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There are two calculators here; one calculates the record time based on the inputted data rate and record size. For example, you can calculate the record time of 150Mbps that a 128GB memory card would give you.
The other calculator gives you the total data rate based on the time recorded on a memory card. So if you can record 2 hours to a 128GB card, that would give you the bit rate.
Calculating Record Time Based On Bit Rate
This calculation finds the record time based on the data rate.
Calculating Bit Rate Based On Record Time
This calculator finds the data rate based on record time.
Often, photographers or videographers get confused by the difference between megabits and megabytes. Our cameras always list their video specs as megabits.
For example, Sony cameras often record 4k at 100Mbps, Nikon at 144Mbps, and Fujifilm at 400 Mbps.
However, the speed of our memory cards is always listed as Megabytes per second. UHS-II cards have a maximum speed of 300MB/s.
So to give some reference when shopping for memory cards.
4k 100Mbps = 12.5MB/s
To get megabytes out of megabits, divide the megabits number by 8.
Your standard UHS-I U3 card has a minimum guaranteed write speed of 30MB/s.
Often, you see people say, I’ll need faster memory cards or bigger hard drives for 4k video.
The data written is entirely based on the video stream’s bitrate. 100Mbps 1080p will produce the same file sizes as 100Mbps 4k.
Now, cameras like the Sony A7sIII, Sony FX3, and Canon R5 allow many different bitrates to choose from, and it becomes more important to buy the appropriate speed of memory card depending on the bitrate needed for your recording.
The Sony A7sIII and Sony FX3 now can write at 600Mbps with H.264 which means you will need a minimum of 75MB/s. Only V90 UHS-II memory cards can guarantee that bitrate.
The tricky thing with Sony cameras is they record slow motion with this mode called S&Q. It will write a 120fps 280Mbps file, which equals 35MB/s. However, the seconds here are in time based on 120fps, not real-time. This means you are dumping way more information to the card than what you think is recording. Pretending real life was 30fps, you would be dumping 35MB/s x 4 to your card, giving you 140MB/s of a data stream. I’m not sure exactly how Sony does it, but this is why you need CFexpress Type-A cards for their 120p 4k. Because the bitrate listed in their specs is based on the 120p time as it plays in slow motion in your editing software.
The Canon R5 can write at a maximum bitrate of 2600Mbps, which is equivalent to 325MB/s. Since that is beyond the spec of our 300MB/s UHS-II memory cards, you must have a CFexpress memory card.
2 comments
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