Frame Rate & Slow-Motion Calculator

How slow can you push a clip before it turns into a stutter? Enter the frame rate you filmed at and the slow-mo speed you want, and see the effective playback frame rate, how smooth it will look, and the slowest speed that still plays cleanly.

Slow motion works by spreading the frames you captured over more time. The more frames per second you start with, the further you can slow the clip before the eye notices the gaps. This tool does the arithmetic and rates the result.

Want smooth slow-mo without the math?

ReplayR records a rolling buffer and plays any saved clip back at adjustable slow-motion, offline.

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Work out your slow-motion smoothness

Capture frame rate

Slow-motion speed

Real clip length (seconds)

Capture high frame rate, slow it on the spot, offline

ReplayR keeps a rolling buffer at your chosen frame rate and lets you scrub any saved clip in slow motion to study technique, no Wi-Fi, no subscription.

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How slow motion actually works

Slowing time means stretching frames

A camera captures a fixed number of still frames per second. Slow motion plays those frames back over a longer span of time. Film one second at 120fps and play it at the standard 30fps timeline, and that one second now lasts four seconds on screen, 0.25x speed, with every one of the 120 frames shown. No frames are invented; the footage is simply spread out.

Effective fps is what your eye judges

The number that decides whether a slow clip looks smooth is the effective playback frame rate: capture fps multiplied by the slow-mo speed. At 60fps and 0.5x you see 30 unique frames per second of playback, which reads as smooth. Push the same clip to 0.25x and you are down to 15 effective fps, which most people see as a visible stutter.

The 24 fps threshold

Motion looks continuous to the human eye from around 24 frames per second, the long-standing cinema standard, and 30 fps is comfortably smooth. Below roughly 15 effective fps the brain starts to perceive individual frames. The practical rule of thumb: keep effective playback at 24 fps or higher, and ideally 30, for review footage you want to look clean.

Higher capture fps buys deeper slow motion

To find the slowest speed that still plays smoothly, divide your capture frame rate by 30. So 60fps holds up to about 0.5x, 120fps to about 0.25x, and 240fps to about 0.125x. Fast actions like a golf impact, a bat swing, or a tennis serve happen in a fraction of a second, which is exactly when you want 120fps or 240fps so you can crawl through the moment frame by frame.

Reference: effective playback fps by capture rate and speed

Capture fps	0.5x	0.25x	0.125x	Slowest smooth (≈30 fps)
30 fps	15 fps	7.5 fps	3.75 fps	1x (real time)
60 fps	30 fps	15 fps	7.5 fps	0.5x
120 fps	60 fps	30 fps	15 fps	0.25x
240 fps	120 fps	60 fps	30 fps	0.125x

Green is smooth (≥30 effective fps), amber is watchable but choppy (15–29 fps), rose stutters (under 15 fps). Assumes a standard 30 fps playback timeline. Recording at higher frame rates needs more light and produces larger files.

How to use this calculator

Step 1: Set capture fps

Pick the frame rate you filmed at, or plan to film at. Check your camera app's video settings if you are not sure; 30 and 60 fps are the common defaults.

Step 2: Choose a speed

Select how slow you want to play it back. 0.5x halves the speed, 0.25x quarters it. The result updates as you change it.

Step 3: Read the rating

Check the effective fps and smoothness rating. If it stutters, either raise the capture frame rate or back off the slow-mo speed until it reads smooth.

Step 4: Capture & review

Film at the frame rate that supports the slow-mo you need. ReplayR keeps a rolling buffer and plays saved clips back in slow motion on your phone, including fully offline.

FAQ

How slow can I play 60fps footage before it stutters?

60fps at 0.5x shows 30 effective fps, which looks smooth. At 0.25x it drops to 15 fps, which most people read as choppy. Divide capture fps by 30 to find the smooth limit: 60 ÷ 30 means about 0.5x is as slow as 60fps goes cleanly.

What frame rate should I record at for slow motion?

Record at the highest frame rate your light and storage allow. 120fps stays smooth to about 0.25x and 240fps to about 0.125x, which suits fast actions like a golf impact or a serve. 60fps is a fine default for general technique review.

Why does slow motion look choppy below 24 fps?

Motion looks continuous to the eye from around 24 fps; 30 is comfortably smooth. Below roughly 15 effective fps the brain sees individual frames as a stutter. Slowing past the point where capture fps times speed falls under 24 shows fewer unique frames per second, so the gaps become visible.

Is my data sent to a server when I use this calculator?

No. Every calculation runs in your browser with JavaScript and nothing is uploaded. Your last capture frame rate and slow-mo speed are saved in your browser's localStorage so the page remembers them, and the Reset button clears them.