SD Card Storage Calculator

Advertised capacity is not what you get to record

Put a "128 GB" card in a computer and it reports 119 GB. Nothing is broken and nobody shorted you. Two separate deductions are at work, one on paper and one in real bytes, and they are worth keeping apart.

The first is a units argument. Card makers define 1 GB as 1,000,000,000 bytes. Windows and Android divide by 1,073,741,824 and still print "GB". Dividing one by the other gives 0.9313, so 128,000,000,000 bytes is 119.2 GiB. That is a 6.9% haircut before a single file is written, and it applies to phones and SSDs just as much as to cards.

The second is real. Formatting and the card's own controller reserve part of the memory for allocation tables, firmware and address space. EMTEC puts the system reserve at about 3%. SanDisk warns that some drives have up to 4% less usable capacity for the same reasons.

This page defaults to treating the number your device shows as the byte budget. That is deliberately cautious, since the 7% unit gap already swallows the 2% to 4% formatting reserve. Leaving 10% to 15% of a card free is normal practice anyway, which is what the other two options in the dropdown are for.

File system matters too. SDHC cards up to 32 GB ship as FAT32, and SDXC and SDUC cards above that ship as exFAT. FAT32 caps a single file at 4 GB, which is roughly 24 minutes of iPhone 4K and under 6 minutes on a camera writing 100 Mbps, so long clips get chopped into pieces. Format in the camera that will use the card, not on a desktop. Phones hide the same advertised-versus-usable gap, and the iPhone storage size calculator works that side of it.

What one photo and one minute of video actually cost

Everything above runs on two small tables. Photos come from megapixels times a per-megapixel figure for the format, which beats a flat "7 MB JPEG" because a 61 MP RAW and a 12 MP HEIC have nothing in common. These are the same constants the photo storage calculator uses, so the two pages never give different answers.

Video comes from bitrate. The HEVC and H.264 rows below are Apple's own in-device estimates from Settings, Camera, Record Video. The camera rows use the identity worth memorising: MB per minute = Mbps x 7.5, because file size is bitrate times duration divided by eight. Read a bitrate off a camera menu, multiply by 7.5, and you have the answer without this page.

Slo-mo is not in the dropdown here to keep it short, but it is expensive: 1080p at 120 fps runs about 170 MB a minute and 240 fps about 480 MB, more per minute than 4K 30 fps costs. The video storage calculator has those modes, plus ProRes and a custom bitrate box.

The caveat both sibling pages make applies here too. Encoders are variable bitrate, so a flat wall and a windy tree line at the same setting do not come out the same size. Treat every total as give or take 15% to 20%.

Splitting one card between photos and video

The mix is what actually decides whether a card is big enough, and it is the question most capacity charts skip. At 4K 30 fps, one hour of video costs about 10.2 GB, roughly what a thousand fine JPEGs from a 24 MP camera cost. So a card that felt roomy for stills lasts an afternoon once you start recording.

Take the default settings above. A 128 GB card gives about 119 GB usable. Shooting 24 MP fine JPEGs at 10 MB each and 4K 30 fps at 170 MB a minute, half and half, you get about 5,950 photos plus 5 hours 50 minutes of video. Give the whole card to stills and it is about 11,900 photos. Give it all to video and it is 11 hours 40 minutes. Same card, three very different days.

A few practical things follow from that:

• Several smaller cards beat one big one when the shoot cannot be repeated. A single card is a single point of failure, and cards fail.
• Leave 10% to 15% free. Cards slow down and cameras get unhappy when they are nearly full, and you want room for one more take.
• Check the speed class as well as the size: U3 or V30 for standard 4K, V60 above about 240 Mbps, V90 for 400 Mbps and RAW video. A big slow card still drops frames.

If you are sizing a phone rather than a card, the phone storage planner asks the same questions in device terms.

When the card is full, the phone is next

Cards fill with the same dead weight a camera roll does. Three takes of one shot. Bursts nobody ever culled. Clips watched once and never opened again. A bigger card postpones the problem, and so does more cloud space, which the iCloud storage cost calculator puts a monthly figure on. The duplicate photo space calculator gives you a sense of how much of it is copies of things you already have.

Swipe44 does that job on the phone. You work through the camera roll by swiping, right to keep and left to delete, from Recents, Random, or one month at a time. Duplicates get found by on-device perceptual hashing and grouped so you can review them side by side. Everything you mark collects in a pending pile, then goes in one batch behind a single system confirmation, with a tally of the space that came back. There is no account, nothing is uploaded, and iCloud originals are never downloaded.

More sizing tools are on the tools page, and the blog covers the storage questions a calculator cannot answer.