Photo Backup Drive Size Calculator
Work out how big an external drive your photo library needs, with years of growth, your backup copies, and the smaller drive a cleanup would buy.
Photo Backup Drive Size Calculator
Your library
iPhone: Settings, General, iPhone Storage, Photos. Android: Settings, Storage. Mac: Finder, right-click the Photos Library, Get Info.
No idea? The photos per year growth calculator works it out from how much you shoot. Zero is a valid answer if you are archiving a library that has stopped growing.
How long, how many copies
Three to five years is the usual planning horizon for a spinning drive.
Each drive holds a full copy of the library, so the per-drive size never changes here. Only the number of drives does.
25% is the middle of the measured range: near-duplicates alone run about 16% of a typical roll and roughly 30% for a heavy burst shooter, before screenshots and old screen recordings.
Enter a library size to see the drive you need.
The drive you would need after a cleanup
Shrink the library first with Swipe44
Drive sizes come in fixed steps, so a library sitting just over a boundary pays for a whole tier it barely uses, and pays twice if it is backed up twice. Swipe44 groups visual duplicates on your phone and lets you clear the rest one swipe at a time, right to keep and left to delete, with one system confirmation before anything goes.
External drive sizes you can actually buy
| Drive | Capacity | Form factor | Covers your library? |
|---|
Capacities are advertised decimal capacity, which is how Apple, Android and the drive makers all report storage. A tier "covers" your library when it clears the capacity per copy at a 70% fill.
What you are actually backing up
The first input is the only number you have to look up, and it is worth reading off the device rather than guessing. On an iPhone it is Settings, General, iPhone Storage, then the Photos row. On Android it is Settings, Storage. On a Mac, right-click the Photos Library in Finder and pick Get Info.
That number is usually bigger than people expect, and carelessness is rarely the reason. Live Photos carry a short video alongside every still. A burst saves every frame it took, not the one you kept. Screen recordings are video, and so are the clips people forget they shot at a concert three years ago. Videos are almost always the majority of the gigabytes even when they are a small minority of the items, which is why counting photos gets you the wrong answer and reading the storage figure gets you the right one.
If all you have is a photo count, the photo storage calculator converts it into gigabytes by format. If you know your library size but not how fast it grows, the photos per year growth calculator gives you the second input.
Why a backup drive should finish 70% full
Walk the default numbers through. A 400 GB library growing 120 GB a year over five years is 400 plus 600, so 1,000 GB of photos at the end of the horizon. Divide that by 0.70 and you get 1,429 GB, about 1.4 TB, of capacity per copy. The smallest drive on the shelf that clears 1,429 GB is a 2 TB, and 1,000 GB on a 2 TB drive is 50.0% full.
You will see 10% to 20% free quoted as the standard advice, and for a drive you are actively editing from that is correct: the concern there is write performance falling off as the free space fragments. A backup drive has a different problem. The margin covers a forecast of photos that do not exist yet, and forecasting low costs you a second shopping trip rather than a slower drive.
Notice that the tier you buy usually leaves more slack than the 70% target asked for, which is fine. The ladder of drive sizes rarely lands exactly on your number. The fill percentage in the results tells you where you really end up, so you can judge the margin yourself instead of taking the formula on faith. Replacing a drive in year three because you sized it for year one costs more than buying one with room to spare.
The arithmetic, start to finish
- Data to back up: library size today plus annual growth times the number of years, linear rather than compounded, because the growth input is already stated in GB per year.
- Capacity per copy: that figure divided by 0.70, so the drive finishes 70% full with 30% free.
- Drive size to buy: the smallest tier in the 500 GB to 24 TB ladder that clears the capacity per copy, compared before any rounding.
- Total across copies: the tier multiplied by the number of drives, where 1 copy is one drive and both 2 copies and 3-2-1 are two.
- Fill percentage: the data you are backing up divided by the tier you bought.
- After a cleanup: the same chain run again with the deletion share taken off today's library only, since a one-off cleanup does not change how many photos you take next year.
Everything here is decimal GB: 1 GB is 1,000,000,000 bytes and 1 TB is 1000 GB. That is the unit iOS, Android and macOS report your library in, and the unit drive makers print on the box, so both sides of the comparison already match. Formatting and filesystem metadata do take a slice of a new drive, but on a data volume that is well under 1% of the capacity, an order of magnitude smaller than the 30% margin, so it is absorbed there rather than modelled as its own number.
Copies before capacity
The 3-2-1 rule is three copies of your data, on two different kinds of media, with one of them offsite. In plain terms: the library on your phone or computer is copy one, a drive on your desk is copy two on a second kind of media, and a drive at a relative's house or a cloud account is copy three, offsite. Offsite means a different building, because a burst pipe or a break-in does not respect which shelf a drive is sitting on.
This matters for capacity because the copy you already have is not one you need to buy. Picking 3-2-1 in this calculator gives you two drives, not three. Other calculators multiply the requirement by three for a 3-2-1 user, which counts the original as a drive you have to purchase and quietly tells you to buy 50% more capacity than the rule asks for. Each drive holds a full copy of the library, so the per-drive size never changes with this setting. Only the drive count does.
If your offsite copy is a cloud service rather than a drive you carry somewhere, the drive count drops back to one, and the cloud side is billed monthly against its own tiers, so it is a separate line item from the one-off drive purchase.
The cheapest terabyte is the one you never buy
Drive sizes jump in wide steps, so a library sitting just over a boundary pays for an entire tier it barely touches. Take a 1,000 GB library growing 100 GB a year over five years: that is 1,500 GB, which needs 2,143 GB per copy at a 70% fill, so a 4 TB drive. Trim 25% of today's library and the requirement becomes 1,250 GB, or 1,786 GB per copy, which a 2 TB drive covers. One cleanup, one tier down, and twice over if you keep two copies.
The gigabytes that make the difference are rarely photographs anyone would miss. They are near-identical burst frames, the three attempts at the same group shot, screenshots of things you dealt with months ago, edited copies sitting beside the originals they were made from, and screen recordings. The duplicate photo space calculator and the camera roll cleanup estimator put numbers on how much of that is sitting in a typical roll.
Clearing it is what Swipe44 does. It groups visual duplicates on the phone using perceptual hashing so you review a group instead of hunting one file at a time, and the rest goes by swiping, right to keep and left to delete, through Recents, Random or a single month. Everything you mark collects in a pending pile and goes in one batch behind a single system confirmation, with a tally of the space you got back. Nothing is uploaded, and iCloud originals are never downloaded. Do that before you order the drive and you may be ordering a smaller one.
Frequently Asked Questions
Common questions about photo backup drive size calculator
How big an external hard drive do I need for photos?
Take what your library weighs today, add what you expect to add each year multiplied by how long you want the drive to last, then divide by 0.70 so the drive finishes 70% full rather than jammed. A 400 GB library growing 120 GB a year over five years is 1,000 GB of photos, which wants about 1.4 TB of capacity, so a 2 TB drive. Most phone-only libraries land on 1 TB or 2 TB. Libraries with years of 4K video or RAW files run to 4 TB and up.
Is 1 TB enough for my photo library?
For JPEGs from a phone, usually yes for a long time. At an average of 4 to 5 MB a photo, a 1 TB drive holds somewhere around 200,000 to 250,000 images. RAW files change the answer completely: at 25 to 40 MB each, 1 TB is more like 25,000 to 40,000 frames. Video is what actually fills drives. Run your own number through the calculator above rather than trusting a photo count, because a library is rarely all one file type. The photo storage calculator turns a photo count into gigabytes if that is the number you have.
How much free space should a backup drive have?
This tool sizes drives to finish 70% full, so 30% free. The usual advice you will read is 10% to 20% free, and that is sound for a drive you are actively working from, where the concern is write performance slowing down as it fills. A backup drive is a different problem: the thing you are padding is a guess about how many photos you will take, and guessing low means buying a second drive. 30% is the cheap insurance, and the calculator shows you the fill percentage you actually end up with so you can judge it yourself.
What is the 3-2-1 backup rule?
Three copies of your data, on two different kinds of media, with one copy kept offsite. The copy already sitting on your phone or computer counts as one of the three, so in practice you are buying two backups, not three. That is why picking 3-2-1 in this calculator gives you two drives rather than three. Offsite means somewhere a burst pipe or a break-in would not reach: a relative's house, a desk drawer at work, or a cloud service.
Why does my 1 TB drive only show 931 GB?
Nothing is missing, the two systems count differently. A drive sold as 1 TB holds 1,000,000,000,000 bytes. Windows divides that by 1,073,741,824 and still calls the result "GB", which gives 931. macOS and iOS divide by 1,000,000,000 and show 1000 GB. The ratio between the two is 0.909. This calculator uses the decimal numbers that Apple and the drive makers use, so if you are reading a Windows properties box, expect it to sit about 9% below the figures here. Formatting itself takes a much smaller slice, well under 1% on a data drive, and the 30% free-space margin covers it.
Does a cloud backup count as one of the three copies?
Yes, and it is the easiest way to satisfy the offsite part of the rule. If your offsite copy lives in the cloud, you only need one physical drive, so use the "1 copy" option for the drive capacity and let the service handle the rest. The calculator leaves cloud out of the capacity number because cloud plans are sold in their own tiers and billed monthly, which is a separate decision from what goes in the cart today.
Should I buy one big drive or two smaller ones?
Two, if you can. A single drive is a single point of failure, and the whole point of the 3-2-1 rule is that one dead drive is an inconvenience rather than a disaster. Drives also get cheaper per terabyte as they get bigger, so the temptation is to buy one 8 TB unit instead of two 4 TB units, and that is exactly the trade you should not make with the only copy of your photos. Buy the tier this calculator gives you, twice.
Will cleaning up my camera roll actually change what I need to buy?
Sometimes, and when it does the saving is a whole drive tier rather than a few dollars. Drive capacities step in doublings, so a library sitting just over a boundary can drop a whole tier once the near-duplicates and the accidental screen recordings are gone, and that saving lands twice if you keep two copies. When it does not change the tier, it still buys you slack, which the calculator shows as a lower fill percentage. The duplicate photo space calculator and the camera roll cleanup estimator will tell you which share of your library is realistically up for deletion.