Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2017

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,
	2017			2016			2015
Numerator:
Net loss	$	(39,886	)	$	(32,641	)	$	(13,153	)
Less: Net loss attributable to non-controlling interests		(1,143	)		(7,150	)		(2,999	)
Plus: Cumulative dividends on redeemable convertible preferred shares		(6,146	)		(3,441	)		(932	)
Plus: Accretion of bridge units and redeemable convertible preferred shares to redemption value		(1,208	)		(996	)		(2,341	)
Net loss attributable to common stockholders of Spero Therapeutics, Inc.	$	(46,097	)	$	(29,928	)	$	(13,427	)

Denominator:
Weighted average common shares outstanding, basic and diluted		2,586,865			312,169			252,807

Net loss per share attributable to common stockholders of Spero Therapeutics, Inc., basic and diluted	$	(17.82	)	$	(95.87	)	$	(53.11	)

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:

	December 31,
	2017		2016		2015
Options to purchase common stock		2,011,296		—		—
Redeemable convertible preferred shares (as converted to common shares)		—		2,229,518		1,257,213
Incentive units		—		413,266		171,758
		2,011,296		2,642,784		1,428,971