I need to replace an old 15.6in Acer Aspire laptop with a Core i3 processor. I want to go down the SSD route, but do I need 128GB or 256GB? I use a laptop for documents, photos, emails, Facebook etc, and some Excel stuff, hence the need for Microsoft Office. No gaming.

My local computer shop has an HP 250 G6 laptop with a Core i5-7200U, 4GB of RAM and a 128GB SSD, for £425, which they said they could upgrade to 256GB in the future if needed. I’ve also looked at a Lenovo IdeaPad 330S with a Core i5-8250U, 8GB of RAM and 256GB SSD for £579, which seems way too expensive for what I need.

I have been advised by a friend in IT that 256GB is needed to run most modern computer programs. Hilary

The laptop industry is slowly changing from PCs with traditional hard disk drives (HDDs) to ones with chip-based solid-state drives (SSDs). The problem, as you have found, is the cost. SSDs are more responsive, but they are also much more expensive per gigabyte of storage space.

The current solution is to compromise by installing smaller – and therefore cheaper – SSDs. Ideally, you’d just replace a 1TB HDD with a 1TB SSD, but even today, a good 1TB SSD can cost around £250. This isn’t going to appear in a £350 laptop. However, 128GB and 256GB SSDs are now affordable. In fact, 128GB SSDs are now cheaper than internal 1TB HDDs (roughly £40 at retail), while some 256GB SSDs are not much more expensive.

The changeover has been helped by two other factors. First, you can now store files in online drives such as Dropbox, Google Drive and Microsoft OneDrive and access them when you need them. Second, SD memory cards have grown in capacity and fallen dramatically in price. If you have gigabytes of photos or music files, you can move them to cheap SD cards – eg 32GB for £7 – and free up valuable SSD space.

SD cards are relatively slow, so it’s not the best idea to use them for large programs. However, they are a great way to store lots of data in a very small space. You could carry 2TB in a purse without even noticing, though the price might put you off.

Check your data

You can find out how much space you need by scanning the hard drive in your Acer laptop. There are dozens of free utilities that will do the job. Popular examples include SpaceSniffer, WinDirStat, TreeSize, SpaceMonger, FolderSizes and SequoiaView. My own favourite is SplashSoft’s RidNacs – the name comes from “scan directory” backwards.

In most cases, the largest folders will be collections of photos, music and videos. It may be worth copying these to SD cards and backing them up to an external hard drive, to avoid transferring them to your new SSD.

Otherwise, the biggest directory will usually be Windows, which may take up 15GB to 30GB. The older the PC, the bigger you can expect it to be, with the most space being consumed by the WinSxS folder.

WinSxS contains some essential Windows components including, for backup purposes, copies of old components and disabled features that are no longer in use. They might be needed if you decide to take your PC back to an earlier condition. Recent versions of Windows include Disme.exe, which calculates the actual size of WinSxS – the instructions are online – and automatically runs a clean-up routine when necessary.

Microsoft Office, in the Program Files (x86) directory, could take up 2GB or 3GB, depending on which programs you have installed.

As a relatively light user, you will probably find that everything you need adds up to 50GB or less, which will fit onto a 128GB SSD. If you need more than 60GB, I’d recommend going for a 256GB SSD, for reasons that will be explained in the next section. In the long run, it will save wasting time on disk management.

Of course, it is better to have 256GB than 128GB, and larger SSDs perform better. But you don’t actually need 256GB to run “most modern computer programs”. You would only need that much space for processing large files, such as re-encoding videos. In most cases, it’s better to have more memory.

Windows 10 will try its best with whatever memory you have – the 32-bit version runs in 1GB, and the 64-bit version in 2GB. While 4GB is adequate for everyday use, 6GB is significantly better, and 8GB gets you into a cruising zone with, usually, memory to spare.

Real drive sizes

Windows and other operating systems work in binary, where a gigabyte 1,073,741,824 bytes. Drive manufacturers work in decimal, where a gigabyte is 1,000,000,000 bytes. This may not sound like a lot of difference – it’s 70MB per GB – but it adds up. Windows will say your 128GB drive is only 119GB, which is why some companies offer 120GB, 250GB and 500GB drives instead of 128GB, 256GB and 512GB.

Microsoft and PC manufacturers also use some of the drive space for other purposes. Windows 10 has a 16MB MSR (Microsoft Reserved Partition) for partition management, and another partition (at least 300MB) to store the Windows Recovery Environment. The manufacturer may have added a small partition to store support tools. When Windows starts, it names the boot drive C: but doesn’t assign letters to the other partitions, so they are invisible.

As a result, your “128GB SSD” will provide less than 119GB of storage for programs and data. Subtract the space used by Windows 10 and you could be left with around 90GB to 100GB. Subtract another 20GB for swap files, caches, Windows updates etc and you’re already down to 70-80GB.

Bear in mind that installing Windows 10’s twice-yearly updates requires about 12GB of free space, preferably more. It’s a lot, but at one point, your PC is holding your current copy of Windows, the new copy it has downloaded, and a backup copy of the current version in case something goes wrong. These are deleted later ...

Possible options

If you like the HP 250 G6 in your local computer shop, you can get the same machine with 8GB of memory and a 256GB SSD with free delivery from eBuyer for £449.47. This looks like your best buy. The Intel Core i5-7200U is last year’s processor, but it has a PassMark score of 4,621. While not up to this year’s i5-8250U (PassMark 7,660), it’s much faster than your antique Core i3-370M (PassMark 2,017), and should be more than enough for your purposes.

There are some decent 15.6in laptops at the psychologically important £399.99 mark, such as the Acer Aspire 3 A315-51 and the Lenovo IdeaPad 330S. However, both of those have 4GB of RAM and 128GB SSDs. It’s really not worth halving both memory and storage to save £50.

Have you got a question? Email it to Ask.Jack@theguardian.com