16. EARNINGS PER SHARE

We calculate earnings per share using the two-class method. The two-class method requires that share-based awards with non-forfeitable dividends be classified as participating securities. In calculating basic earnings per share, this method requires net income to be reduced by the amount of dividends declared in the period for each participating security and by the contractual amount of dividends or other participation payments that are paid or accumulated for the period. Undistributed earnings for the period are allocated to participating securities based on the contractual participation rights of the security to share in those current earnings assuming all earnings for the period are distributed. Recipients of certain of our restricted stock unit awards have contractual participation rights that are equivalent to those of common stockholders. Therefore, we allocate undistributed earnings to these restricted stock unit participating securities and common stock based on their respective participation percentage.

The two-class method also requires the denominator to include the weighted average number of shares of restricted stock unit participating securities when calculating basic earnings per share. Basic and diluted earnings per share for the three and nine months ended September 30, 2013, include the weighted average share impact of the 35.2 million shares issued in connection with the Merger from January 28, 2013, the date of issuance. For both the three and nine months ended September 30, 2013 there were 0.1 million weighted average restricted stock unit participating securities that were included in the denominator and for the three and nine months ended September 30, 2012 there were 0.2 million and 0.4 million weighted average restricted stock unit participating securities. Diluted earnings per share also include the additional share equivalents from the assumed conversion of stock based awards including options and certain restricted stock units. Conversion of stock options and certain restricted stock units are calculated using the treasury stock method, subject to anti-dilution provisions.

In computing diluted earnings per share for the three and nine months ended September 30, 2013, common stock equivalents of 0.5 million shares and 0.3 million shares, respectively, were not included due to their anti-dilutive effect. For both the three and nine months ended September 30, 2012, common stock equivalents of 0.2 million shares were not included due to their anti-dilutive effect. Computations of basic and diluted earnings per share for the three and nine month periods ended September 30, 2013 and 2012 are presented in the following table:

	Three Months Ended September 30,				Nine Months Ended September 30,

*(In millions, except per share data)*	2013		2012		2013		2012
Numerator
Net income attributable to Axiall	$	39.0	$	39.3	$	108.3	$	88.2
Less: Net income attributable to participating securities		-		0.2		0.1		0.8

Net income attributable to common stockholders	$	39.0	$	39.1	$	108.2	$	87.4


Denominator
Weighted average common shares outstanding—Basic		69.9		34.5		66.4		34.4
Plus: Dilutive effect of stock options and awards		0.5		0.4		0.4		0.2

Weighted average common shares outstanding—Diluted		70.4		34.9		66.8		34.6


Earnings per common share from net income attributable to Axiall's common stockholders:
Basic	$	0.56	$	1.13	$	1.63	$	2.54
Diluted	$	0.55	$	1.12	$	1.62	$	2.53