The basis amount that results from the difference between the current clean price of a bond and the clean price at which the bond is purchased through taking a long position in a futures contract. The gross basis is thus calculated as follows:

Where P_c is the bond clean price, P_fut is the futures price, and CF_bond is the conversion factor.

Put another way, the gross basis captures the difference between the running yield on the bond and the current repo rate (money market rate).