FixedUpdate vs Update in Unity - Visual Side-by-Side Comparison

Concept Flow - FixedUpdate vs Update

Game Loop Start

↓

Update Called Every Frame

↓

Process Input, Animations

↓

FixedUpdate Called at Fixed Intervals

↓

Physics Calculations

↓

Render Frame

↓

Loop Back to Start

The game loop calls Update every frame for general tasks, and FixedUpdate at fixed time steps for physics updates.

Execution Sample

Unity

void Update() {
    MovePlayer();
}

void FixedUpdate() {
    ApplyPhysics();
}

Update moves the player every frame; FixedUpdate applies physics at fixed intervals.

Execution Table

Step	Time (seconds)	Method Called	Action	Notes
1	0.00	Update	MovePlayer called	First frame update
2	0.02	FixedUpdate	ApplyPhysics called	Physics step 1
3	0.03	Update	MovePlayer called	Second frame update
4	0.04	FixedUpdate	ApplyPhysics called	Physics step 2
5	0.06	FixedUpdate	ApplyPhysics called	Physics step 3 (FixedUpdate called twice due to frame delay)
6	0.06	FixedUpdate	ApplyPhysics called	Physics step 4 (FixedUpdate called twice due to frame delay)
7	0.06	Update	MovePlayer called	Third frame update
8	0.08	FixedUpdate	ApplyPhysics called	Physics step 5
9	0.09	Update	MovePlayer called	Fourth frame update
10	0.10	FixedUpdate	ApplyPhysics called	Physics step 6
11	0.12	Update	MovePlayer called	Fifth frame update
12	0.12	FixedUpdate	ApplyPhysics called	Physics step 7
Exit	End	Stop	Game loop continues	Execution trace ends here

💡 Game loop continues indefinitely; FixedUpdate runs at fixed intervals, Update runs every frame.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	After Step 9	After Step 10	After Step 11	Final
frameCount	0	1	1	2	2	2	2	3	3	4	4	5	5
fixedUpdateCount	0	0	1	1	2	3	4	4	5	5	6	7	7

Key Moments - 3 Insights

Why does FixedUpdate sometimes run multiple times between two Update calls?

Because FixedUpdate runs at fixed time intervals independent of frame rate, if frames are slow, FixedUpdate can run multiple times to catch up, as shown in step 5 and 6 where FixedUpdate runs twice before the next Update.

Can physics calculations be done inside Update instead of FixedUpdate?

No, physics should be done in FixedUpdate because it runs at consistent intervals, ensuring stable and predictable physics behavior, unlike Update which varies with frame rate.

Why is Update called more frequently than FixedUpdate in some cases?

Update is called every frame, which can vary in time, while FixedUpdate is called at fixed intervals, so if frame rate is high, Update runs more often than FixedUpdate.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of fixedUpdateCount after step 5?

A 3

B 4

C 2

D 5

💡 Hint

Check the fixedUpdateCount value in the variable_tracker after step 5.

At which step does FixedUpdate run twice before the next Update?

A Step 4

B Step 5

C Step 6

D Step 7

💡 Hint

Look at the execution_table rows where FixedUpdate is called twice with the same time.

If the frame rate drops, how does FixedUpdate behave compared to Update?

A FixedUpdate runs less often than Update

B Both run at the same frequency

C FixedUpdate runs more often than Update to catch up

D FixedUpdate stops running

💡 Hint

Refer to the explanation in key_moments about FixedUpdate running multiple times when frames are slow.

Concept Snapshot

Update runs every frame and handles input and animations.
FixedUpdate runs at fixed time intervals for physics.
FixedUpdate can run multiple times per frame if frame rate is low.
Use Update for frame-dependent tasks.
Use FixedUpdate for physics and consistent timing.

Full Transcript

In Unity, the game loop calls Update every frame to handle tasks like input and animations. FixedUpdate runs at fixed time intervals to handle physics calculations. This means FixedUpdate can run multiple times between two Update calls if the frame rate is slow, ensuring physics stays stable. Update frequency depends on frame rate and can vary. Physics should always be done in FixedUpdate for consistent behavior. The execution table shows Update and FixedUpdate calls over time, with variable tracking for frameCount and fixedUpdateCount. Key moments clarify why FixedUpdate runs multiple times sometimes and why physics belongs there. The visual quiz tests understanding of these timings and counts.