Free calculator
Speed Change Calculator
Footage shot at 48 fps and played at 24 fps runs at 50% speed and lasts twice as long. Pick the rate you shot at and the rate you play at to get the playback speed, the change in running time and the duration factor.
Free, no sign-up. Runs in your browser.
Built on the same formulas as the Speed Change Calculator in the CineMath app. Save, share and get the Pro tools in the app.
How it works
speed % = target fps ÷ source fps × 100Below 100% is slow motion (overcranking); above 100% is fast motion (undercranking).
Where
- source fps
- the rate the camera captured(fps)
- target fps
- the rate the timeline or project plays(fps)
duration % = source fps ÷ target fps × 100How long the shot runs compared with real time. The duration factor is the same ratio written as a multiplier: ×2.000 means twice as long. Speed % × duration % always equals 10,000.
Why NTSC rates are entered as 23.976
The calculator uses 23.976, 29.97, 59.94 and 119.88 as written, the same values the CineMath app uses, so 24 fps played at 23.976 is exactly 99.90% and 29.97 played at 23.976 is exactly 80.00%. The true NTSC rates are 24000/1001 and so on, about 0.0001% higher. That moves the last decimal in a handful of cases: 30 fps played at 23.976 runs 125.13% as long here and 125.12% with 24000/1001. Drop-frame is a way of labeling timecode, not a different rate, so 29.97 DF and 29.97 NDF give the same answer.
Worked example
120 fps slow motion in a 23.976 fps timeline
- Speed: 23.976 ÷ 120 × 100 = 19.98%
- Duration: 120 ÷ 23.976 × 100 = 500.50%
- Duration factor: 120 ÷ 23.976 = ×5.005
- 10 s of action × 5.005 = 50.05 s on the timeline
120 fps in a 23.976 timeline plays at 19.98% speed and runs 500.50% as long.
Quick reference
Common frame-rate conforms
| Shot at | Played at | Speed | Duration |
|---|---|---|---|
| 48 fps | 24 fps | 50.00% | 200.00% |
| 60 fps | 24 fps | 40.00% | 250.00% |
| 120 fps | 24 fps | 20.00% | 500.00% |
| 240 fps | 24 fps | 10.00% | 1000.00% |
| 59.94 fps | 23.976 fps | 40.00% | 250.00% |
| 119.88 fps | 23.976 fps | 20.00% | 500.00% |
| 50 fps | 25 fps | 50.00% | 200.00% |
| 24 fps | 23.976 fps | 99.90% | 100.10% |
| 25 fps | 24 fps | 96.00% | 104.17% |
| 23.976 fps | 25 fps | 104.27% | 95.90% |
Save, share and get the Pro tools in the CineMath app
CineMath is a free download for iPhone, iPad and Mac, with this calculator built in. In version 1.3, coming soon to the App Store, CineMath Professional saves calculations to your history, shares them as links, and adds the Camera Database and Data Planning.
Questions
How do I calculate slow motion from frame rates?
Divide the playback rate by the capture rate. 120 fps played at 24 fps is 24 ÷ 120 = 20% speed, so the action runs 5 times longer: a 2-second moment fills 10 seconds of screen time.
What frame rate do I shoot for 50% slow motion?
Twice your timeline rate: 48 fps for a 24 fps timeline, 50 fps for 25 fps, and 59.94 fps for 29.97 fps. For 40% speed shoot 2.5 times the rate (60 fps for 24), and for 20% shoot 5 times (120 fps for 24).
What is overcranking?
Shooting at a higher frame rate than the one you play back at, so motion is slowed down. The name comes from hand-cranked cameras. Undercranking is the opposite: shooting at a lower rate, such as 12 fps for a 24 fps timeline (200% speed), to speed action up.
Why does 24 fps footage in a 23.976 timeline play slightly slow?
23.976 ÷ 24 = 99.90% speed, so every hour of 24 fps footage runs about 3.6 seconds longer. That is the 1000/1001 NTSC offset, and the calculator warns when you mix NTSC fractional rates (23.976, 29.97, 59.94, 119.88) with whole rates.
Does a speed change affect the audio?
If the sound is conformed with the picture, its pitch moves too. 99.90% speed lowers pitch by about 1.7 cents, which few people hear; 96% (25 fps played at 24) lowers it by about 71 cents, nearly three-quarters of a semitone. Beyond 2× or 0.5× the calculator warns that you will need pitch correction and probably frame interpolation.
What is the difference between speed % and duration %?
They are the same change seen from two sides. Speed % says how fast the footage plays (50% = half speed); duration % says how long it runs compared with real time (200% = twice as long). Editing apps ask for one or the other, so the calculator shows both.