The formula
File size (MB) = bitrate (Mbps) × duration (seconds) ÷ 8, because one byte is eight bits. Rearranged, bitrate (Mbps) = file size (MB) × 8 ÷ duration (seconds). Add the audio bitrate to the video bitrate if you want the total: a 128 kbps audio track adds 0.128 Mbps.
Worked examples
A 10-minute 1080p video at 8 Mbps: 8 × 600 ÷ 8 = 600 MB. A one-hour recording at 5 Mbps: 5 × 3,600 ÷ 8 = 2,250 MB, about 2.25 GB. To fit a 60-second clip into 100 MB, the total bitrate must be at most 100 × 8 ÷ 60 = 13.3 Mbps.
Recommended upload bitrates
YouTube's recommended bitrates for standard (SDR) uploads are about 5 Mbps for 720p at 30 fps (7.5 Mbps at 60 fps), 8 Mbps for 1080p at 30 fps (12 Mbps at 60 fps), and 35–45 Mbps for 4K at 30 fps (53–68 Mbps at 60 fps). Higher bitrates keep more detail in fast motion; lower bitrates save space but can show blocky compression artifacts.
Why the exported file can differ
Most encoders use variable bitrate (VBR), so the actual bitrate rises in complex scenes and drops in simple ones, and the target is only an average. Audio tracks, the container (MP4, MOV, MKV), subtitles and metadata add to the size. Codecs matter too: H.265/HEVC and AV1 reach similar quality to H.264 at a noticeably lower bitrate.
MB vs MiB
The calculator uses decimal units: 1 MB = 1,000,000 bytes and 1 Mbps = 1,000,000 bits per second. Windows shows sizes in binary units (1 MiB = 1,048,576 bytes) while labeling them MB, so a 600 MB file appears as about 572 MB there.