to a hair

A reference to the very small diameter of a strand of hair: compare hair's breadth.

• (Received Pronunciation) IPA^(key): /tuː‿ə ˈhɛə/
• (General American) IPA^(key): /tu‿ə ˈhɛ(ə)ɹ/
• Rhymes: -ɛə(ɹ)

to a hair

1. (idiomatic) To a high degree of precision; with the utmost exactness; to a nicety.

   Synonym: to the turn of a hair

   • c. 1602 (date written), William Shakespeare, The Famous Historie of Troylus and Cresseid. […] (First Quarto), London: […] G[eorge] Eld for R[ichard] Bonian and H[enry] Walley, […], published 1609, →OCLC, [Act III, scene i]:

     Pand[arus]. Youle remember your brothers excuſe? / Par[is]. To a hayre.

   • 1912, Arthur Conan Doyle, “‘A Procession! A Procession!’”, in The Lost World […], London, New York, N.Y.: Hodder and Stoughton, →OCLC, page 298:

     As to our own representative, the well-known athlete and international Rugby football player, E. D. Malone, he looks trained to a hair, and as he surveyed the crowd a smile of good-humored contentment pervaded his honest but homely face.

• hair's breadth

• “to a hair, phrase” under “hair, n.”, in OED Online ⁠, Oxford, Oxfordshire: Oxford University Press, March 2021.
• “to a hair”, in Collins English Dictionary.