Unity Frame Rate

Intro

In Unity, since Update() is called once per frame, if motion is coded without accounting for frame time, object speed will vary with FPS

Frame-Dependent Animation

• If you intend for an object to move 10 units/sec but don’t use Time.deltaTime

float speed = 10f; // “10 units/sec”
void Update() {
    transform.position += Vector3.forward * speed; // NO deltaTime
}

• Unity interprets speed as units per frame, not per second (e.g. speed = 10 units/frame NOT 10 units/sec)
• Example: 60 FPS
  • Update() runs 60 times/sec and each frame moves 10 units → total per second which is way faster than intended:

distance/sec = 10 units/frame × 60 frames/sec = 600 units/sec

• To force the desired 10 units/sec at 60 FPS, you would theoretically set speed to 0.166:

speed = 1 frame × (1 sec / 60 frames) × 10 units/sec ≈ 0.166 units/frame

• But if Unity drops to 30 FPS, then, it only moves 4.98 units

distance/sec = 0.166 units/frame x 30 frames/sec = 4.98

Frame-Independent Animation

• To make motion frame-rate independent, multiply by Time.deltaTime:

float speed = 10f; // units/sec
void Update() {
    transform.position += Vector3.forward * speed * Time.deltaTime;
}

• Now Unity scales movement by the time since the last frame, so total distance per second stays correct regardless of FPS
• Example:
  • 60 FPS → distance/frame ≈ 0.167 units, total/sec ≈ 10 units
  • 30 FPS → distance/frame ≈ 0.333 units, total/sec ≈ 10 units

deltaTime @30 FPS = time/frame = 1 sec/30 frames = 0.0333 sec/frame

• deltaTime is basically the unit conversion from unit/sec (real-world speed) to units/frame (Unity world speed)

# transform.position += Vector3.forward * speed * Time.deltaTime;
distance/frame = speed * deltaTime
               = 10 units/sec * 0.0333 sec/frame = 0.333 units/frame 

• Verified with Dimensional Analysis:

total distance per sec = 1 sec * 30 frames/sec * 0.333 units/frame = ~10 units