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H.264 vs H.265 (HEVC) Codec Comparison

Overview

This document summarizes the technical differences between H.264 (AVC) and H.265 (HEVC) video codecs and considerations for conversion.

Technical Specifications Comparison

Characteristic H.264 (AVC) H.265 (HEVC)
Release Year 2003 2013
Block Size 4×4, 8×8, 16×16 Up to 64×64 (CTU)
Compression Efficiency Baseline ~50% improvement
Encoding Complexity Low High (2-10x)
Hardware Support Almost all devices Primarily newer devices
Licensing Clear patent pool Complex patent situation

Compression Efficiency

Quality Comparison at Same Bitrate

H.265 provides higher quality at the same bitrate compared to H.264:

  • PSNR Basis: H.265 averages 2-3dB improvement
  • SSIM Basis: H.265 approximately 0.01-0.02 improvement
  • VMAF Basis: H.265 5-10 points improvement

File Size at Same Quality

"A 1GB H.264 video can be reduced to about 500MB when converted to H.265 without visible quality loss." — codestudy.net

Quality Measurement Metrics

VMAF (Video Multimethod Assessment Fusion)

  • Machine learning-based quality metric developed by Netflix
  • Results most similar to human visual perception
  • Score range: 0-100 (95+ recommended)

PSNR (Peak Signal-to-Noise Ratio)

  • Traditional objective quality metric
  • Generally 40dB or higher indicates excellent quality
  • Useful for measuring fine detail loss

SSIM (Structural Similarity Index)

  • Measures structural similarity
  • Range: 0-1 (0.99+ recommended)
  • Effective for measuring subjective quality loss

When H.264 is Better Than H.265

According to research, H.264 can produce better results in the following situations:

  1. Noisy Sources: H.265's larger block size can lose noise detail
  2. Very High Bitrate: Differences are minimal with sufficient bitrate
  3. Fast Motion Content: H.264's smaller blocks are better at preserving fine motion
  4. Already Compressed Content: Re-compression causes additional loss

"At QP=27, H.265 and H.264 have similar bitrates (9.66Mbps vs 9.96Mbps), but H.265's PSNR may be lower (40.19 vs 41.8)." — NETINT Technologies

Quality Preservation Strategies During Conversion

Use Case H.264 CRF H.265 CRF Description
Visually Lossless 17-18 20-22 Indistinguishable from original
High Quality Archive 20-21 23-25 Recommended setting
General Viewing 23 26-28 Default value
Size Priority 26-28 30-32 Visible degradation possible 
ffmpeg -i input.mp4 \
  -c:v libx265 \
  -preset slow \
  -crf 20.6 \
  -g 600 \
  -keyint_min 600 \
  -tune fastdecode \
  output.mp4

Optimal settings achieving VMAF 95 in March 2025 benchmark — scottstuff.net

Limitations of Lossless Conversion

Important: "Lossless" conversion from H.264 to H.265 is technically impossible.

  • Both codecs use lossy compression algorithms
  • Re-encoding inevitably causes additional quality loss
  • "Visually lossless" is the realistic goal

Methods to Minimize Generation Loss

  1. Use High Quality Settings: CRF 20-22 recommended
  2. Use slow/slower Preset: Better compression efficiency
  3. Maintain Original Resolution: No upscaling/downscaling
  4. Maintain Original Framerate: No frame interpolation

Compatibility Considerations

macOS/iOS Support

Feature H.264 H.265
macOS Playback All versions High Sierra+
iOS Playback All versions iOS 11+
Photos App Full support Full support
iCloud Sync Full support Full support

Hardware Decoding

  • Intel Mac: 6th generation and later (Skylake)
  • Apple Silicon: All M-series (M1, M2, M3, M4)

References