offhand

offhand — сразу, без раздумий, без подготовки

Example 1: “Do you know how often the buses run?” “Sorry, not offhand.” — ...С ходу / так сразу и не могу сказать.

Example 2: “Do you know Ann’s date of birth?” “Not off the top of my head.” — Сразу и не вспомнить.

See also
[off the top of one’s head]
[offhand]
[wing it]
[play it by ear]
[экспромтом]
[at the drop of a hat]
навскидку
с ходу/сразу (ответить)