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.
What Is Video Bitrate, and Why It Matters for Memory Cards
Video bitrate is just how much data your camera writes to the card every second while recording. Higher bitrate means more information per frame, which generally means cleaner footage — less banding in skies, better detail in shadows, fewer compression artifacts in fast motion. It also means bigger files and a faster card requirement.
The number you see on the spec sheet — 100Mbps, 400Mbps, 2600Mbps — is measured in megabits per second. Memory cards are rated in megabytes per second. Those are not the same thing and this is where most people get tripped up. One byte is eight bits, so to go from the camera spec to the card spec you divide by eight. A 400Mbps recording writes 50MB/s to the card. A 2600Mbps recording writes 325MB/s — which is why the Canon R5 needs CFexpress and not UHS-II SD.
The calculator above handles the math both ways. Plug in a card size and a bitrate and you get record time. Plug in a record time and a card size and you get the effective bitrate — handy when a camera spec sheet is missing or when you’re trying to figure out what a proxy workflow is actually writing. The important takeaway: always verify your card meets or exceeds the bitrate divided by eight, with headroom. A V30 card promises 30MB/s minimum, which is only safe up to roughly 240Mbps. Above that, move up to V60, V90, or CFexpress depending on the camera.
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.
H.264 vs H.265 — What Bitrate Do You Actually Need?
The same resolution and frame rate can look completely different depending on the codec. H.264 is the older format and needs more bitrate to hit the same perceived quality as H.265 (HEVC). A rough rule of thumb: H.265 can roughly match H.264 at around half the bitrate, which is why newer cameras quietly dropped their H.264 numbers once H.265 was available.
That matters for the calculator because the same card lasts twice as long when you’re shooting H.265 at equivalent quality. A Sony camera writing H.264 at 600Mbps (75MB/s) is roughly equivalent in final image quality to H.265 at around 300Mbps (37.5MB/s). On a 128GB card, that’s the difference between about 28 minutes and about 57 minutes of recording.
The catch is edit-side. H.264 is lighter to decode, so timeline playback is smoother on modest machines. H.265 gives you smaller files but can bog down older editors without proxies or hardware acceleration. If your machine handles H.265 natively, shoot H.265 and let the calculator tell you how much longer the card lasts. If it doesn’t, stick with H.264 and plan card size accordingly.
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.
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