でいうと is an expression in Japanese that is commonly used to ask questions, or to give information about how one thing corresponds with another. It is a combination of the 格助詞（かくじょし） (case marking particle) で meaning ‘with’, and 言（い）うと meaning ‘if said’, or ‘if we say’.
This structure will be seen after nouns, and can be translated as ‘if said with (A), (B)’.

• このバイクは、車（くるま）で言（い）うとランボルギーニみたいなものだ。
  If we say it’s a car, this bike is something like a Lamborghini.
• 日本（にほん）で言（い）うとパンみたいなもの？
  If we say Japan, is it something like bread? (Would this be considered bread in Japan)
• 一言（ひとこと）で言（い）うと彼（かれ）は本当（ほんとう）に凄（すご）い人（ひと）だ。
  If said in a word, he is an amazing person.

でいうと will often be seen in sentences with に当（あ）たる ‘to be equivalent to’, another common grammar structure used for drawing connections between things.

• 「What's up」は日本語（にほんご）でいうと、「元気（げんき）」に当（あ）たる。
  If said with Japanese, 'what's up' is equivalent to 'Genki'.
• コンピューターでいうとRAMに当（あ）たる。
  If we say it’s a computer, it is equivalent to Ram.