State the units used to measure data (bit, byte, KB, MB, GB, TB), convert between them, and calculate and compare file and storage sizes.

What this dot point is asking

WJEC asks you to know the standard units that digital data is measured in, from the smallest (the bit) up to large capacities (the terabyte), to convert confidently between them, and to use those conversions to calculate file sizes and how much will fit on a device. These conversions underpin every image and sound calculation, and "how many files fit" questions appear regularly.

The units of storage

Storage sizes are quoted in a fixed sequence of units, each a step up from the last.

You should be comfortable placing these in order and stating how they relate, because conversions are built on this ladder.

The 1000 versus 1024 point

There are two conventions, and the exam tells you which to use.

Converting between units

Converting is a matter of multiplying or dividing by 1000 (or 1024) for each step on the ladder.

So 3 GB is $3 \times 1000 = 3000$ MB, and 6000 KB is $6000 \div 1000 = 6$ MB. Doing one step at a time avoids losing track of the powers of ten.

Calculating how many files fit

The classic capacity question combines conversion with a division.

Combining a file size with a conversion

Many exam questions chain a size calculation onto a conversion, so you produce a size in bits, turn it into bytes, then express it in a sensible unit. The order is always the same: work out the raw size, divide bits by 8 to reach bytes, then move up the ladder by dividing by 1000 (or 1024) for each step to reach KB, MB or GB. Quoting an answer of "8,000,000 bytes" is correct but a marker often wants it expressed as 8 MB, so practise finishing the conversion to the most readable unit. Keeping each stage on its own line means that if one division is wrong, the marker can still award method marks for the steps that are right, which is why showing working matters as much here as in the image and sound calculations.

Estimating and sense-checking

Because the units differ by factors of a thousand, a single misplaced conversion changes an answer by a huge amount, so a quick sense-check is valuable. If a phone is advertised as 128 GB and your working says it holds three photos, something has gone wrong by a factor of a thousand. A useful habit is to ask whether the final figure is the right order of magnitude: a typical photo is a few megabytes, a song a few megabytes, an hour of video a few gigabytes. Comparing your answer with these everyday sizes catches the most common error, converting in the wrong direction, before you write it down.

Why this matters

Every file-size question, for images, sound or documents, ends in one of these units, and "will it fit?" decisions are everyday digital reasoning: choosing a memory card, knowing whether a video will upload, or deciding if a backup will fit on a drive. Getting the conversions right is also essential for compression questions, where you compare the size before and after.