Calculation details
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What does this calculator estimate?
A 120-second track at 0.75x speed becomes 160 seconds, a 33.3 percent increase; at 1.25x it becomes 96 seconds, a 20 percent decrease. New duration = original / playback speed.
- New duration = original duration / playback speed.
- Time change percent = (1 / speed - 1) x 100.
- Slower playback deepens pitch unless pitch correction is applied.
How playback speed changes duration
Playback speed multiplies the rate of the audio: 2× halves the duration, 0.5× doubles it. Changing speed naively also changes pitch — proper time-stretch algorithms (like phase vocoders) change duration while preserving pitch.
Limitations to watch for
The math is exact for duration, but quality depends on the player or editor: simple speed changes shift pitch (chipmunk effect), while time-stretch processing can add artifacts at extreme ratios. Very long or extreme speeds (4×+) degrade noticeably.
How to use it in practice
Use it to plan podcast editing (remove pauses, tighten episodes), video narration, or study playback. For pitch-preserving stretch, use a DAW or editor with time-stretch rather than a simple speed control.
['Enter the original clip duration in seconds.', 'Enter the playback speed (e.g., 1.0, 1.25, 0.75).', 'The tool computes the new duration and percentage change.']
How this calculator works
Formula
New duration = original duration ÷ playback speed. Time change % = (1 ÷ speed − 1) × 100: a speed above 1.0 shortens the clip, below 1.0 lengthens it.
Worked example
A 120-second clip played at 1.25× speed: new duration = 120 ÷ 1.25 = 96 seconds — 20% shorter.
Assumptions to verify
- Playback speed is constant across the clip.
- The player applies the speed uniformly.
- Duration math ignores fade-in/out trimming.
Frequently asked questions
How does playback speed affect duration?
New duration = original ÷ speed. A 2-minute clip at 1.5× becomes 80 seconds.
What's the difference between speed change and time stretch?
Speed change alters pitch along with duration; time stretch preserves pitch while changing duration. Podcast and video editors use time-stretch algorithms.
How much time does 1.25× save?
20% — a 30-minute episode becomes 24 minutes. At 1.5× it's 33% shorter.
Why does audio sound chipmunk-like at higher speeds?
Simple speed controls raise the pitch. Time-stretch processing corrects this but can add artifacts at extreme speeds.
What speeds are safe for listening?
1.25–1.5× is comfortable for most speech; 2× works for many listeners. Beyond 2×, comprehension drops and artifacts grow.
Can I use this for video?
Yes — the same math applies to video playback duration. Frame-rate and audio sync are handled by the player.
Is the duration math exact?
Yes — it's a simple ratio. Quality of the result depends on the player's speed algorithm.
Cite this tool
BoringToolsKit. “Audio Time Stretch Calculator.” boringtoolskit.com/audio-time-stretch-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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