How many photos fit on a memory card?
It is one of those questions you ask yourself at the worst possible moment. You are somewhere, you have just finished a nice series, and your camera tells you there are 47 shots remaining. Is that enough? Should I switch cards now? Do I even have a second card with me?
I have been in that situation a few times. Not because I am careless, but because I did not know exactly what to expect from the card that was in my camera. I knew the capacity, but not what that capacity meant in practice.
That is precisely where most photographers go wrong. They buy a memory card based on capacity, 64 GB or 128 GB, without knowing how many photos that will give them with their specific camera and settings.
Why the answer is more complicated than you think
A memory card stores bytes. A photo takes up a certain number of bytes. Divide the capacity by the file size and you know how many photos fit on it. That is the theory.
In practice it is more complicated, because the file size of a photo is not fixed. A RAW file from a Fujifilm X-T5 is larger than a RAW file from an entry-level mirrorless camera, simply because the sensor has more megapixels. A compressed RAW file is smaller than an uncompressed RAW file from the same camera. And a JPEG is much smaller than a RAW, but its size also varies depending on the subject: a photo of a busy scene with lots of detail contains more data than a photo of a plain wall.
On top of that, the stated capacity of a memory card is never quite accurate. A 64 GB card actually has slightly less usable space, because some of it is taken up by the file system.
The result is that your camera's estimate, those 47 shots I mentioned, is a rough indication. Sometimes it is fairly accurate, sometimes it is way off.
What this means in practice
I shoot with the Fujifilm X-T5, a 40-megapixel camera. RAW files from that camera are large. An uncompressed RAW file can easily reach 80 MB. With compression I end up at around 40 to 50 MB per photo.
On a 128 GB card that gives you roughly 2,500 to 3,000 compressed RAW files. That sounds like a lot, but if you spend a day doing street photography and shoot 500 photos, you will have filled your card after five such days. And if you also record video, it goes much faster.
For JPEG shooters it is a different story. A high-quality JPEG from the same camera might be 15 to 20 MB. On that same 128 GB card you can fit 6,000 to 8,000 files. More than enough for most situations.
The point is: knowing what to expect helps you bring the right card and switch at the right moment, not when your camera tells you it is already too late.
Which card do you need
Capacity is one thing, speed is another. A memory card has two speeds that matter: read speed and write speed. Write speed determines how fast your camera can write data to the card. If you shoot in quick succession, also known as burst shooting, or if you record video at high quality, you need a card that writes fast enough to keep up with your camera.
A slow card in a fast camera means your buffer fills up. Your camera is then temporarily unable to shoot while it waits for the card to finish writing the data. At the wrong moment, that is a real problem.
For most photographers, a V30 card, which stands for a minimum write speed of 30 MB per second, is sufficient. If you shoot a lot of 4K or 8K video, you will need a V60 or V90.
The memory card calculator
At apps.michielheijmans.com there is a memory card calculator that works all of this out for you. You enter your camera, your file format and your card capacity, and you get a realistic estimate of how many photos will fit on it. The tool is free and works directly in your browser.
It is also useful when you want to buy a new card and need to know what capacity you need for a particular type of shoot. You flip the calculation around: how many photos do I want to be able to take, and which card suits that?
The calculator is linked to the print size tool and the crop factor calculator, for when you are already looking into the technical side of your gear.
My own approach
I always work with two cards in my camera. Not because I am afraid one will fail, although that is also a reason, but because I write my RAW files to both cards simultaneously. A double backup, on the spot, without any extra steps.
Something else I do: I format my cards after every import. Not delete, but format, because that is faster and more reliable. And I use cards from a brand I trust. Not the cheapest from an unknown brand, but not necessarily the most expensive either. Somewhere in the middle, from a manufacturer that has been around for years.
They are small habits, but they have given me peace of mind more than once at the moment it counts.
Preparation is not a guarantee
A full card at the wrong moment is annoying but solvable. You switch, you might miss a few seconds. Worse is a card that fails altogether. That is rare, but it does happen.
The best preparation is not one large card, but several smaller ones. If one fails, you lose part of your images, not everything. And if you know how many photos to expect per card, you also know when it is time to switch, without waiting for your camera to force your hand.
Planning a big shoot, a wedding, a day abroad, a workshop? Work it out beforehand. It takes two minutes and saves a lot of stress at the moment you can least afford it.