¶1*265Solicitor General Hughes, with whom Assistant Attorney General Youngquist, Messrs. Sewall Key and Randolph C. Shaw, Special Assistants to the Attorney General, Clarence M. Charest, General Counsel, Bureau of Internal Revenue, and Allin H. Pierce, Special Attorney, Bureau of Internal Revenue, were on the brief, for petitioner.
¶2Mr. Armwell L. Cooper, with whom Messrs. Ellison A. Neel, Wm. E. Kemp, Wallace Sutherland, and John P. Cooper were on the brief, for respondent.
¶4delivered the opinion of the Court.
¶5The Kansas City Structural Steel Company, .a Missouri concern, appealed to the United States Board of T.ax Appeals from determinations by the Commissioner of Internal Revenue which made an increase of $7,656.74 in *266the company’s 1918 income tax and of $15,953.36 in its 1920 income tax.1 These .additions were due wholly to changes made by the Commissioner in the inventory valuation of material carried in stock. The Company valued at a constant price all the material which did not exceed in quantity what was said to be the normal stock on hand.2 The Commissioner revalued this at current market prices. The changes resulted in increasing the December, 1918, inventory by $165,849.46 and the December 31, 1920, inventory by $117,113.61. The Board of Tax Appeals sustained the Commissioner’s action. 11 B. T. A. 877. Its decision was reversed by the United States Circuit Court of Appeals for the Eighth Circuit. 33 F. (2d) 53. This Court granted writs of certiorari, 280 U. S. 543.
¶6Section 203 of the Revenue Act of 1918, Feb. 24, 1919, c. 18, 40 Stat. 1057, 1060, provides: “That whenever in the opinion of the Commissioner the use of inventories is necessary in order clearly to determine the income of any taxpayer, inventories shall be taken by such taxpayer upon such basis as the Commissioner, with the approval of the Secretary, may prescribe as conforming as nearly as may be to the best accounting practice in the trade or-business and as most clearly reflecting the income.”3 Regulations 45 (1920 edition, as amended by Treasury *267Decision 3296) provides, in Article 1581, that “inventories at the beginning and end of each year are necessary in every case in which the production, purchase, or sale of merchandise is an income-producing factor.” Article 1582 declares that the basis of valuation “ most commonly used by business concerns and which meets the requirements of the revenue act is (a) cost or (b) cost or market, whichever is lower ”; that “ goods taken in the inventory which have been so intermingled that they cannot be identified with specific invoices will be deemed to be . . . the goods most recently purchased ”; that the “ taxpayer must satisfy the commissioner of the correctness of the prices adopted ”; and that: “ id) Using a constant price or nominal value for a so-called normal quantity of materials or goods in stock ” is not in accord with the regulations.4
¶7The Company is engaged in the fabrication and erection of steel plates for buildings, bridges, tanks, etc. It does not carry finished products in stock, but fabricates the plates for specific structures or contracts. It orders material from the mills for each structure or contract; but it also keeps a supply on hand in order “ to insure the prompt and orderly execution of contracts in view of delay, etc., incident to. shipments from the mills and other exigencies affecting the availability for use when needed of material ordered for a particular job.” Material is taken from this supply as and when needed; and the stock is subsequently replenished.5 On December 31, *2681916, the quantity in stock was 5,554 tons. The Company then inventoried it at cost — $1.70 per hundredweight f. o. b. Pittsburgh. At the close of each year thereafter until 1921, the Company inventoried its stock on hand up to 5,554 tons at that price, regardless of its actual cost or the market, and the excess, if any, at cost or market price, whichever was lower. In the tax years in question, the market was much higher. It is not shown what the actual cost of the stock then on hand was, or that any of it had cost as little as $1.70.6 The Commissioner therefore revalued the entire stock at market price, with the consequent increase in the taxes complained of.
¶8First. Whether in a particular business inventories are necessary for the determination of income is a practical question left by the statute to the judgment of the Commissioner. On that question, he and the Company did not differ. In every year, it, without any question or protest, used inventories in making its return. The dispute was merely on the method of valuation to be adopted for that part of the stock which it calls its normal stock. Throughout, the Company valued at cost or market prices all stock in excess of 5,554 tons; and since 1921 has so valued all the stock on hand.
¶9It is not contested that if inventories are necessary in order to determine the Company’s income, the “base stock ” method does not fulfill the desiderata. The Federal income tax system is based upon an annual accounting period · Case Law">annual accounting period. This requires that gains or losses be accounted for in the year in which they are realized. The purpose of the inventories is to assign to each period its profits and losses. In years of rising prices, the “base stock” *269method causes an understatement of income; for it disregards the gains actually realized through liquidation of low price stock on a high price market. In times of falling prices, it causes an overstatement of income; for it ignores the losses which result from the consumption of high price stock. This method may, like many reserves which business men set up on their books for their own purposes, serve to equalize the results of operations during a series of years. But it is inconsistent with the annual accounting required by Congress for income tax purposes. It results in offsetting an inventory gain of one year against an inventory loss of another, obscures the true gain or loss of the tax year and, thus, misrepresents the facts. It does not conform with the general or best accounting methods and is apparently obsolete.7 The Company disclaims any defense of the base stock method; and the lower court disapproved it.
¶10Second. It is urged, however, that the inventory requirement is not applicable to the Company’s stock to the extent of 5,554 tons; that the Company is not a dealer, manufacturer or producer, but rather a contractor or builder; that its income results from the performance of its construction contracts; that the material in its stand-by stock has no relation to these contracts, the contract prices, or the Company’s profits; that the material from this stock is only borrowed for specific jobs and is promptly replaced in kind; that it is not an income pro*270ducing factor, but is like the Company’s machinery and equipment; and that any accretion to the value of this material is of no consequence until a final liquidation. The contentions are inconsistent with the Company’s practice and are unsound.
¶11The Company’s purchase and production of steel plates is obviously an income producing factor. Throughout the years, the Company has varying amounts of material" on hand. The value of the particular material used, at the time of use, plainly affects its profits. That the material is replaced in kind and its amount kept within some limits is not exceptional and is of no significance. Most concerns strive ordinarily to carry no more stock than is required for the safe and profitable conduct of the business. They plan neither to run short nor to overstock. They replace supplies as they are consumed. And the cost or value of the new material is properly reflected in the later inventories and returns. There is nothing peculiar about the 5,554 tons, — except that that happened to be the amount of stock on hand on December 31, 1916. It is not a permanent stock, like machinery or equipment. Nor is it merely depleted by borrowing and promptly restored to that fixed size. On the contrary, the stock has fluctuated from about 3,000 tons in 1918 to 11,000 tons in 1920.8 There is no stand*271by stock set aside and earmarked as such. The material is all commingled and is indiscriminately used in production, as and when needed.' No reason is given for excepting 5,554 tons — no more and no less. To draw an artificial line at that amount would distort the computation of income in the accounting periods, although the errors might be equalized in a series of years. Since inventories are properly deemed necessary, the exception of that or any amount is nothing but the use of the discarded “ base stock ” method.
¶12The Company’s case falls far short of meeting the heavy burden of proving that the Commissioner’s action was plainly arbitrary. Compare Lucas v. American Code Co., 280 U. S. 445, 449; Williamsport Wire Rope Co. v. United States, 277 U. S. 551, 559.
¶13Reversed.
¶14The Chief Justice did not take part in this case.