Methodology, Formulas & Parameter Reference
Network transmission and codec rate control measure throughput in bits per second (kbps or Mbps, using decimal multiples of \(10^3\) or \(10^6\)), whereas operating systems and upload limits measure storage in bytes (MB or MiB, where \(1 \text{ Byte} = 8 \text{ bits}\)). Confusing bits with bytes or ignoring audio streams and container muxing overhead is the primary reason a file targeted at 50 MB finishes rendering at 53.4 MB and gets rejected by an upload form.
To guarantee that an encoded video stays strictly below a maximum size \(S_{\text{MB}}\) (in binary megabytes, where \(1 \text{ MiB} = 1024 \times 1024 = 1,048,576 \text{ bytes} = 8,388,608 \text{ bits}\)) over a duration of \(T\) seconds with a safety overhead factor \(m\) (typically \(1.5\%\) to \(3\%\) for MP4/MKV packet headers and seek tables), the total available bitrate in kilobits per second (\(1 \text{ kbps} = 1000 \text{ bits/s}\)) is calculated as:
\[ B_{\text{total (kbps)}} = \frac{S_{\text{MB}} \times 8388.608 \times (1 - m)}{T} \]
Subtracting the audio stream bitrate \(B_{\text{audio (kbps)}}\) yields the maximum permissible video stream bitrate: \(B_{\text{video}} = B_{\text{total}} - B_{\text{audio}}\). When hitting an exact file size ceiling matters more than encoding speed, single-pass CRF cannot be used because scene complexity varies unpredictably. Instead, two-pass Variable Bitrate (2-Pass VBR) analyzes the complexity of every frame in Pass 1 (writing statistics to a log file) and distributes the exact bit budget \(B_{\text{video}}\) across the timeline in Pass 2.
| Resolution | Frame Rate | H.264 (AVC) Target | H.265 (HEVC) / AV1 Target | Typical Audio Bitrate |
|---|---|---|---|---|
1280x720 (720p) |
30 fps | 3,500 to 5,000 kbps | 2,000 to 3,200 kbps | 128 kbps AAC |
1920x1080 (1080p) |
30 fps | 6,000 to 8,000 kbps | 3,500 to 5,000 kbps | 192 kbps AAC |
1920x1080 (1080p) |
60 fps | 9,000 to 12,000 kbps | 5,500 to 7,500 kbps | 192 kbps AAC |
2560x1440 (1440p) |
60 fps | 18,000 to 24,000 kbps | 11,000 to 15,000 kbps | 256 kbps AAC |
3840x2160 (4K UHD) |
60 fps | 45,000 to 68,000 kbps | 28,000 to 42,000 kbps | 320 kbps AAC |
Production Workflow Verification & Quality Control
Before committing a full batch transcode or publishing a final master, verify the generated parameters against a five-second test slice of your source timeline. In post-production pipelines, container metadata headers, variable frame rate (VFR) timebases from mobile or screen-capture recordings, and non-square pixel aspect ratios can alter how downstream players interpret stream flags. Running every calculation and file conversion locally in browser memory ensures your production assets remain private while providing deterministic numbers you can paste directly into terminal automation scripts, NLE export presets, or camera prep checklists.