Video bitrate real test - run: 2026-10-25 11:14 UTC
host: Linux 6.8.0-139-generic x86_64
ffmpeg: ffmpeg version 6.1.1-3ubuntu5 Copyright (c) 2000-2023 the FFmpeg developers

=== 1. source clip (Wikimedia Commons, CC BY 4.0) ===
$ ffprobe -v error -show_entries format=duration,size,bit_rate -show_entries stream=codec_name,codec_type,width,height,r_frame_rate -of default=noprint_wrappers=1 '/home/ubuntu/rt/bit/source.webm'
codec_name=vp8
codec_type=video
width=1280
height=720
r_frame_rate=30000/1001
codec_name=vorbis
codec_type=audio
r_frame_rate=0/0
duration=38.272000
size=4469695
bit_rate=934300
exit=0
=== 2. our 10 second working file, one source, constant frame rate ===
$ ffmpeg -v error -t 10 -i '/home/ubuntu/rt/bit/source.webm' -vf 'fps=30' -c:v libx264 -crf 22 -preset medium -an source10.mp4 -y
exit=0
$ ffprobe -v error -show_entries format=duration,size,bit_rate -show_entries stream=codec_name,width,height,r_frame_rate,nb_frames -of default=noprint_wrappers=1 source10.mp4
codec_name=h264
width=1280
height=720
r_frame_rate=30/1
nb_frames=300
duration=10.000000
size=1576669
bit_rate=1261335
exit=0
=== 3. encode the same file eight ways ===
$ ffmpeg -v error -i source10.mp4 -c:v libx264 -crf 18 -preset medium -an crf18.mp4 -y
exit=0
$ ffmpeg -v error -i source10.mp4 -c:v libx264 -crf 22 -preset medium -an crf22.mp4 -y
exit=0
$ ffmpeg -v error -i source10.mp4 -c:v libx264 -crf 28 -preset medium -an crf28.mp4 -y
exit=0
$ ffmpeg -v error -i source10.mp4 -c:v libx264 -b:v 500k -maxrate 500k -bufsize 1000k -preset medium -an br500.mp4 -y
exit=0
$ ffmpeg -v error -i source10.mp4 -c:v libx264 -b:v 1000k -maxrate 1000k -bufsize 2000k -preset medium -an br1000.mp4 -y
exit=0
$ ffmpeg -v error -i source10.mp4 -c:v libx264 -b:v 2000k -maxrate 2000k -bufsize 4000k -preset medium -an br2000.mp4 -y
exit=0
$ ffmpeg -v error -i source10.mp4 -c:v libx264 -b:v 4000k -maxrate 4000k -bufsize 8000k -preset medium -an br4000.mp4 -y
exit=0
=== 4. what each encode actually came out as ===
file          seconds      bytes  container bit_rate   measured avg bps   busiest second bps  avg Mbps
crf18.mp4      10.000  1,995,894           1,596,715          1,596,715            2,473,832     1.597
crf22.mp4      10.000  1,426,160           1,140,928          1,140,928            1,654,840     1.141
crf28.mp4      10.000    688,983             551,186            551,186              801,936     0.551
br500.mp4      10.000    541,433             433,146            433,146              827,440     0.433
br1000.mp4     10.000  1,061,836             849,468            849,469            1,613,056     0.849
br2000.mp4     10.000  2,088,645           1,670,916          1,670,916            2,625,568     1.671
br4000.mp4     10.000  4,064,647           3,251,717          3,251,718            4,964,992     3.252

Formula check: size in bytes = bitrate in bits per second x seconds / 8
  crf18.mp4    predicted    1,995,894 bytes  actual    1,995,894 bytes  difference +0.00%
  crf22.mp4    predicted    1,426,160 bytes  actual    1,426,160 bytes  difference +0.00%
  crf28.mp4    predicted      688,982 bytes  actual      688,983 bytes  difference +0.00%
  br500.mp4    predicted      541,432 bytes  actual      541,433 bytes  difference +0.00%
  br1000.mp4   predicted    1,061,835 bytes  actual    1,061,836 bytes  difference +0.00%
  br2000.mp4   predicted    2,088,645 bytes  actual    2,088,645 bytes  difference +0.00%
  br4000.mp4   predicted    4,064,646 bytes  actual    4,064,647 bytes  difference +0.00%
=== 5. the file we ship ===
-rw-rw-r-- 1 ubuntu ubuntu 1061836 Oct 25 11:15 br1000.mp4
-rw-rw-r-- 1 ubuntu ubuntu 1426160 Oct 25 11:15 crf22.mp4
-rw-rw-r-- 1 ubuntu ubuntu 1576669 Oct 25 11:15 source10.mp4
=== done ===

=== 6. the site's own video bitrate calculator on the same numbers (production site, headless Chromium) ===
$ node scenario-calc.cjs https://koldflux.com/tools/calculators/video-bitrate 1 10 128
  TARGET FILE SIZE (MB) 1 | DURATION (s) 10 | AUDIO BITRATE (kbps) 128
  TOTAL BITRATE BUDGET 839 kbps
  VIDEO BITRATE TO SET 711 kbps
  VIDEO BITRATE IN MBPS 0.71 Mbps
  page formula: video kbps = (target MB x 1048576 x 8 / seconds / 1000) - audio kbps
  no console or page errors

$ ffmpeg -i source10.mp4 -c:v libx264 -b:v 711k -maxrate 711k -bufsize 1422k -preset medium -c:a aac -b:a 128k -ar 44100 -ac 2 tool-711k.mp4
  tool-711k.mp4: h264 1280x720, video bit_rate 608585, duration 10.000000 s, size 765165 bytes = 0.730 MB
  The file is 27.03% under the 1.000 MB target: the encoder spends less than the budget it is given.