What this estimator works out
This estimator converts card capacity into a photo count at your average file size, holding back a reserve.
The reserve matters more than it looks. A card filled completely can fail to write the final frame, and cards perform better with headroom.
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Before you rely on a card
- Average file size should come from your own files, not from a specification. RAW size varies with subject detail and ISO.
- A 10% reserve is sensible. Cards also lose usable capacity to formatting overhead.
- Card capacity is quoted in decimal gigabytes; the camera reports binary, so a 64 GB card shows as about 59.6 GB.
- Video changes the arithmetic entirely — it is measured in gigabytes per minute rather than per frame.
A single large card puts an entire shoot on one point of failure. Cards do fail, and they fail without warning. Splitting across two or more cards, and changing cards between sessions rather than filling one, limits what any single failure costs — which for an irreplaceable occasion is the whole argument.
How this calculator works
Capacity converted, less reserve, divided by file size:
Photos = card size in GB × 1,024 × (1 − reserve ÷ 100) ÷ average file size in MB, rounded downMultiplying by 1,024 converts gigabytes to megabytes so the units match the file size. The result rounds down, since a partly written frame is not a photograph.
Worked example: a 64 GB card
Using the default figures — a 64 GB card, 25 MB average files, 10% reserve:
- Capacity: 2,359 photos
Over two thousand frames, which for most shooting is more than a day. But shoot RAW plus JPEG at 33 MB combined and it falls to 1,787; shoot a camera producing 45 MB files and it is 1,310. And a few minutes of 4K video can consume more than a hundred stills. The photo count is the least demanding thing a card is asked to hold.
Common mistakes
- Relying on a single card for an irreplaceable occasion. Cards fail without warning.
- Filling a card completely. Leave headroom.
- Using a manufacturer's file size figure. Check your own files.
- Formatting in a computer rather than the camera. Camera formatting produces the file structure it expects.
- Ignoring the speed rating. Capacity is irrelevant if the card cannot keep up with the buffer or with video.
Frequently asked questions
What do the card speed ratings mean?
UHS speed class and Video Speed Class state a guaranteed minimum sustained write speed — V30 means at least 30 MB/s, for example. That minimum is what matters for video and for continuous shooting, far more than the headline maximum read speed printed largest on the card. Buying to the camera's stated requirement is the reliable approach.
How long do memory cards last?
Flash memory has a finite number of write cycles, but in normal photographic use cards usually fail from physical damage, contact wear or controller failure long before wearing out. Treating them as consumables and replacing working cards every few years is cheaper than losing a shoot.
Should I format or delete?
Format, in the camera, before each shoot. Formatting rebuilds the file structure the camera expects and clears the residue that deleting individual files leaves behind. Always confirm the previous shoot is backed up first, since formatting is not recoverable by ordinary means.
What if a card fails?
Stop using it immediately — continuing to write reduces the chance of recovery. Card recovery software can retrieve deleted or corrupted files in many cases, and professional recovery services exist for physical failures. Neither is guaranteed, which is why a second card and prompt backup matter more than either.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — consumer rights on electronic goods and product standards
- MoneyHelper — budgeting for equipment
Sources are checked at publication and can change — how I choose and check references.