According to wiki and official test, what differs H.265 from H.264 is the compression efficiency. H.265 (aka HEVC) doubles the coding efficiency compared with its predecessor H.264. This means H.265 video saves around 50% of the bit rate at the same quality of coding. Specifically, the average bit reduction for H.265 is 64% at 4K UHD, 62% at 1080p, 56% at 720p and 52% at 480p. For instance, the H.265 replaces the 16x16 pixel macroblocks used with H.264 and uses larger block structures of up to 64x64 samples. Therefore, by reducing the design of flow rate, H.265 further lowers the cost of storage and transmission and delivers better visual quality, compared with H.264.
HEVC vs H.264: Bandwidth Use
H.265 is superior to H.264 regarding to the bandwidth usage. Because H.265's algorithm uses the efficient coding, H.265 promises approximately 40-50% of reduction in transmission bandwidth needed to compress the video (e.g. in 720p) over H.264 at the same video quality. So you can enjoy 4k video smoothly even at 1-2 Mbs bandwidth constrained network connection. On the contrary, the UHD videos encoded with h.264 will be watched at a stuttering mode.
HEVC vs H.264: Quality
The big difference between H.265 and H.264 lies in the video quality at the same bit rate. In H.264, the border areas of the block are likely to be distorted, because each macroblock is fixed and the data is independent to each other. While H.265 offers sharper detailed on faces and fabrics and smooth gradient areas with less blocking and fewer artifacts, since the new standard determines the size of code unit based on regional information. So H.265 is better than H.264 when it comes to compress a video with better image quality.
HEVC vs H.264: File Size
The great compression ratio also has a great relationship with the digital storage requirement of video streams and transmission. The reduced bandwidth leads to smaller file size. Test shows a video encoded with H.264 is almost 1-3 times larger than that with H.265. This is favorable for hard drive storage or the device with limited storage space required to house the video data.
HEVC vs H.264: Performance Comparison
H.265 vs H.264, which one is more suitable for you for videos playback? Of course, H.264 codec is applicable for almost all common devices. But things are not feasible to H.265. For devices that can decode HEVC video, performance is a concern. Here we tested the performance of various devices on average CPU usage, median clock speed and battery life for a reference of final decision of H.265 and H.264.