How much SSD capacity you actually need

Size the drive for what you will hold in two years, not for what you hold today. Start from what you use now, add the growth you can already see coming, and leave the drive room to work. The rest is how to run that method.

How much space you are using now

Your current usage is the one number in this decision you can measure. Start there, not with a size that sounds about right.

On Windows, open Settings, then System, then Storage. It gives used and free space per drive and breaks the used part down by kind: apps and games, documents, temporary files, system files. On macOS, open the Apple menu, then System Settings, then General, then Storage, for the same picture. The breakdown is the part that matters: a single used-space figure tells you much less than knowing how much of it is games.

Read that breakdown twice. Once for what moves with you — the operating system, the applications you install on any machine, the files you would carry to a new one. Once for what does not: caches, temporary files, and downloaded installers. Those come back, but they are not a standing claim on the drive, and counting them inflates your figure.

Then add what your current drive has already pushed out: the game you uninstalled to make room, the footage parked on an external disk, the photo library you stopped importing to. That is demand the used-space figure cannot show, because the current drive already failed to meet it.

One subtraction before you shop. A drive sold as 1 TB counts a terabyte as a trillion bytes. Windows counts in units of 1024 and labels the result GB anyway, so a drive reads smaller there than the box says; macOS counts the way the manufacturer does, so it does not. Formatting and a recovery partition take a little more on either. The number on the box is not the number you get to fill.

Why a drive needs room to work

Flash cannot overwrite in place. It is written in small pages and erased in much larger blocks, and a page cannot take new data until the whole block around it has been erased. So when you change a file, the drive writes the new version to a clean page elsewhere and marks the old one dead. Dead pages pile up, and reclaiming them means copying the still-live pages out of a block and erasing that block whole.

That housekeeping is garbage collection, and free space is the room it runs in. With clean blocks in hand, the drive does it in the background between your requests. With almost none, it has to do it in front of your write, and each block it reclaims means writing the drive does on its own behalf before your data can land.

The same squeeze narrows the fast write path. Drives run a slice of their flash at one bit per cell, where writing is much quicker, and let incoming writes land there before folding them into their denser final form. How large that slice can be usually depends on how much of the drive is free, so it shrinks as the drive fills, and a long copy drops to the slower native rate sooner. TLC, QLC, and DRAM-less SSDs works through the arithmetic behind that, and how much room each kind of flash gives up to run at one bit.

Neither effect arrives at a threshold you can look up. Both grow as free space shrinks, gently across most of the drive and steeply near the end of it. For sizing, that means the last stretch of a drive's capacity is not storage. It is working room. Leave it out of the figure you shop against instead of planning to fill it.

Is one larger drive better than two smaller ones

Usually, and for reasons that have nothing to do with the flash.

Slots are finite, and often there is only one. Many laptops have one M.2 slot, holding the drive the machine boots from. Desktops have more, but on some boards the second M.2 slot borrows its lanes from the SATA ports, so filling it switches ports off. NVMe or SATA covers how to find out what your machine has before you plan around a slot that may not exist.

A second drive also does not relieve the first. The operating system and installed applications sit on the boot drive, and moving them is work. What moves easily is games, media, and archives. A boot drive bought too small stays too small, with an emptier drive beside it.

Headroom does not pool either. Each drive needs its own working room, so two drives ask for it twice, and the small boot drive kept near full is exactly the one that pays for being full.

Two drives earn their place when the second holds bulk that is written in long runs and read occasionally, on a machine with a bay or port going unused. That is a separate purchase from this one.

When a bigger drive stops being worth the money

The largest capacity in a range is usually the one that costs more per terabyte than the size below it, and the reason is that it is the hardest one to build. A drive's board is a fixed size with room for a fixed number of flash packages, so the top capacity has to use the densest packages on sale, which are the newest and the least plentiful, and it sells in the smallest numbers.

That premium is not attached to a particular capacity forever. It sits on whatever the top of the range happens to be, and moves up as larger drives arrive and yesterday's ceiling becomes an ordinary size.

So the check is arithmetic between the two sizes you are choosing between: divide each price by its capacity and compare. If the larger drive comes out lower per terabyte, the extra room is cheap and worth taking. If it comes out higher, you are looking at the top-of-range premium, and the money is buying space you would be buying only because it exists. How to choose an SSD sets out what to measure that figure against before you decide the price itself is fair, and when an SSD deal is actually a deal goes further into the comparison itself: what a per-terabyte figure can and cannot tell you, and why a strong price on one size is not a strong price on the next.

How do we show a drive's different sizes

A model sold at several capacities is one product with several sizes here, not several unrelated products. Each size is tracked on its own, with its own current price, its own history, and its own all-time low. Prices for the sizes move independently, so folding them together would misprice every one of them.

On the browse page a model appears once, at the cheapest size we currently hold a price for, and the capacity shown beside that price belongs to that size, not to the model in general. Read it as a starting price. A row showing a small capacity is not telling you the model stops there.

The drive's own page is where the sizes separate. Where we track more than one, there is a control for each size, and choosing one switches the price history and the list of offers to that size. Pick the size you mean to buy before you read the price.

Every SSD we track is on one page.