How Many Photos Fit on a Memory Card?

RAW file size is almost pure arithmetic

An uncompressed RAW stores one value per pixel at the sensor's bit depth. At 14 bits that's 1.75 bytes per pixel:

24 MP × 1.75 = 42 MB a frame

Compressed RAW — what most cameras write by default — lands near 1.1 MB per megapixel, so about 26 MB for the same sensor.

The useful consequence: RAW barely varies between frames. What you get on a blank wall is about what you get on a forest.

JPEGs are a different story

JPEG compression discards detail the eye is unlikely to miss — so how much it can discard depends entirely on the scene.

A blank sky compresses to a fraction of what a forest full of leaves does, on the same camera at the same quality setting. A factor of several between individual frames is completely normal.

0.3 MB per megapixel is a fair average for fine quality, but it's an average in a way the RAW figures aren't. A card of JPEGs won't fill at a steady rate.

Then video arrives and none of it matters

At 100 Mbps, 4K eats 45 GB an hour.

One minute of 4K costs roughly what 28 RAW photographs do.

A photographer who has never come close to filling a card can be genuinely surprised the first time they record a few clips on a shoot. That's why the tool subtracts the video first and counts frames into what's left — the video decides the answer, and the stills are the remainder.

Format, at a glance

Same 24 MP camera, same 128 GB card, no video:

Nearly a sixfold spread from one menu setting.

⚠️ And if your camera is set to RAW+JPEG, it writes both — about 33 MB a frame rather than 26. It's a common setting to have enabled without noticing, and one of the few things besides video that meaningfully changes how fast a card fills.

Capacity is not speed

These get confused constantly, and they feel identical when you hit them.

Burst length is limited by the camera's buffer and by how fast the card can absorb data. A card with hundreds of gigabytes free can still stop a burst dead if its sustained write speed can't keep up — and most cameras specify a minimum sustained write speed for 4K that a slow card will fail regardless of capacity.

How much it holds and how fast it accepts data are separate specifications. Check both before a shoot that matters.

Does a 128 GB card hold 128 GB?

Almost. Capacity is quoted in decimal gigabytes — 128 billion bytes — and the file system's own structures use a small share.

It's about a percent, not the ~7% gap quoted for hard drives (that one is decimal-versus-binary units, a different thing entirely). Either way it isn't why your card fills sooner than expected. That's video, or RAW+JPEG.

One big card or several small ones?

This is a backup question rather than a capacity one, and it deserves a deliberate answer.

Cards fail, and when one does everything on it goes at once. A single 512 GB card is a single point of failure for an entire trip; four 128 GB cards mean a failure costs you a quarter of it.

Plenty of working photographers deliberately shoot smaller cards and change them, accepting the inconvenience as insurance. No calculator can weigh that for you — but it should be a choice rather than an accident.