Units: Data capacity and calculation of data capacity requirements

The idea

Data capacity is the amount of storage a file, program or medium needs. It is measured in bits and bytes, with 8 bits = 1 byte. OCR J277 uses decimal (base-10) units: 1 kB = 1000 bytes, 1 MB = 1000 kB, 1 GB = 1000 MB, 1 TB = 1000 GB. To calculate capacity, multiply the number of items by the size of each item, then convert to a sensible unit. For example, 500 photos each of 4 MB require 500 × 4 MB = 2000 MB = 2 GB. If a file is 20,000 bits, divide by 8 to get 2500 bytes, then by 1000 to get 2.5 kB. Always state the unit and show your conversion steps clearly. Do not use binary prefixes such as KiB or MiB in J277 answers.

What earns the marks

• State that data capacity is measured in bits and bytes, with 8 bits = 1 byte.
• Use decimal units: 1 kB = 1000 bytes, 1 MB = 1000 kB, 1 GB = 1000 MB, 1 TB = 1000 GB.
• Calculate total capacity by multiplying the number of items by the size of one item.
• Convert between units correctly, for example divide by 1000 to move to a larger decimal unit, and give the final unit.
• Apply capacity calculations to contexts such as estimating storage for images, audio, video or text files.
• Convert between bits and bytes by multiplying or dividing by 8 as appropriate.

Common mistakes and how to put them right

• Using 1024 instead of 1000 for kB, MB, GB or TB.

  Put it right: OCR J277 uses decimal units, so 1 kB = 1000 bytes, 1 MB = 1000 kB, and so on.

• Forgetting to convert the final answer to the requested unit.

  Put it right: After multiplying, divide by the correct factor and label the result, for example 2000 MB ÷ 1000 = 2 GB.

• Mixing bits and bytes.

  Put it right: Check whether file sizes are in bits or bytes; if needed, divide or multiply by 8 before combining with byte-based capacities.

Exam tips

• Write down the multiplication and any conversion as a clear chain, for example 500 × 4 MB = 2000 MB = 2 GB, so the method is visible.
• Check the unit in the question and the unit expected in the answer; convert only at the end to reduce rounding errors.
• For multi-step problems, keep track of the unit at each stage and include it in your answer.