instance size / memory
- for PostgreSQL, consider PgBouncer

More guides

Estimates only; validate against your DB parameter group and real workload.