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About this tool
Work out speed percentage, new duration, frame count and audio pitch shift when conforming footage to a different timeline frame rate.
Conforming footage means reinterpreting every recorded frame as one timeline frame, and this calculator gives the exact result: speed = timeline fps ÷ source fps, and the clip lasts source fps ÷ timeline fps times as long. Sixty-frame footage on a 24 fps timeline therefore plays at 40% speed and runs 2.5 times longer, with the frame count unchanged. The tool also covers the other direction — set a target speed and it works out how many frames the timeline needs versus how many were actually recorded, which is what determines whether you get clean slow motion or blended frames.
Open Frame Rate Retime Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
The two operations produce different frame counts, and the tool shows exactly how many frames get created or discarded.
Flags when fewer than 24 unique frames a second reach the screen, the point where motion starts to look stepped.
Gives the shift in semitones from 12 × log₂(speed), so you know whether the sound can travel with the picture.
40%. The speed is the timeline rate divided by the source rate, so 24 ÷ 60 = 0.4. The clip also lasts 60 ÷ 24 = 2.5 times as long, and every recorded frame is still shown, which is why this is clean slow motion rather than interpolated slow motion.
Divide the timeline rate by the slow-motion factor you want. For 25% speed on a 24 fps timeline you need 24 ÷ 0.25 = 96 fps, so you would shoot 100 or 120. Shooting at less than that forces the software to duplicate or blend frames.
It is 24 × 1000/1001, a 0.1% slowdown introduced when colour was added to NTSC broadcast and still used for delivery to that ecosystem. Mixing 24 and 23.976 material in one sequence produces that same 0.1% speed difference, which is around 3.6 seconds of drift per hour.
Only if the audio is retimed with the picture. Playing a recording at a speed factor f shifts its pitch by 12 × log₂(f) semitones, so 40% speed drops the pitch by about 15.9 semitones — well over an octave. In practice editors keep the audio at its native rate or use a time-stretch that preserves pitch.
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