Net Loss per Share (Tables)

3 Months Ended

Mar. 31, 2018

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,
	2018			2017
Numerator:
Net loss	$	(10,644	)	$	(6,411	)
Less: Net loss attributable to non-controlling    interests		—			(535	)
Plus: Cumulative dividends on redeemable    convertible preferred shares		—			(1,236	)
Plus: Accretion of bridge units and redeemable    convertible preferred shares to redemption value		—			(18	)
Net loss attributable to common stockholders    of Spero Therapeutics, Inc.	$	(10,644	)	$	(7,130	)
Denominator:
Weighted average common shares outstanding,    basic and diluted		14,369,182			330,075
Net loss per share attributable to common stockholders    of Spero Therapeutics, Inc., basic and diluted	$	(0.74	)	$	(21.60	)

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,
	2018		2017
Options to purchase common stock		2,129,082		—
Incentive units		—		422,399
		2,129,082		422,399