do one's sums

do one's sums (third-person singular simple present does one's sums, present participle doing one's sums, simple past did one's sums, past participle done one's sums)

1. (UK, informal) To figure out whether one can afford to pay for something.

• “do your sums” in Longman Dictionary of Contemporary English, Longman.
• “do your sums” (UK) in Macmillan English Dictionary.