Be able to calculate the number of bits required to store a piece of…

The idea

ASCII stores each character as a fixed-length code, so the storage requirement is simply the number of characters multiplied by the bits per character. In the standard ASCII set each character uses 7 bits, giving 128 possible codes; many systems store each character in one 8-bit byte, so a question must state which convention it uses. Method: count every character, including spaces, punctuation and line breaks if they are part of the data, then multiply by the stated bits per character. For example, the text "Hello" has 5 characters, so at 7 bits per character it needs 5 × 7 = 35 bits, and at 8 bits per character it needs 5 × 8 = 40 bits. Compare this with the compressed size to judge whether compression saves space.

What earns the marks

• Identify that ASCII uses a fixed number of bits per character, commonly 7 bits (128 codes) or 8 bits when stored in a byte.
• Count all characters in the data, including spaces and punctuation, before multiplying.
• Multiply the character count by the stated bits per character to obtain the total number of bits.
• Recognise that the answer depends on the bits-per-character convention given, so state the assumption used.
• Use the calculated uncompressed size as a baseline when comparing with a compressed size.

Common mistakes and how to put them right

• Counting only letters and ignoring spaces or punctuation.

  Put it right: Count every character that must be stored.

• Multiplying by 8 when the question specifies 7-bit ASCII.

  Put it right: Use the bits-per-character value stated in the question.

• Reporting the answer in bytes when bits are requested.

  Put it right: Keep the total in bits, or convert only if asked.

Exam tips

• Underline the bits-per-character value in the question before calculating.
• Write the character count and the multiplication explicitly so method marks are visible.
• Check whether the question asks for bits or bytes and give the unit in your answer.