Bitrate Calculator: Set Quality and Predict File Size

Video bitrate is the amount of compressed video data written or transmitted each second, normally expressed in kilobits per second (kbps) or megabits per second (Mbps). A higher value gives the encoder more bits to describe each frame, but it does not guarantee better quality.

Resolution, frame rate, motion, noise and codec efficiency all affect the required bitrate. A static 1080p interview can survive on less than fast gameplay at the same resolution and fps. Grain is especially expensive because an encoder must account for constantly changing detail.

Do not confuse bitrate with file size. Bitrate is a rate. File size is that rate multiplied by runtime, with audio and container overhead included.

Codec choice shifts the useful range as well. H.265 can usually preserve comparable detail at a lower bitrate than H.264, though playback support and encoding time may decide the job. The practical differences are covered in H.264 versus H.265.

A video file size calculator multiplies the total bitrate by runtime and converts bits into bytes. To work backward from a target file size, multiply the size in decimal megabytes by 8,000 and divide by the duration in seconds.

Use total bitrate when predicting output size:

For a 10-minute file at 6,000 kbps video plus 192 kbps audio, the estimate is 464.4 MB:

That calculation assumes decimal units, where 1 MB equals 1,000,000 bytes. Windows may display a binary value while labelling it MB, so its number can look smaller. Container metadata, subtitles and muxing overhead add more uncertainty. Reserve 1 to 3 percent when a hard upload limit matters.

Variable bitrate complicates the prediction only if the stated value is a peak or an encoder target it fails to reach. Read the finished file's average overall bitrate with MediaInfo↗, rather than trusting a maximum bitrate copied from export settings.

Start with the upload limit, subtract a small overhead allowance, and calculate the total bitrate available across the entire runtime. Then subtract the audio bitrate to obtain the target video bitrate.

The subtraction matters. People commonly spend the whole budget on video, add 192 kbps audio, and wonder why the finished file misses the platform limit.

1. Convert the runtime to seconds. A 12-minute video lasts 720 seconds.
2. Reduce the target size by 2 percent for container and metadata overhead. A 500 MB ceiling becomes a 490 MB working target.
3. Calculate total bitrate: 490 × 8,000 ÷ 720 = 5,444 kbps.
4. Subtract audio. With 192 kbps AAC, the video target becomes 5,444 - 192 = 5,252 kbps.
5. Enter 5,252 kbps as the average video bitrate. For two-pass variable bitrate, set a sensible maximum above it rather than treating the maximum as the average.
6. Encode a representative section before committing to a long output. Check motion, gradients, text and dark scenes, since those reveal damage sooner than a clean talking head.

A command-line check avoids relying on a player's rounded display. FFprobe can report the container duration and overall bitrate when those fields are present:

If the required value looks implausibly low, the arithmetic is probably fine. The target is wrong. Shorten the runtime, permit a larger file, lower the resolution or use a more efficient codec. A bitrate calculator cannot manufacture detail from an inadequate bit budget.

Choose a streaming bitrate that stays below both the platform limit and the connection's stable upload capacity. Leave headroom for network variation, audio and other traffic instead of setting the encoder to the best result from one fast speed test.

Constant bitrate is commonly required or recommended for live delivery because a platform needs a predictable incoming stream. It does not make the connection stable. If upload throughput drops below the combined video and audio output, viewers get dropped frames, buffering or a forced quality reduction.

For uploaded 1080p SDR video, 8 Mbps at 24, 25 or 30 fps and 12 Mbps at 48, 50 or 60 fps are reasonable H.264 starting points. They are delivery settings, not universal quality thresholds. A noisy concert recording may need more during an intermediate encode, while a clean animation may look acceptable with less.

Live streaming needs a stricter decision rule:

• Base the setting on sustained upload speed, not the connection's advertised maximum.
• Include the audio stream when calculating total output.
• Prefer a stable lower value over a higher setting that repeatedly saturates the connection.
• Reduce frame rate before crushing intricate 1080p frames into an obviously inadequate bitrate.

OBS documents its connection and dropped-frame checks in the stream connection troubleshooting notes↗. If dropped frames appear only while live, test the route and ingest server before dismantling encoder quality settings.