Net Loss per Share (Tables)

9 Months Ended

Sep. 30, 2022

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 September 30,						Nine Months Ended September 30,
	2022			2021			2022			2021
Numerator:
Net loss	$	(11,675	)	$	(22,521	)	$	(73,186	)	$	(60,516	)
Net loss attributable to common stockholders	$	(11,675	)	$	(22,521	)	$	(73,186	)	$	(60,516	)
Denominator:
Weighted average common shares outstanding, basic and diluted		35,882,076			32,132,500			33,834,198			30,417,305
Net loss per share, basic and diluted	$	(0.33	)	$	(0.70	)	$	(2.16	)	$	(1.99	)

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 September 30,				Nine Months Ended September 30,
	2022		2021		2022		2021
Options to purchase common stock		4,015,661		4,747,203		4,015,661		4,747,203
Unvested restricted stock units		1,145,760		445,040		1,145,760		445,040
Series B convertible preferred stock (as converted to common shares)		—		938,000		—		938,000
Series C convertible preferred stock (as converted to common shares)		—		2,214,000		—		2,214,000
Series D convertible preferred stock (as converted to common shares)		—		3,215,000		—		3,215,000
		5,161,421		11,559,243		5,161,421		11,559,243