Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2021

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):

	Year Ended December 31,
		2021			2020
Numerator:
Net loss	$	(89,756	)	$	(78,280	)
Deemed Dividend		—			(549	)
Net loss attributable to common stockholders	$	(89,756	)	$	(78,829	)
Denominator:
Weighted average common shares outstanding,    basic and diluted		30,895,756			22,386,122
Net loss per share, basic and diluted	$	(2.91	)	$	(3.52	)

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:

	Year Ended December 31,
		2021	2020
Options to purchase common stock		4,890,716	3,682,233
Unvested restricted stock units		513,690	30,561
Series A convertible preferred stock (as converted to common shares)		—	—
Series B convertible preferred stock (as converted to common shares)		938,000	1,000,000
Series C convertible preferred stock (as converted to common shares)		2,214,000	2,287,000
Series D convertible preferred stock (as converted to common shares)		3,215,000	3,215,000
Total		11,771,406	10,214,794