Master the math that controls file size, learn when to choose CRF or two-pass ABR, and grab ready-to-use tables that tie directly into the FitToMB calculator.
Bitrate is how many kilobits per second (kbps) your audio and video streams consume. Multiply bitrate by duration (minus audio) and you get your file size.
Approx. size (MB) ≈ (video_kbps + audio_kbps) x duration_sec ÷ 8 ÷ 1000
The FitToMB calculator subtracts audio automatically, so you only need to enter duration and desired MB. Then send the result straight to the compressor.
Think of containers as boxes, codecs as the compression method, and bitrate as the budget you allocate for quality.
Formats like MP4 or MKV bundle audio, video, captions, and metadata. MP4 is the universal pick for inbox previews and social uploads.
H.264, HEVC, and AV1 describe how each frame is encoded. Newer codecs pack more detail per bit but may limit playback compatibility.
The rate of data. More kbps typically equals sharper video-until you hit diminishing returns based on resolution and motion.
Constant bitrate (CBR) spends the same number of bits per second regardless of motion. It's predictable but inefficient. Variable bitrate (VBR) adjusts based on scene complexity and is standard for quality-focused exports.
FitToMB uses two-pass ABR: the first pass analyzes motion, the second allocates bits precisely to hit your target MB. Use this when limits are strict (10, 25, 50 MB). Choose CRF when hosting on your own site or YouTube where the final size can float.
Rule of thumb: If a platform has a hard cap, use two-pass ABR. If quality is the only constraint, use CRF and let size land where it wants.
Follow these steps to compute the exact video bitrate for a target size:
Worked examples:
| Duration | 10 MB target | 25 MB target | 50 MB target |
|---|---|---|---|
| 30 seconds | ≈ 2,570 kbps video | ≈ 6,570 kbps video | ≈ 13,240 kbps video |
| 60 seconds | ≈ 1,240 kbps video | ≈ 3,240 kbps video | ≈ 6,570 kbps video |
| 90 seconds | ≈ 800 kbps video | ≈ 2,130 kbps video | ≈ 4,350 kbps video |
Bitrate is the lever that controls size, but resolution and frame rate dictate how efficiently bits are used.
Need platform heuristics? The marketer guide and sharing guide document channel-specific rules of thumb.
After exporting, verify that your bitrate matches the target you calculated:
ffprobe -v error -select_streams v:0 -show_entries stream=bit_rate -of default=noprint_wrappers=1:nokey=1 output.mp4Attachment caps translate directly into bitrate ceilings. For Gmail's 25 MB limit, a 60-second clip should land around 3,300 kbps video + 96 kbps audio. Discord's 10 MB cap allows ~1,300 kbps video for the same length.
Use the Gmail calculator and Gmail workflow for inbox specifics, then reference the general sharing guide for Discord and chat heuristics.
Not once you exceed what the codec and display can use. Evaluate scenes on calibrated monitors to avoid overspending bits.
Content complexity drives efficiency. Calm talking heads can look pristine at 2000 kbps while sports footage needs far more.
CRF targets perceptual quality. Complexity shifts the resulting bitrate, so final size floats from clip to clip.
Drop audio from 128 kbps to 96 kbps before sacrificing video. Below 96 kbps, stereo music starts to suffer.
Use the FitToMB bitrate calculator. Enter duration, MB target, and audio bitrate to receive instant video kbps.
Two-pass ABR ensures you hit the cap. CRF is ideal for streaming or archiving when size can float.
Run ffprobe or MediaInfo, or check FitToMB's job history to confirm average bitrate and codec.
Plan for Gmail's 25 MB envelope and Yahoo's 25 MB limit. The Yahoo guide and Gmail calculator include headroom.