Net Loss per Share (Tables)

6 Months Ended

Jun. 30, 2019

Earnings Per Share [Abstract]

Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders

Basic and diluted net loss per share attributable to common stockholders of Spero Therapeutics, Inc. was calculated as follows (in thousands, except share and per share amounts):

	Three Months Ended June 30,						Six Months Ended June 30,
	2019			2018			2019			2018
Numerator:
Net loss	$	(13,150	)	$	(9,956	)	$	(18,222	)	$	(20,600	)
Denominator:
Weighted average common shares outstanding,    basic and diluted		17,667,620			14,376,529			17,445,600			14,372,876
Net loss per share, basic and diluted	$	(0.74	)	$	(0.69	)	$	(1.04	)	$	(1.43	)

Summary of Shares Excluded From the Computation of Diluted Net Loss Per Share Attributable to Common Stockholders

The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Options to purchase common stock		3,098,481		2,236,949		3,098,481		2,236,949
Unvested restricted stock units		50,000		—		50,000		—
Series A convertible preferred stock (as converted to common shares)		1,720,000		—		1,720,000		—
Series B convertible preferred stock (as converted to common shares)		1,000,000		—		1,000,000		—
		5,868,481		2,236,949		5,868,481		2,236,949