Why do I need to deduct audio bitrate?

File size is shared between video and audio streams. If your audio track uses 128 kbps, that bitrate is unavailable for video. This calculator subtracts the audio budget so the video bitrate you receive exactly fills the remaining space.

Which codec should I choose?

H.264 is the most compatible choice for web and devices. H.265/HEVC delivers roughly the same quality at about half the bitrate, but requires hardware support to play smoothly on older devices. VP9 is similar to H.265 and widely supported in browsers. AV1 is the most efficient — roughly 30% smaller than H.265 — but encoding is very slow without hardware acceleration.

What is the codec efficiency multiplier?

Newer codecs compress more efficiently than H.264. H.265 uses a 0.6× multiplier (you need 40% less bitrate for equivalent quality), VP9 uses 0.65×, and AV1 uses 0.5×. H.264 is the baseline at 1×. The calculator applies these ratios so you can compare apples-to-apples across codecs.

Is this the same as two-pass encoding?

This calculator gives you the average (CBR/target) bitrate to use with -b:v in FFmpeg or the equivalent setting in other encoders. Two-pass encoding uses this target but distributes bits more intelligently across complex and simple scenes. For the tightest file-size control, use two-pass with the bitrate this tool outputs.

What if the result is below 500 kbps?

Very low bitrates cause severe compression artifacts — blocky motion, smeared detail, color banding. If the result is below 500 kbps, consider reducing resolution (e.g. 1080p → 720p) before encoding, which lowers the bitrate needed to look acceptable.

Transcode Bitrate Target Calculator

Name: Transcode Bitrate Target Calculator
Availability: InStock
Author: Nham Vu

Enter target file size and video duration to get the exact video bitrate for your transcode, with codec efficiency and audio deduction.

Transcode Parameters

Target File Size

Video Duration

Hours

Minutes

Seconds

Codec

Efficiency multiplier adjusts the effective bitrate for equivalent visual quality.

Audio Bitrate (kbps)

Number of Audio Tracks

Multi-language outputs often carry 2+ audio streams.

FFmpeg Flag

Use with ffmpeg -i input.mp4 [codec flags] -b:v <value> -b:a <audio> output.mp4

Codec Comparison (same file size)

Codec	Video bitrate	Efficiency

Higher efficiency codec = better quality at the same bitrate.

Fill in the parameters and click Calculate.

Summary