The term capital surplus generally refers to the amount in excess of par value paid for capital stock when it was originally issued by the corporation. As an example, if a newly incorporated corporation issues 10 shares of stock, par value $1 per share, for a price of $5 each, the corporation will have $40 of capital surplus (10 shares x $5 purchase price minus 10 shares x $1 par value per share). The remaining $10 of the purchase price will go into the corporation's legal capital.
Once the shares are issued by the corporation, the amount of the capital surplus account normally does not change. The fact that the shares may be sold for more or less than this amount in the secondary market later does not change the capital surplus amount.
Capital surplus and earned surplus together form surplus, which is how some states like Delaware measure a corporation's ability to pay dividends. Delaware allows a corporation to pay dividends "out of its surplus" which means that a Delaware corporation can pay dividends so long as its net assets would not be less than its capital.
The amount of the capital surplus will normally be reflected on the corporation's balance sheet as "additional paid-in capital," or sometimes "capital in excess of par," or "APIC."