Video bitrate guide

Bits, seconds and the size formula

Bitrate is usually quoted in kilobits per second (kbps) or megabits per second (Mbps); 1 Mbps is 1,000 kbps. File size follows directly: size in megabytes = bitrate in kbps × seconds ÷ 8 ÷ 1,000, because there are eight bits in a byte. A ten-minute video at 5,000 kbps is 5,000 × 600 ÷ 8,000, or 375 MB, plus audio. Turned around, the formula tells you how long a file can be at a given bitrate before it hits a limit. VideoBB accepts files up to 4 GB: at 8 Mbps that is about 66 minutes, at 12 Mbps about 44 minutes, and at 40 Mbps about 13 minutes.

Note the units. Bitrates are bits; storage is bytes; and file managers often show gigabytes as 1,024 megabytes while encoders count in thousands. The differences are small enough to ignore for planning and large enough to explain why a file lands a few percent off your estimate.

VideoBB's ladder, in bits per pixel

Target video bitrate per rendition, and the bits spent on each pixel of each frame at 30 fps. Frame widths for the smaller renditions assume 16:9.

Reading the bits-per-pixel column

Bits per pixel, bitrate divided by pixels per frame divided by frames per second, is the most useful single measure of how hard an encoder is working. The ladder spends fewer bits on each 4K pixel than on each 1080p pixel, and far more on each 144p pixel. Small frames need more information per pixel to keep edges and text readable. Large frames are full of smooth areas that compress cheaply. The figures shift with frame rate. At 60 fps the same bitrate halves the bits per pixel. That is why high-frame-rate content wants a higher rate than the ladder's 30 fps baseline. See the frame rate guide for that trade-off.

CBR, VBR and CRF

Constant bitrate (CBR) spends the same budget on every second, whatever the content. It suits live streaming and hardware with fixed pipes, and it wastes bits on static scenes. Variable bitrate (VBR) sets an average and lets the encoder move budget from easy seconds to hard ones. Two-pass VBR does this best, because the first pass maps where the hard parts are. Constant rate factor (CRF) is the mode most editors show as a quality slider. It drops the bitrate target altogether and asks for steady visual quality. The file size then lands wherever it needs to.

For a master you will upload, CRF is usually the right choice: it spends what the content needs and no more. For a file that must fit a size limit, two-pass VBR with a calculated average is the tool. CBR is for live encoders and for delivery to systems that require it.

What eats bitrate

Content is the variable the ladder cannot control. These are the things that make a second of video expensive to encode well.

• Motion, especially motion that covers the whole frame: pans, handheld shake, sport.
• Noise and film grain, which look like detail to the encoder and change every frame.
• Fine texture: foliage, fabric, crowds, water, and confetti, the classic worst case.
• Rapid cuts, since every cut is a new keyframe that must be described from scratch.
• Fades and dissolves, where every pixel changes a little on every frame.
• Small text and thin lines, which blur first when the budget is tight.

Upload bitrate: the two-times rule of thumb

The transcoder decodes your file and re-encodes it, so any softness or blocking in the source is inherited by every rendition and then compressed again. Giving the source a healthy margin over the top rendition's target keeps that second generation clean. For most content, aim for about twice the top rendition's bitrate. That is around 10 Mbps for a 1080p master, 16 to 20 Mbps for 1440p, and 30 to 40 Mbps for 4K. Content from the list above deserves the high end; a static presenter can go lower. There is little point going far beyond, because the rendition's target is fixed and the file only gets slower to upload.

Long recordings need a choice. At 30 to 40 Mbps, the 4 GB limit holds only about 13 to 17 minutes of 4K. For anything longer, a clean 1080p master at about 10 Mbps beats a 4K file starved of bits. The free resize tool can make that 1080p copy from your 4K export.

The mistake to avoid is exporting at the rendition target itself. A 1080p master at 5 Mbps becomes a 1080p rendition at 5 Mbps of an already-compressed picture, and the result looks like a 3 to 4 Mbps file. If you are unsure what a file's bitrate is, check it before uploading; the blurry after upload guide shows how, and preparing video for upload gives export presets by editor.

Check and set bitrate with FFmpeg

Two commands cover most needs. FFmpeg is free and available for every platform.

1. Read the bitrate of a file

   ffprobe -v error -show_entries format=bit_rate,duration -of default=nw=1 in.mp4 prints the overall bitrate in bits per second and the duration.

2. Encode a master with CRF

   ffmpeg -i in.mov -c:v libx264 -crf 18 -preset slow -pix_fmt yuv420p -c:a aac -b:a 192k out.mp4 gives a high-quality H.264 file whose size follows the content.

3. Hit a size target instead

   Work out the average from the formula, then use -b:v with -pass 1 and -pass 2 for a two-pass VBR encode. If you just need a smaller copy to email or share, the free video compressor can target a file size for you. Keep the full-quality master for upload.

4. Compare before uploading

   Scrub the output next to the source at fullscreen on the busiest scene. If it looks worse, lower the CRF or raise the bitrate and encode again.

Codec changes the exchange rate

Everything above assumes H.264, which is what VideoBB's ladder produces and what plays everywhere. Newer codecs get more quality from the same bits. H.265 and AV1 are commonly described as needing something like 30 to 50 percent less bitrate for similar quality. The saving depends heavily on content and encoder settings, so treat it as an expectation rather than a promise. For an upload master this is mostly irrelevant, since the file is re-encoded anyway, but it matters if you store masters or serve files yourself. The codec comparison covers the trade-offs, and the hosting costs guide follows bitrate through to the bandwidth bill.