Exhibit 99.1

UNAUDITED PRO FORMA COMBINED CONSOLIDATED FINANCIAL INFORMATION

Introduction

On August 31, 2016, Westlake Chemical Corporation (the “Company”), completed the acquisition of, and acquired all of the remaining equity interest in, Axiall Corporation, a Delaware corporation (“Axiall”). Prior to the acquisition, the Company held 3.1 million shares in Axiall. Pursuant to the terms of the Agreement and Plan of Merger (the “Merger Agreement”), dated as of June 10, 2016, by and among the Company, Axiall and Lagoon Merger Sub, Inc., a Delaware corporation that is a wholly-owned subsidiary of the Company (“Merger Sub”), Merger Sub merged with and into Axiall (the “Merger”), with Axiall surviving the Merger as a wholly-owned subsidiary of the Company.

Set forth below is the unaudited pro forma combined consolidated statement of operations for the year ended December 31, 2016. The pro forma financial information has been derived from the historical consolidated financial statements of the Company and Axiall and gives pro forma effect to the Merger and related financing transactions entered concurrently with the Merger.

The unaudited pro forma combined consolidated statement of operations, including the notes thereto, should be read in conjunction with the Company’s audited consolidated financial statements for the fiscal year ended December 31, 2016 included in the Company’s Annual Report on Form 10-K.

The unaudited pro forma combined consolidated statement of operations gives effect to the Merger in accordance with the acquisition method of accounting for business combinations. The pro forma adjustments included herein are directly attributable to the Merger, factually supportable, and expected to have a continuing impact on the Company’s consolidated results. The unaudited pro forma combined consolidated statement of operations for the year ended December 31, 2016 is presented as if the Merger and related financing transactions occurred on January 1, 2016.

Management believes that all significant adjustments necessary to reflect the effects of the Merger and related financing transactions are included in the accompanying unaudited pro forma combined consolidated statement of operations and are deemed to be reasonable. The unaudited pro forma financial statement do not reflect the impact of any potential cost savings or efficiencies the Company may have already achieved or may achieve from the combination of the two entities.

Below is the unaudited pro forma combined consolidated statement of operations and notes thereto which give effect to the Merger and related financing transactions.

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WESTLAKE CHEMICAL CORPORATION

UNAUDITED PRO FORMA COMBINED CONSOLIDATED STATEMENT OF OPERATIONS

YEAR ENDED DECEMBER 31, 2016

(in thousands of dollars, except share amounts and per share data)

See notes to the unaudited pro forma combined consolidated financial statement.

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WESTLAKE CHEMICAL CORPORATION

NOTES TO THE UNAUDITED PRO FORMA COMBINED CONSOLIDATED FINANCIAL STATEMENTS

(in thousands of dollars, except share amounts and per share data)

The accompanying unaudited pro forma combined consolidated statement of operations was prepared using the acquisition method of accounting. The unaudited pro forma combined consolidated statement of operations is based on the historical financial statements of the Company and Axiall after giving effect to the Merger and related financing transactions as if the Merger had occurred on January 1, 2016.

The Merger has been accounted for using the acquisition method of accounting in accordance with the Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) 805, Business Combinations (“ASC 805”). ASC 805 requires, among other things, that assets acquired and liabilities assumed be recognized at their fair values, as determined in accordance with ASC 820, Fair Value Measurements, as of the Merger date. Under the acquisition method of accounting, the assets acquired and liabilities assumed are recorded at the effective time of the Merger at their respective fair values and added to those of the Company.

Total consideration transferred for the Merger was $2,539,360. The Merger was accounted for under the acquisition method of accounting. The following table summarizes the consideration transferred at the date of acquisition. The allocation of the consideration transferred is based on management’s estimates, judgments and assumptions. When determining the fair values of assets acquired, liabilities assumed and noncontrolling interests of the acquiree, management made significant estimates, judgments and assumptions. Management estimated that consideration paid exceeded the fair value of the net assets acquired. Therefore, goodwill of $942,096 was recorded. The goodwill recognized is primarily attributable to synergies related to the Company’s vinyls integration strategy that are expected to arise from the Merger.

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Adjustments included in the columns labeled “Pro Forma Adjustments” in the unaudited combined consolidated statement of operations are as follows:

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Exchange Offers

In September 2016, the Company issued $624,793 aggregate principal amount of 4.625% senior notes due 2021 (the “4.625% Westlake 2021 Senior Notes”) and $433,793 aggregate principal amount of 4.875% senior notes due 2023 (the “4.875% Westlake 2023 Senior Notes” and, together with the 4.625% Westlake 2021 Senior Notes, the “Westlake Exchange Notes”) upon the closing of the Company’s offers to exchange any and all of the $688,000 aggregate principal amount of the outstanding 4.625% senior notes due 2021 issued by Eagle Spinco Inc., a wholly-owned subsidiary of Axiall, and the $450,000 aggregate principal amount of the outstanding 4.875% senior notes due 2023 issued by Axiall (together, the “Subsidiary Notes”). In the exchange offers, $624,793 aggregate principal amount of 4.625% Westlake 2021 Senior Notes and $433,793 aggregate principal amount of 4.875% Westlake 2023 Senior Notes were issued by the Company, leaving outstanding $63,207 aggregate principal amount of the Eagle Spinco, Inc. 4.625% 2021 senior notes and $16,207 aggregate principal amount of the Axiall 4.875% 2023 senior notes.

3.60% Senior Notes due 2026 and 5.0% Senior Notes due 2046

On August 10, 2016, the Company completed its private offering of $750,000 aggregate principal amount of 3.60% senior notes due 2026 (the “3.60% 2026 Senior Notes”) and $700,000 aggregate principal amount of 5.0% senior notes due 2046 (the “5.0% 2046 Senior Notes”).

Credit Agreement

On August 23, 2016, the Company and certain of its subsidiaries entered into an unsecured five-year revolving credit facility (the “Credit Agreement”) in an aggregate principal amount of up to $1,000,000. The Credit Agreement replaced the Company’s existing $400,000 senior secured third amended and restated credit facility, dated as of July 17, 2014. At December 31, 2016, the Company had $325,000 of borrowings outstanding under the Credit Agreement. Borrowings under the Credit Agreement will bear interest, at the Company’s option, at either (a) LIBOR plus a spread ranging from 1.00% to 1.75% that will vary depending on the credit rating of the Company or (b) Alternate Base Rate plus a spread ranging from 0.00% to 0.75% that will vary depending on the credit rating of the Company. The Credit Agreement also requires an undrawn commitment fee ranging from 0.10% to 0.25% that will vary depending on the credit rating of the Company. The interest rate on the outstanding revolving credit facility was 2.19% at December 31, 2016.

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