Introduction
The director wants a hero shot of a glass shattering on a marble floor, played back at one-eighth speed. Your RED V-RAPTOR can shoot 240fps at 4K, and the timeline is 24fps. You need to know exactly how long the real-time event has to last for the final slow-motion clip to fill an 8-second beat in the edit. Shoot too little, and the editor is left short. Shoot too much, and you're burning storage on footage that never makes the cut.
The slow motion calculator gives you the playback duration and slowdown factor from your capture frame rate and timeline frame rate. Enter your two frame rates and real-time duration. Get the exact clip length you'll have in the edit.
What This Tool Calculates
The calculator takes three inputs: the capture frame rate (the fps your camera is actually recording at), the playback frame rate (the fps of your editing timeline), and the real-time duration of the event in seconds.
It returns the slowdown factor (how many times slower the footage plays), the final clip duration in seconds, and the real-time duration you need to capture if you want a specific clip length in the edit.
The Formula and How It Works
The math is a simple ratio, but the mistakes made with it are surprisingly common. The slowdown factor equals the capture frame rate divided by the playback frame rate. The final clip duration equals the real-time duration multiplied by the slowdown factor.
For a camera running at 240fps playing back at 24fps: Slowdown Factor = 240 / 24 = 10x. A 1-second real-time event becomes a 10-second clip. Inversely, to fill an 8-second edit beat, you need 8 / 10 = 0.8 seconds of real-time action.
To achieve a specific slowdown effect, you can also solve for the required capture rate: Capture FPS = Playback FPS times Desired Slowdown Factor. For 4x slow motion on a 24fps timeline: 24 times 4 = 96fps. For 8x slow motion: 24 times 8 = 192fps.
One often overlooked detail: most cameras reduce resolution as frame rate increases. A RED V-RAPTOR shoots 8K at 120fps but drops to 4K at 240fps. Always check your camera's resolution limits at the desired capture rate. The slowdown factor calculation is correct regardless of resolution, but your delivery specs may require a minimum pixel count.
Real-World Examples
Glass Shatter for Automotive Commercial
A commercial director wanted a wine glass shattering in slow motion as a metaphor for "breaking free." The Phantom Flex4K captured at 1000fps on a 24fps timeline. Real-time shatter duration was approximately 0.15 seconds. The calculator returned a slowdown factor of 41.67x and a final clip duration of 6.25 seconds. This was enough to fill the 5-second beat in the storyboard with a 1.25-second handle for the editor to choose the best in and out points.
Wildlife Documentary Water Splash
A nature documentary crew filming a kingfisher diving into a lake used a Sony FX6 at 120fps for a 24fps delivery. The bird's dive took roughly 0.3 seconds in real time. The calculator showed a 5x slowdown factor and a 1.5-second clip. The director wanted at least 3 seconds of usable slow motion, so the crew switched to a Phantom VEO 4K at 960fps, giving a 40x factor and a 12-second clip from the same 0.3 seconds of action.
Sports Highlight Reel at 60fps Delivery
A sports broadcast team shooting a basketball game at 240fps needed slow-motion replays for a 60fps timeline (common in sports broadcasting). The calculator returned a slowdown factor of 4x. A 2-second real-time dunk sequence became an 8-second replay. The producer used this number to plan that each slow-motion replay would consume roughly 8 to 10 seconds of broadcast time, factoring in lead-in and lead-out.
Common Slow-Motion Frame Rates by Camera and Resolution
| Camera | Max FPS | Resolution at Max FPS | Slowdown at 24fps |
|---|---|---|---|
| ARRI ALEXA 35 | 120fps | 4.6K | 5x |
| RED V-RAPTOR | 240fps | 4K | 10x |
| Sony FX6 | 120fps | 4K | 5x |
| Phantom Flex4K | 1000fps | 4K | 41.7x |
| Sony A7S III | 120fps | 4K | 5x |
| Blackmagic Pocket 6K G2 | 120fps | 2.8K | 5x |
| iPhone 16 Pro | 240fps | 1080p | 10x |
Pro Tips and Common Mistakes
Pro Tips
- Always capture more real-time duration than you think you need. A 10x slowdown sounds generous, but the usable portion after trimming handles and selecting the best moment often cuts the clip in half.
- At frame rates above 120fps, most cameras switch from a rolling shutter readout to a faster readout mode that can introduce visible banding under artificial lighting. Test with your actual location's lights before the shoot day.
- When shooting slow motion outdoors, your shutter speed increases proportionally. At 240fps with a 180-degree shutter, your shutter speed is 1/480. You'll likely need ND filtration to avoid stopping down past your creative aperture.
- Match your slow-motion frame rate to a clean multiple of your timeline rate. 120fps into 24fps = 5x (clean). 100fps into 24fps = 4.167x (creates a frame cadence issue that can appear as subtle stutter in playback).
Common Mistakes
- Confusing the capture frame rate with the playback frame rate. If you enter 24fps as the capture rate and 240fps as the playback rate, you get 0.1x, which is a 10x speed-up, not slow motion. Capture rate is always the larger number for slow-motion work.
- Not accounting for the reduction in resolution at high frame rates. A client expecting 4K delivery won't accept footage shot at 240fps on a camera that drops to 1080p at that frame rate. Always verify the resolution at your chosen capture rate.
- Forgetting that audio is not usable from high-frame-rate clips. Most cameras disable audio recording above their base frame rate. Plan for wild sound or Foley to cover slow-motion sequences in post.
Frequently Asked Questions
What frame rate do I need for 8x slow motion?
Multiply your timeline frame rate by 8. For a 24fps timeline, you need 192fps. For a 30fps timeline, you need 240fps. For a 60fps timeline, you need 480fps. Check your camera's maximum frame rate at your required resolution before committing to a shot.
Can I create slow motion from 24fps footage in post?
Software like Twixtor and DaVinci Resolve's optical flow can interpolate frames to create artificial slow motion. The results are acceptable for subtle slowdowns (2x) but often produce visible artifacts on fast-moving subjects, detailed edges, or complex backgrounds. Capturing at a higher frame rate always produces cleaner results.
Why does my slow-motion footage look darker than normal speed footage?
At higher frame rates, the shutter is open for less time per frame. At 240fps with a 180-degree shutter, each frame gets only 1/480th of a second of exposure, compared to 1/48th at 24fps. That's 3.3 stops less light. Compensate by opening the aperture, increasing ISO, or adding more light.
How do I calculate slow motion for a variable frame rate camera?
Use the maximum frame rate the camera actually achieved during the shot. Variable frame rate cameras (like those with a ramp-up feature) change speed mid-shot. Calculate the slowdown at the peak rate for the slowest portion, and at the baseline rate for the normal-speed portion.
Start Calculating
Slow motion is one of the most powerful tools in a filmmaker's visual vocabulary, but it requires precise planning to execute. Knowing your clip duration before you roll means no wasted takes and no surprises in the edit.
Run your next slow-motion shot through the calculator above. What frame rate and camera combination do you reach for when a director asks for slow motion?
