Pygame Clock: tick(), FPS and Delta Time

pygame.time.Clock keeps your game loop at a steady speed. Make one clock before the loop and call tick() once per frame:

clock = pygame.time.Clock()

while running:
    ...
    pygame.display.flip()
    clock.tick(60)

What tick() does

clock.tick(60) does two things:

1. It caps the frame rate. If the frame finished early, it waits, so the loop runs at most 60 times a second. Without it a simple game runs hundreds of frames a second: everything moves too fast and the processor works flat out.
2. It returns the time since the last tick, in milliseconds. At 60 frames a second that is about 16 or 17.

tick() cannot make a slow computer faster. If drawing a frame takes 40 milliseconds, the game runs at 25 frames a second whatever number you pass.

Showing the frame rate

clock.get_fps() returns the average frame rate over the last few ticks:

import pygame

pygame.init()
screen = pygame.display.set_mode((480, 300))
clock = pygame.time.Clock()
font = pygame.font.Font(None, 40)
running = True

while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

    screen.fill((18, 20, 32))
    fps = clock.get_fps()
    label = font.render(f"FPS: {fps:.0f}", True, (120, 255, 160))
    screen.blit(label, (20, 20))
    pygame.display.flip()
    clock.tick(60)

pygame.quit()

It shows 0 for the first frames, because there is nothing to average yet.

Delta time

If you move a player 5 pixels every frame, its speed depends on the frame rate: 300 pixels a second at 60 FPS, but only 150 at 30 FPS. Delta time fixes that. Measure how long the last frame took and move by speed × time:

import pygame

pygame.init()
screen = pygame.display.set_mode((480, 300))
clock = pygame.time.Clock()

x = 0.0
speed = 200  # pixels per second
dt = 0.0
running = True

while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

    x += speed * dt
    if x > 480:
        x = -40

    screen.fill((18, 20, 32))
    pygame.draw.rect(screen, (255, 170, 60), (int(x), 130, 40, 40))
    pygame.display.flip()
    dt = clock.tick(60) / 1000

pygame.quit()

clock.tick(60) / 1000 turns milliseconds into seconds, so speed is in pixels per second. The box crosses the screen in about 2.4 seconds on any computer. Two details:

• Store the position as a float (x = 0.0), not in a pygame.Rect. A Rect keeps whole numbers, so 0.5 pixels a frame would be dropped every time. Copy the float into the Rect, or use pygame-ce's pygame.FRect, when you draw.
• The first frame has dt = 0, because no frame has been timed yet.

For a small game a fixed tick(60) and per-frame speeds are fine. Delta time is worth it when frame rates vary, for example on old laptops or when you let players choose 30, 60 or 144 FPS.

Clock methods

For things that happen every few seconds (spawning an enemy, a countdown), see pygame timers.