don’t count your chickens before they are hatched

An instruction not to make, or act upon, an assumption (usually favourable) which might turn out to be wrong. The metaphorical phrase to count one’s chickens is also used.

c 1570 T. HOWELL New Sonnets C2 Counte not thy Chickens that vnhatched be.

1579 S. GOSSON Ephemerides of Phialo 19 I woulde not haue him to counte his Chickens so soone before they be hatcht, nor tryumphe so long before the victorie.

1664 S. BUTLER Hudibras II. iii. To swallow Gudgeons ere th’are catch’d, And count their Chickens ere th’are hatch’d.

1829 SCOTT Journal 20 May (1946) 69 I see a fund..capable of extinguishing the debt..in ten years or earlier... But we must not reckon our chickens before they are hatchd.

1906 in Lady D. Nevill Reminiscences xxii. A victory may be snatched, But never count your little chicks, Before they’re safely hatched.

1935 ‘R. HULL’ Keep it Quiet xxiv. To Cardonnel, he wrote his congratulations, adding that as to convicting the thief, he feared that they were far from proof, and ought not to count their chickens.

1964 RIDOUT & WITTING English Proverbs Explained 42 Mr. Smith hoped to be made manager before the end of the year... ‘Don’t count your chickens before they are hatched,’ warned his wife.

2002 New Scientist 5 Jan. 17 Ultimately it’s a question of how much oil is down there, and how quickly it can be extracted. No one really knows, but the geological evidence suggests that the US might be counting its chickens before they’re hatched.