BD-rate — Bjøntegaard Delta rate — is the standard metric for comparing the compression efficiency of two video encoders or codecs. Proposed by Gisle Bjøntegaard at an MPEG meeting in 2001, it answers the question "how much less bitrate does codec A need to reach the same quality as codec B?". A BD-rate of -30 % means codec A produces files 30 % smaller than codec B at every quality level — the standard way industry papers describe codec generations beating their predecessors.
The method is mathematically tidy. You encode the same source video at multiple bitrates with both codecs, measure quality at each (PSNR, SSIM or VMAF), plot the rate-quality curves, then compute the average percentage difference between them. The result is a single number summarising the codec's efficiency advantage across the relevant quality range. HEVC vs H.264 typically shows BD-rate around -50 % (HEVC needs half the bits). AV1 vs HEVC is around -30 %. VVC vs HEVC is around -50 %. AV2 vs AV1 targets around -30 %.
For a product team, BD-rate is the language industry papers and vendor presentations use to claim codec efficiency. When you read "30 % BD-rate gain over the previous generation", that means at any given quality target, the new codec needs 30 % less bitrate — directly translating to 30 % less CDN spend. The metric has limitations: it's only as good as the underlying quality metric (BD-rate measured with PSNR can show a different number than the same comparison with VMAF), and it depends on what range of bitrates and content you tested. Take vendor BD-rate claims with measured scepticism, but they're the industry's shorthand for codec advancement and worth understanding.
