Earnings per Share

3 Months Ended

Mar. 31, 2021

Earnings Per Share [Abstract]

Earnings per Share

Basic and diluted earnings per share are calculated as follows:

	For the Three Months Ended March 31,
(In millions)	2021		2020
Numerator:
Net income attributable to Biogen Inc.	$	410.2	$	1,399.1
Denominator:
Weighted average number of common shares outstanding	151.9		172.8
Effect of dilutive securities:
Time-vested restricted stock units	0.2		0.1
Market stock units	0.1		0.1
Performance stock units settled in stock	0.1		0.1
Dilutive potential common shares	0.4		0.3
Shares used in calculating diluted earnings per share	152.3		173.1

Amounts excluded from the calculation of net income per diluted share because their effects were anti-dilutive were insignificant.