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Bitrate × Time = Size, But Which Bitrate?

The core formula is simple: file size (MB) ≈ total bitrate (Mbps) × duration (seconds) / 8 — dividing by 8 converts bits to bytes. The part people get wrong is "total bitrate": it is the sum of the bitrate of the video stream and the bitrate of the audio stream, calculated separately and then added together, not just the video number alone. A 1080p60 video encoded at 12 Mbps video plus a 192 kbps (0.192 Mbps) AAC audio track actually totals 12.192 Mbps — audio is usually small enough next to video that it is easy to ignore, but it is never truly zero.

Concrete example: a 10-minute (600-second) clip at the 12 Mbps YouTube recommends for 1080p60, plus 128 kbps audio, totals 12.128 Mbps. Size = 12.128 × 600 / 8 = 909.6 MB ≈ 0.9 GB. Scale that up to an hour of 4K60 at a 68 Mbps video bitrate plus 320 kbps audio: (68.32 × 3600) / 8 ≈ 30,744 MB, or about 30 GB — which is why an hour of high-resolution raw footage can fill most of a memory card. Container overhead (the MP4 or MKV wrapper holding the streams together) is typically well under 1% and can be ignored for practical estimates.

The formula assumes a constant bitrate (CBR), where every second of video uses roughly the same number of bits. Most modern encoders default to variable bitrate (VBR) instead, which spends more bits on complex, high-motion scenes and fewer on static ones, targeting an average rather than a fixed rate. VBR is more efficient for a given visual quality, but it also means the real exported file can come in noticeably larger or smaller than a simple calculation predicts, depending on how complex the actual footage is — a talking-head video will undershoot the estimate, while fast action or heavy grain will overshoot it.

Things Worth Knowing

• Reverse mode answers the practical question: to fit a 90-minute event under a 4 GB limit, roughly (4 × 8 × 1000) / 5400 ≈ 5.9 Mbps total is affordable — 1080p30 territory, not 1080p60.
• H.265/HEVC and AV1 typically deliver comparable visual quality to H.264 at roughly half the bitrate, so the same file-size target buys noticeably better quality with a modern codec.
• If a hard size ceiling matters (upload limits, storage quotas), set a peak/max bitrate cap in the encoder rather than relying on an average VBR target, since VBR by definition can exceed its average on complex scenes.
• Common live-streaming targets: 1080p60 ≈ 6 Mbps (the Twitch ceiling is roughly 6-8), 1080p30 ≈ 4.5 Mbps, 720p60 ≈ 4 Mbps — with upload speed ideally at least 1.5 times the chosen bitrate for stability.