Please note that densities, which involve weight (mass) to volume ratios, are further defined as to whether the weight is "in air" or "in vacuo." Density, kg/l (kilograms per litre), for example, is given "in vacuo." Accordingly, when a litre volume (at the same temperature) is multiplied by this density, the resultant weight is "in vacuo." To determine weight "in air," the kg/l density has to first be corrected for the buoyancy of air. To determine kg/l at 15°C (in air), subtract 0.0011 from kg/l (in vacuo).