UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15 (d) of

the Securities Exchange Act of 1934

Date of Report (date of earliest event reported): August 6, 2015

AIRGAS, INC.

(Exact name of registrant as specified in its charter)

259 North Radnor-Chester Road, Suite 100

Radnor, PA 19087-5283

(Address of principal executive offices)

Registrant’s telephone number, including area code: (610) 687-5253

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

On August 11, 2015, Airgas, Inc. (the “Company”) issued $400 million aggregate principal amount of 3.050% Notes due August 1, 2020 (the “Notes”) pursuant to a shelf registration statement on Form S-3 (File No. 333-188772) (the “Registration Statement”) filed with the U.S. Securities and Exchange Commission, which became effective on May 22, 2013, and a related prospectus supplement dated as of August 6, 2015.

The Notes were issued under an Indenture dated May 27, 2010 (the “Base Indenture”), between the Company and U.S. Bank National Association, as Trustee, as supplemented by the Sixth Supplemental Indenture, dated August 11, 2015 (the “Supplemental Indenture” and together with the Base Indenture, the “Indenture”). The Notes were sold pursuant to an Underwriting Agreement dated August 6, 2015 (the “Underwriting Agreement”), among the Company and Goldman, Sachs & Co., Merrill Lynch, Pierce, Fenner & Smith Incorporated, U.S. Bancorp Investments, Inc. and Wells Fargo Securities, LLC, as representatives of the several underwriters named therein.

The Notes bear interest at a fixed annual rate of 3.050%, payable semi-annually on February 1 and August 1 of each year, commencing February 1, 2016. The Indenture contains covenants which, subject to certain exceptions, limit the ability of the Company to, among other things, incur liens or engage in sale/leaseback transactions. Upon a change of control triggering event (as defined in the Indenture), the Indenture requires the Company to make an offer to repurchase the Notes at 101% of their principal amount, plus accrued and unpaid interest. The Company has the option to redeem the Notes up to the date that is one month prior to the maturity date of the Notes, in whole or in part, at 100% of the principal amount plus a make-whole premium, plus accrued and unpaid interest, and on or after the date that is one month prior to the maturity date of the Notes, in whole or in part, at 100% of the principal amount, plus accrued and unpaid interest.

The Underwriting Agreement and Supplemental Indenture are filed as exhibits to this Current Report on Form 8-K and shall be incorporated by reference into the Registration Statement and any amendments thereto.

The information set forth in Item 1.01 is incorporated herein by reference.

On August 11, 2015, Cahill Gordon & Reindel LLP, counsel to the Company, issued an opinion and consent (attached hereto as Exhibit 5.1 and 23.1, respectively, and incorporated herein by reference). The opinion and consent are filed as exhibits to this Current Report on Form 8-K and shall be incorporated by reference into the Registration Statement and any amendments thereto.

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Dated: August 11, 2015

Exhibit Index