Net Loss per Share

6 Months Ended

Jun. 30, 2019

Earnings Per Share [Abstract]

Net Loss per Share

13. Net Loss per Share

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	)

The Company excluded potentially dilutive securities from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders of Spero Therapeutics, Inc. is the same. 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