There are various FFmpeg encoders that support variable bit rate / constant quality encoding (learn more about rate control modes here). This gives you a much better overall quality when file size or average bit rate are not constrained (e.g. in a streaming scenario). Variable bit rate is usually achieved by setting -q:v (or -q:a for audio) instead of -b:v (or -b:a), which just sets a target bit rate.

The problem is that every encoder uses a different range of values to set the quality—and they’re hard to memorize. This is an attempt to summarize the most important ones.

Notes for reading this table:

  • Qmin stands for the setting to be used for achieving lowest quality and Qmax for highest. These are not just lowest and highest values.
  • Qdef is the default value chosen if no other is specified. This means that (most?) encoders will use one or the other VBR mode by default, e.g. libx264. I wasn’t able to research whether this applies to all encoders.
  • Some encoders use private options instead of the regular -q. Read the second column Param for the correct option to use.
Video
Encoder Param Qmin Qmax Qdef Recommended Notes
libx264 -crf 51 0 23 18–28 Values of ±6 result in double/half avg. bitrate. 0 is lossless.
Specifying -profile:v lets you adjust coding efficiency. See H.264 Encoding Guide.
libx265 -crf 51 0 28 24–34 Values of ±6 result in double/half avg. bitrate. 0 is lossless.
Specifying -profile:v lets you adjust coding efficiency. See H.265 Encoding Guide and x265 docs.
libvpx-vp9 -crf 63 0 31 (1080p) 15–35 Recommended CRF 31 for 1080p HD. Use -b:v 0 for pure CRF mode. Two-pass encoding recommended for best quality. See VP9 Encoding Guide.
libaom-av1 -crf 63 0 n/a 20–35 0 is lossless. CRF 23 is roughly equivalent to x264 CRF 19. Slower than other encoders but excellent compression. See AV1 Encoding Guide.
libsvtav1 -crf 63 0 35 20–40 Much faster than libaom-av1 with similar quality. Use -preset (0–13) to control speed/quality tradeoff. See AV1 Encoding Guide.
h264_videotoolbox -q:v 1 100 n/a 70–85 macOS hardware encoder. Scale 1–100, with 100 being highest quality. Constant quality only available on Apple Silicon (ffmpeg 4.4+). Also supports -b:v for bitrate mode.
hevc_videotoolbox -q:v 1 100 n/a 70–85 macOS hardware HEVC encoder. Scale 1–100, with 100 being highest quality. Constant quality only available on Apple Silicon (ffmpeg 4.4+). Also supports -b:v for bitrate mode.
libxvid -q:v 31 1 n/a 3–5 2 is visually lossless. Doubling the value results in half the bitrate. Don't use 1, as it wastes space.
No VBR by default—it uses -b:v 200K unless specified otherwise.
libtheora -q:v 0 10 n/a 7 No VBR by default—it uses -b:v 200K unless specified otherwise.
mpeg1, mpeg2, mpeg4, flv, h263, h263+, msmpeg+ -q:v 31 1 ? 3–5 2 is visually lossless. Doubling the value results in half the bitrate.
-q:v works for mpeg4, but haven't tested others.
prores_ks -profile:v
-q:v		 Profile: 0
Quality: 32		 Profile: 5
Quality: 0		 Profile: 2
Quality: n/a		 Profile: Depends
Quality: 9–13		 Recommended software ProRes encoder. Profiles: 0=Proxy, 1=LT, 2=Standard, 3=HQ, 4=4444, 5=4444 XQ. Quality controlled via -q:v (0=best, 32=worst), recommended 11. Only encoder supporting 4444/XQ and alpha channels. See VFX Encoding Guide.
Note: prores and prores_aw encoders also exist but lack profile and quality controls.
prores_videotoolbox -profile:v 0 5 auto Depends macOS hardware ProRes encoder. Profiles: 0=Proxy, 1=LT, 2=Standard, 3=HQ, 4=4444, 5=XQ. Profile auto-selected based on input format.
Audio
Encoder Param Qmin Qmax Qdef Recommended Notes
libfdk_aac -vbr 1 5 ? 4 (~128kbps) Highest quality AAC encoder. Not available in standard builds—requires custom compilation with --enable-libfdk-aac --enable-nonfree.
libopus -b:a 6–8K (mono) – 96K (for stereo) – -vbr on is default, -b:a just sets the target, see FFmpeg documentation.
libvorbis -q:a 0 10 3 4 (~128kbps) Make sure not to use vorbis, which is the (bad) internal encoder.
libmp3lame -q:a 9 0 4 2 (~190kbps) Corresponds to lame -V. See FFmpeg Wiki.
aac_at -q:a
-aac_at_mode		 n/a n/a auto VBR mode with -q:a macOS AudioToolbox AAC encoder. High quality, supports multiple VBR modes (vbr, cvbr, abr). Use -aac_at_mode vbr with -q:a for quality-based VBR. Generally better quality than native aac encoder.
aac -q:a 0.1 2 ? 1.3 (~128kbps) Native FFmpeg AAC encoder (second-best quality). VBR mode via -q:a is "experimental and [likely gives] worse results than CBR" according to FFmpeg Wiki. Ranges from 18 to 190kbps. Prefer libfdk_aac or aac_at (macOS) for better quality.

Please let me know if there's anything wrong or if you're missing some values. Thanks to @LordNeckbeard and @evilsoup on Super User for providing additional input on this.

Updates:
  • January 2025 – Update libvpx to libvpx-vp9; add AV1 encoders (libaom-av1, libsvtav1); add VideoToolbox encoders (h264_videotoolbox, hevc_videotoolbox, prores_videotoolbox); update ProRes (prores_ks) with quality settings; add aac_at (macOS); clarify native aac VBR is experimental
  • April 2019 – Clarify Opus options
  • March 2019 – Clarify range of AAC; add Opus defaults
  • November 2018 – Remove unsupported encoders
  • August 2017 – Clarification on aac encoder, reorder encoders.