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About this tool
Find the exact speed multiplier needed to hit a required runtime, plus how much to cut if the speed change is too aggressive.
To make a video fit a fixed runtime you divide: speed multiplier = current length ÷ target length. A 3 minute 12 second cut that has to land at 90 seconds needs 2.13×. This calculator does that arithmetic, accepts durations as seconds, m:ss or h:mm:ss, and lets you subtract material you are willing to cut before the speed change is applied. It also solves the problem in reverse — given a speed ceiling such as 1.25× or 1.5×, it tells you exactly how many seconds you would have to remove instead.
Open Video Duration Target Speed Calculator on AltFTool — it loads instantly in your browser.
Upload or drag in the file you want to work with.
Adjust the settings and preview the result.
Download the finished file to your device.
Subtracts the material you are cutting first, so the multiplier reflects the edit you are actually making.
Shows the seconds you would need to remove to stay under 1.25× and under 1.5×, the usual comfort thresholds for speech.
Reads 90, 1:30 or 00:01:30 and rejects impossible values like 1:70 rather than silently misreading them.
Divide the current length by the target length. A 180-second video that has to be 60 seconds needs 180 ÷ 60 = 3.00×, or 300% speed. If you are also cutting material, subtract that first: (180 − 30) ÷ 60 = 2.50×.
For spoken content, up to about 1.25× is usually unnoticeable, 1.25× to 1.5× is audibly quick but followable, and beyond 2× narration stops working as narration. Music and action tolerate far less. These are editing thresholds rather than measured limits, so judge it against your own material.
Yes, unless you use a pitch-preserving time stretch. The shift is 12 × log₂(multiplier) semitones, so 2× speed raises the audio by a full octave, and 1.25× raises it by about 3.9 semitones — enough to sound distinctly odd on a voice.
Cut first. Speeding up compresses everything equally, including the pauses that make speech intelligible, whereas cutting removes the parts that were not carrying the point. This tool shows the exact trim needed to keep the speed under 1.25× or 1.5× so you can compare the two approaches directly.
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