Net Loss per Share (Tables)

3 Months Ended

Mar. 31, 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 March 31,
	2022			2021
Numerator:
Net loss	$	(32,829	)	$	(19,423	)
Net loss attributable to common stockholders	$	(32,829	)	$	(19,423	)
Denominator:
Weighted average common shares outstanding, basic and diluted		32,606,715			29,414,148
Net loss per share, basic and diluted	$	(1.01	)	$	(0.66	)

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 March 31,
	2022		2021
Options to purchase common stock		5,775,068		4,583,140
Unvested restricted stock units		1,163,647		—
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
		13,305,715		10,950,140