The formula behind every video file size
A video's size comes down to one line: size in bytes = bitrate in bits per second × length in seconds ÷ 8, with the audio bitrate added to the video bitrate. Ten minutes at 8 Mbps with 128 kbps audio works out to 8.128 million × 600 ÷ 8 = 609.6 MB.
Resolution and codec don't appear in the formula at all. They decide how good the picture looks for the bits you spend, not how many bits there are. A 4K file and a 720p file at the same bitrate and length are the same size, the 4K one just has to stretch each bit further.
Most encoders use variable bitrate, spending more on busy scenes and less on still ones, so treat the result as the average you'll land near. The container adds a little on top, usually a percent or two.
Working backwards from a size limit
Chat apps, email and upload forms often cap file size. The Bitrate for a size tab turns the limit into a bitrate: it multiplies the size by 8, divides by the length, sets aside the audio and keeps 3% in reserve for the container and encoder overshoot. A 3-minute clip squeezed into 25 MB with 128 kbps audio leaves about 950 kbps for the picture.
If the number comes out very low, lower the resolution rather than starving a big frame of bits, a clean 480p video looks far better than a blocky 1080p one. Cutting the parts nobody needs helps too: the video trimmer does that without re-encoding. When you're happy with the plan, the video compressor hits the size for real and runs a second pass automatically if the first one lands over.
MB or MiB? Why your computer shows a smaller number
Drive makers, macOS and most apps count in thousands: 1 MB is 1,000,000 bytes and 1 GB is 1,000,000,000. Windows counts in 1,024s but still writes "MB" and "GB", so the same file looks about 7% smaller there. An hour at 50 Mbps is 22.5 GB on the box and about 21 GiB in Windows Explorer. The calculator shows both so nothing comes as a surprise.
Planning storage for a shoot
Multiply the recording bitrate by the hours you'll roll and the number of cameras. One hour at 50 Mbps is 22.5 GB per camera; ProRes 422 HQ at Apple's 1080p target of 220 Mbps needs about 99 GB per hour, and roughly four times that in 4K.
The second card on the Storage tab flips the question: type a card or drive size to see how long it holds. Formatting takes a small slice of the capacity, so leave some headroom rather than planning to the last minute.
Bitrate also decides how fast a memory card must write. Mbps is megabits, not megabytes, so divide by 8: a 240 Mbps camera mode needs a card that sustains at least 30 MB/s.
What bitrate should I upload to YouTube?
The Upload guide tab lists YouTube's published recommendations: for standard dynamic range, about 8 Mbps for 1080p at 24 to 30 fps and 12 Mbps at 48 to 60 fps, 16 and 24 Mbps for 1440p, and 35 to 45 or 53 to 68 Mbps for 4K. HDR uploads need roughly 25% more. Treat these as approximate guidance rather than hard rules: YouTube re-encodes every upload, so going higher is fine, while going much lower costs quality you can't get back.
Upload at the frame rate you filmed at, and check what an existing file uses with the video info tool. Tap any figure in the table to see how big the file will be.