How I make ffmpeg hit an exact file size (the bitrate math nobody explains)

Short summary

Hitting an exact video file size is arithmetic, not guesswork: bitrate = target_bits / duration. Reserve ~2% for container overhead, subtract audio bitrate, and divide the remainder by duration to get video bitrate. Use ffmpeg two-pass encoding so the encoder distributes bits intelligently and the average lands on your target. The author wraps this math in a Rust CLI called DeepShrink.

• •File size = bitrate × duration; flip the equation to solve for bitrate from a target size
• •Reserve ~2% for container overhead and subtract audio budget before calculating video bitrate
• •Two-pass ffmpeg encoding ensures the average bitrate actually hits your target

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