What Is Bitrate? Bits per Second, CBR vs VBR

Bits per second, and what the number means

Bitrate answers a single question: how much data does this stream consume every second? A 5 Mbps video moves five million bits of data per second of playback. Those bits encode everything the player needs to reconstruct the picture and sound, so more bits per second means the encoder can preserve finer texture, sharper edges, and smoother gradients. The unit is bits per second, scaled up to kilobits and megabits for readability.

The catch is that bits are not free. Every bit per second of bitrate has to be stored on disk and, for streaming, delivered over the network in real time. That ties bitrate to two costs at once: file size, which grows in direct proportion to bitrate, and required bandwidth, since a viewer must download the stream at least as fast as it plays. Picking a bitrate is therefore always a balance between how good you want it to look and how much data you are willing to store or send.

CBR versus VBR

Constant bitrate (CBR) locks the data rate to a fixed value for the whole file. Every second gets the same budget of bits, whether it shows a still title card or a fast chase scene. Its big advantage is predictability: the file size and the bandwidth needed are easy to calculate and stay steady, which is why CBR suits live streaming and situations with a strict, unchanging bandwidth limit. The downside is waste and starvation: simple scenes get more bits than they need, and complex ones may not get enough.

Variable bitrate (VBR) lets the rate rise and fall with the content. The encoder spends extra bits where the picture is hard to compress, such as rain, smoke, or rapid motion, and saves bits during calm, static shots. Aiming for a target average, VBR generally delivers better quality per byte than CBR, which is ideal for stored files you download or watch later. Its cost is unpredictability: the instantaneous rate spikes, so it is less suited to strict real-time bandwidth caps unless you constrain the peaks.

Choosing a bitrate for quality and size

There is no single correct bitrate, only the right one for a given resolution, frame rate, and codec. As a starting point, published presets suggest roughly 8 Mbps for 1080p at 30 fps and about 12 Mbps at 60 fps, with 4K several times higher. A modern codec such as H.265 or AV1 reaches the same quality at a lower bitrate than older H.264, because it compresses more efficiently. Always match the bitrate to the codec, not just the resolution.

Beyond a certain point, adding bitrate stops improving what the eye can see while continuing to grow the file. This is the region of diminishing returns, and finding it is the whole art of encoding. The practical approach is to pick a preset for your target, encode a short test clip, and check whether the quality holds on the screens your audience uses. If it looks perfect, try trimming the bitrate; if artifacts appear in busy scenes, raise it a step. That loop lands you on an efficient setting quickly.

Frequently asked questions

What is the difference between bitrate and bandwidth?

Bitrate describes the data rate of the media file itself, while bandwidth describes how fast a network connection can carry data. To stream smoothly, your available bandwidth must be at least as high as the video's bitrate, with a margin for overhead and variation.

Does a higher bitrate always look better?

Up to a point, yes, but there are diminishing returns. Past the level your codec and resolution need, extra bits stop making a visible difference and only grow the file. The goal is the lowest bitrate that still looks clean on your audience's screens.

When should I use CBR instead of VBR?

Use CBR when you need a predictable, steady data rate, above all for live streaming and connections with a strict bandwidth cap. Use VBR for stored files you download or watch later, where better quality per byte matters more than a constant rate.

How do I convert Mbps to megabytes per second?

Divide the megabits per second by 8, because one byte is eight bits. So 8 Mbps equals 1 megabyte per second, and 16 Mbps equals 2 megabytes per second. This is the same divide-by-8 step used to turn bitrate into file size.