How to Fix Unity FixedUpdate Running Too Often or Too Rarely

FixedUpdate running a variable number of times is the fixed timestep catching up. Here is how to handle it.

How to fix it

1. Do not assume one per frame

FixedUpdate runs as many times as needed to keep the fixed timestep, which can be several times in a slow frame or none in a fast one. Code assuming exactly one FixedUpdate per render frame breaks.

2. Separate physics and input

Put physics in FixedUpdate (it runs in sync with the physics step) and read per-frame input in Update. Reading input in FixedUpdate can miss or double inputs because it does not match the frame rate.

3. Use fixedDeltaTime

In FixedUpdate, use Time.fixedDeltaTime for time-based calculations, since deltaTime there is the fixed step. This keeps physics consistent regardless of how many times FixedUpdate runs in a frame.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every Unity error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.