Earnings per Share (Tables)

3 Months Ended

Mar. 31, 2026

Earnings Per Share [Abstract]

Basic and diluted earnings per share

Basic and diluted shares outstanding used in our earnings per share calculation are calculated as follows:

	For the Three Months Ended March 31,
(In millions)	2026		2025
Numerator:
Net income attributable to Biogen Inc.	$	319.5	$	240.5
Denominator:
Weighted average number of common shares outstanding	147.2		146.1
Effect of dilutive securities:
Time-vested restricted stock units	1.0		0.5
Performance stock units settled in stock	0.2		—
Dilutive potential common shares	1.2		0.5
Shares used in calculating diluted earnings per share	148.4		146.6