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How Long Can You Record on 64GB, 128GB, 256GB, and 1TB?

Recording time is capacity divided by bitrate, once both are in the same units. This guide works the arithmetic for four common card sizes at a documented 4K recording bitrate, and covers what a card's speed class does and does not promise.

Published 2026-08-11 · Updated date pending

The short answer

Recording time is capacity divided by total bitrate, once both are expressed in the same units. At a documented 4K recording bitrate of approximately 100 Mbps plus a 128 Kbps audio track:

Card Recording time
64 GB 1:25:13
128 GB 2:50:26
256 GB 5:40:53
1 TB 22:11:37

Those are ceilings on a fully available card, and your camera’s bitrate is the one that matters. Enter your own capacity and bitrate in the Video Recording Time Calculator.

The formula

usableBytes   = capacityBytes x cardCount x (1 - reserveFraction)
payloadBits   = usableBytes x 8
totalBitrate  = videoBitrate + audioBitrate
durationSecs  = payloadBits / totalBitrate

The multiplication by eight is the step that trips people up: capacity is counted in bytes and bitrate in bits, and NIST’s note on SI and binary prefixes fixes the relation at one byte being eight bits. The reserve is a percentage of the card you do not intend to fill; it defaults to zero, which gives you the theoretical ceiling.

Worked example: 64 GB at a documented 4K bitrate

Sony’s help guide for the FDR-AX30 series documents an XAVC S 4K mode at approximately 100 Mbps at 3,840 x 2,160 pixels. Take that with a 128 Kbps audio track and a 64 GB card:

usableBytes  = 64 000 000 000 bytes
payloadBits  = 64 000 000 000 x 8   = 512 000 000 000 bits
totalBitrate = 100 000 000 + 128 000 = 100 128 000 bps
durationSecs = 512 000 000 000 / 100 128 000
             = 5 113.454777... s
             = 1:25:13   (85.2242 minutes)

Roughly an hour and twenty-five minutes. The other three capacities are the same division with a different numerator:

128 GB  1 024 000 000 000 / 100 128 000 = 10 226.909555... s =  2:50:26
256 GB  2 048 000 000 000 / 100 128 000 = 20 453.819111... s =  5:40:53
  1 TB  8 000 000 000 000 / 100 128 000 = 79 897.730903... s = 22:11:37

Doubling the capacity doubles the time exactly, because nothing else in the equation changes. Every line reproduces in the Video Recording Time Calculator.

At that bitrate the card fills at 750,960,000 bytes per minute — about 751 MB every minute.

Lower the bitrate and the times multiply

The relationship is a simple inverse: halve the bitrate and you double the time. On the same 64 GB card, keeping the 128 Kbps audio track:

100 Mbps  512 000 000 000 / 100 128 000 =  5 113.454778 s = 1:25:13
 60 Mbps  512 000 000 000 /  60 128 000 =  8 515.167642 s = 2:21:55
 50 Mbps  512 000 000 000 /  50 128 000 = 10 213.852538 s = 2:50:13

Sony documents all three of those bitrates: approximately 100 Mbps and approximately 60 Mbps for XAVC S 4K, and approximately 50 Mbps for XAVC S HD.

This is why “how long does a 128 GB card record” has no answer without a bitrate, and why this site will not publish a table of hours per card. Such a table would silently assume a bitrate, and would be wrong for every camera that uses a different one.

What a card’s speed class does and does not promise

Capacity tells you how much fits. Speed class tells you whether the card can keep up while it is being written.

The SD Association defines three overlapping schemes: Speed Class 2, 4, 6 and 10; UHS Speed Class 1 and 3; and Video Speed Class V6, V10, V30, V60 and V90. It states that Speed Class standards define minimum sequential card access performance.

On the numbers, the SD Association states that Video Speed Classes are “primarily defined by the card’s minimum sustained transfer speeds in megabytes per second (the number after ‘V’)”, and that UHS Speed Class 1 allows 10 MB/s file transfer rates while UHS Speed Class 3 allows 30 MB/s.

That gives you a check worth doing before a shoot. A 100 Mbps recording mode needs 100,000,000 / 8 = 12,500,000 bytes per second, which is 12.5 MB/s. A class rated at 30 MB/s of sustained write has margin over that; one rated at 10 MB/s does not.

Note what the class does not promise: it is a minimum sustained write figure, not a read speed, and not a guarantee about any particular camera’s recording mode. Your camera’s manual states the card requirement for each of its modes.

Usable capacity is smaller than printed capacity

Three things reduce a card below its label, and the arithmetic above knows about none of them:

  1. Units. Capacity is printed in decimal gigabytes — NIST fixes giga at 10^9 — while some operating systems report binary gibibytes, where one GiB is 1,073,741,824 bytes. A 64 GB card is about 59.6 GiB. Nothing has been lost; the unit changed.
  2. Formatting. The file system occupies space before you record anything.
  3. Existing content. Whatever is already on the card.

The calculator’s reserve field exists for these. Set a reserve percentage and the usable capacity shrinks accordingly before the division. Leave it at zero and you get the ceiling, which is the honest label for what the arithmetic produces.

The SD Association states that the SDXC standard “supports memory capacities above 32GB up to 2TB”, which is the standard’s range, not a statement about any card you can buy.

What this does not tell you

  • It assumes the bitrate is constant. A variable-bitrate recording mode averages around its figure. Sony’s own documentation distinguishes modes stated as a maximum from modes stated as an average, which is a distinction worth reading for.
  • It assumes the card is empty and the reserve is what you set. Use the reserve field rather than mentally subtracting.
  • It does not model clip-length or file-system limits. Some cameras and file systems split recordings into multiple files. That affects file count, not total time.
  • It does not model dropped frames. A card too slow for a mode is a different failure from a card too small, and speed class is the relevant specification.
  • It does not model heat, battery or media management. These end recordings before the card is full more often than capacity does.

Frequently asked questions

How long does 128 GB record in 4K?

At a documented approximately 100 Mbps 4K mode plus 128 Kbps audio, 2 hours 50 minutes and 26 seconds. At a different bitrate it is a different answer — the Video Recording Time Calculator takes your own figure.

Why does my 64 GB card show less than 64 GB?

Because of units and formatting. NIST’s note gives the two systems: 64 GB is 64,000,000,000 bytes, which is about 59.6 GiB in the binary units some operating systems display. Formatting then takes a further share.

What speed class do I need?

Compare the sustained write your card guarantees with the bytes per second your recording mode needs. A 100 Mbps mode needs 12.5 MB/s. The SD Association states that UHS Speed Class 3 allows 30 MB/s and that a Video Speed Class number is the minimum sustained write in MB/s. Your camera’s manual is the authority for its own modes.

Does using two cards double the recording time?

Yes, in total, and the calculator has a card-count field for it. It does not make an individual clip longer, and it does not help if the constraint is write speed rather than capacity.

Should I plan to fill the card completely?

The reserve field exists so you do not have to. Setting a reserve models the margin you want to keep, and the result is the recording time you can actually rely on.

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