Every drive you buy shows up smaller than the box says. A 1 TB drive reports 931 GB, a 2 TB drive reports 1,863 GB, and nothing has gone wrong: the shop counts in powers of ten and Windows counts in powers of two. This storage calculator does both sums.
This works out what a drive will actually show, then adds up your own files and tells you which size to buy. Almost every number in it is one you typed; the only two supplied here are the conversion itself and Microsoft's published minimum for Windows.
In this guide
The storage calculator
The number the shop shows: 1 TB, 2 TB, 4 TB.
Pick yes for the drive you boot from.
Check the biggest ones you actually keep; modern releases run from about 40 GB to well over 150 GB.
A phone JPEG is a few MB; a camera raw file is 25 MB and up.
Roughly 8 GB an hour for 1080p at a typical bitrate, and several times that for 4K.
Applications, documents, downloads.
Room for updates and for the drive to work in. 15 is a margin, not a rule.
How that is worked out, line by line
- Games560 GB
- Photos29 GB
- Video80 GB
- Your files add up to719 GB
- With Windows on it783 GB
- Plus the margin you asked for901 GB
- Space left on the drive you picked31 GB
On those numbers you are in the 1 TB is enough band, which is the section below.
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Where every number here comes from
No part of this was measured on a bench here. This site works from published specifications and from what owners report, and every number below is either one you typed in or a published figure with its source named next to it.
The figures this page supplies
- What one advertised TB shows as: 931.3226 GiB. Drive makers count a terabyte as 1,000,000,000,000 bytes, the SI decimal meaning. Windows counts in powers of two and calls 2^30 bytes a 'GB' when IEC 80000-13 calls it a GiB. 1,000,000,000,000 / 2^30 is 931.32, which is the whole of the missing space.
- Windows 11's published minimum: 64 GiB. Microsoft's own system requirement for Windows 11 is a 64 GB or larger storage device. It is a minimum for the installer, not a measurement of a working install, and an install that has taken a few feature updates will be larger.
- Megabytes in a gigabyte: 1,024. The binary convention Windows displays in: 2^10 MiB to the GiB.
Why each step is the step it is
- What the drive will actually show:
advertised_tb * gib_per_tb. The advertised decimal capacity converted to the binary units Windows displays. Nothing is lost or hidden; the two are counting in different bases. - Games:
games * game_gb. Your own count multiplied by your own average. - Photos:
photos * photo_mb / mb_per_gb. Your own count and size, converted from MB. - Video:
video_hr * video_gb_hr. Your own hours multiplied by your own size per hour. - Your files add up to:
games_gib + photos_gib + video_gib + other_gb. The four lines above, added. Every one of them is a number you supplied. - With Windows on it:
content_gib + windows * win_gb. Microsoft's published minimum added when this is the boot drive. - Plus the margin you asked for:
need_gib * (1 + free_pct / 100). The margin is yours to set and is applied to the total. It is room to work in, not a manufacturer requirement. - Space left on the drive you picked:
usable_gib - total_gib. What the drive shows, less what you need. A negative number means the drive is too small. - Buy at least:
atLeast(total_gib / gib_per_tb, [0.25, 0.5, 1, 2, 4, 8, 16]). Your requirement converted back into the advertised sizes drives are sold in, rounded up to the next one on sale.
What this cannot tell you
Every calculator has an edge, and a page that hides its edge is worse than no page at all. Here is this one's.
- Most of this is your own numbers multiplied together. The only figures supplied here are the decimal-to-binary conversion and Microsoft's published Windows minimum; nothing is measured and no drive has been tested.
- Game sizes move. A title can double after a year of updates, and a store page is the only current figure.
- A fresh format costs a little more again for the file system's own records, and a drive sold as 1 TB may hold slightly more or less than exactly 10^12 bytes.
- It says nothing about speed, endurance or how long a drive lasts. Two drives of the same size are not the same drive.
Drives on this site in that range
These follow from the answer above and nothing else. Each heading below covers one span of results, so read the one the calculator pointed you at and ignore the rest.
1 TB is enough
A boot drive with an operating system, applications and a handful of games. This is the size most builds start at, and the size most builders outgrow in about two years.
Best NVMe SSD 2026: 5 Gen4 and Gen5 Picks Ranked covers this range in full.
2 TB
A games library that stays installed, or photos and video alongside the system. The usual answer for a machine that is used for more than one thing.
Best NVMe SSD 2026: 5 Gen4 and Gen5 Picks Ranked covers this range in full.
Samsung 990 Pro vs WD Black SN850X: Best Value in 2026 covers this range in full.
More than 2 TB
Video, raw photos or a library that never gets deleted. Past this point a fast drive for the system and a larger slow one for the bulk is usually cheaper than one large fast drive.
SSD vs HDD in 2026: Which Drive Is Worth Your Money covers this range in full.
If the number came out just above a size you can buy, look at the margin you asked it to keep free before you spend the difference: it is a setting on this page, not a rule. A storage calculator that hides that setting is deciding for you.
Questions about the storage calculator
Why does a 1 TB drive show as 931 GB?
Because the box and Windows count in different bases. Drive makers use the decimal terabyte, 1,000,000,000,000 bytes, while Windows counts in powers of two and still writes GB, so one advertised terabyte shows as 931.32 GB. A 2 TB drive shows 1,863 GB and a 4 TB drive 3,725 GB. Nothing is missing: 1,000,000,000,000 bytes divided by 2 to the power 30 is 931.32, which accounts for the gap, and formatting the drive costs a little more for the file system's own records.
How much storage does Windows 11 use?
Microsoft's published minimum is a 64 GB storage device, and that is the figure this calculator adds when the drive is the one Windows boots from. It is the installer's minimum rather than a measurement of a working install: one that has taken a few feature updates will be larger, and Windows also keeps some space in reserve for updates. On the example this page opens with, the files come to 719 GB, Windows takes that to 783 GB, and the 15% margin to 901 GB.
How much storage does a gaming PC need?
Add up the games you keep installed, then everything else, then a margin. The example on this page keeps 8 games at 70 GB each, 560 GB before a single photo; with 5000 photos, 10 hours of video, 50 GB of everything else, Windows and a 15% margin it comes to 901 GB, which a 1 TB drive holds with 31 GB to spare. Keep 20 games of that size and the total is 1,867 GB, just past the 1,863 GB a 2 TB drive shows, so the answer jumps to 4 TB. Game sizes grow with updates, so go by the store pages of the ones you keep.
Should the drive be an SSD or a hard drive?
The storage calculator cannot settle that: it works out size, and speed, endurance and how long a drive lasts are not in its arithmetic. Two drives of the same size are not the same drive. The space you need is the same whichever kind you buy, so settle the number here first, then choose the kind of drive to hold it.
How much free space should I leave on a drive?
This calculator keeps 15% free by default, and that is a margin, not a rule: it is a box on the page, so change it. Windows also sets space aside for updates on its own, which is part of why a drive never has its whole capacity free. On the example, the 15% margin turns 783 GB of files and Windows into 901 GB; at 5% the same files need 822 GB, and at 30% they need 1,018 GB.
Sources
- IEC 80000-13, prefixes for binary multiples: Defines the gibibyte as 2^30 bytes, which is what Windows shows when it writes GB.
- Microsoft: Windows 11 system requirements: States the 64 GB minimum storage requirement used here.
- Microsoft: how reserved storage works in Windows: Explains the space Windows sets aside for updates, which is part of why a drive never has its whole capacity free.
SSD vs HDD in 2026: Which Drive Is Worth Your Money
All of my current picks in one place: the Everyday Tech Reviews Amazon storefront.
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