Video Encoding Lab

explore practical encoding tradeoffs before exporting

Worked example — Video Encoding Lab

Effective bitrate	6 Mbps
Estimated size	90 MB
Bits per pixel-second (BPPs)	0.0965
Raw pixel rate	62,208,000 px/s
Total frames	3,600
Quality score	69.2 / 100 · acceptable

These are the values the calculator shows when the page opens. Change any input above to recompute them.

Loading interactive simulation...

why bitrate fails on fast scenes first 🖖

Every frame carries an entropy 'demand' — how many bits it truly needs to look clean, driven by resolution, frame rate, motion, and chroma detail. Your chosen bitrate is a fixed 'budget': spread evenly across the whole clip in CBR mode, or aimed at a target quality per scene in CRF mode. When a fast, detailed scene's demand spikes above that budget, the encoder has no choice but to round more aggressively to stay within its bit allowance — which is exactly why blocking and smearing show up first on the busiest scenes, not the calm ones.

Smaller files, same look, slower to make 🖖

Newer codecs squeeze the same picture into fewer bits, and this lab puts numbers on it: at matched quality it models H.265 at 72% of the H.264 size and AV1 at 58%. So moving to H.265 saves roughly a quarter, and moving to AV1 a little over 40%. The catch this lab hints at is that those savings cost compute — AV1 can take many times longer to encode than H.264. Rule of thumb: pick the newest codec your devices can play and your time budget can afford.

Why 60 fps doesn't cost double 🖖

Intuitively, twice the frames should need twice the bitrate — but they don't. The closer two frames sit in time, the more alike they are, so the encoder's motion compensation only stores the tiny differences between them. Going from 30 to 60 fps at matched quality usually adds only about 30–50% more bits, not 100%. The extra frames are cheap because they're nearly copies of their neighbors.

Problem solved in full

Compression of two minutes of 1080p30 at 6 Mbit/s 5 steps
Two minutes of 1080p30 at 6 Mbit/s comes to 90 MB. Work out how much compression that represents, and find the one number that lets you compare it with a 4K stream.
1. Start with what the camera produces. Resolution times frame rate gives the pixel rate, and it is a big number: 62.2 million pixels every second.
  
  $1920 \times 1080 \times 30 = 62208000 px/s, 30 \times 120 = 3600 frames$
2. File size is bitrate times duration, with the only trap being bits against bytes. 90 MB for two minutes.
  
  $size = \frac{6 Mbit/s \times 120 s}{8} = 90 MB$
3. Bitrate alone cannot be compared across formats — 6 Mbit/s is generous for 720p and thin for 4K. Dividing by the pixel rate removes the resolution and leaves bits per pixel-second, which can.
  
  $BPPs = \frac{6 \times 1 0 ^{6}}{62 208 000} = 0.0965 bits per pixel-second$
4. Uncompressed 4:2:0 at 8 bits averages 12 bits per pixel, so the raw stream would be 746 Mbit/s.
  
  $raw 4:2:0 8-bit = 12 bits/px \Rightarrow 746 Mbit/s$
5. Against that, 6 Mbit/s is a compression ratio of about 124 to 1 — and the picture is still watchable, which is the actual claim modern codecs are making.
  
  $\frac{746}{6} = 124 \times compression; 4K at the same BPPs = 24.0 Mbit/s$
Answer

The tool prints 62,208,000 px/s, 3,600 frames, 90 MB and 0.0965 bits per pixel-second. That last figure is the portable one. Hold it fixed and 4K needs 24 Mbit/s — four times the pixels, four times the bitrate, same visual budget per pixel. It is also how you tell whether a streaming service is being generous: divide their bitrate by the pixel rate and compare, rather than arguing about megabits for formats that are not the same size.

$BPPs = \frac{bitrate}{w h f} is resolution-free; 124 \times compression here$

References (1)

What the codec efficiency factors in this lab approximate: Y. Han et al., "A Technical Overview of AV1." Proceedings of the IEEE 109(9), 1435–1462, 2021 — AV1's coding tools and its measured gain over earlier standards.

By the 42Math team · Updated 2026-05-24

Reviewed against published sources. Corrections welcome. Editorial policy · Corrections

All Computing lessons →

Example problems

1080p stream baseline - 1080p streaming baseline: 6 Mbps H.264 CBR
mobile short clip - Mobile/social short with H.265 CRF balance
fast sports scene - Fast motion sports needs high bitrate headroom
4K AV1 archive - 4K archive-oriented AV1 profile with smaller size