Pressure is defined as the magnitude of the force acting normal to a unit area ($P = F/A$). Although force is a vector, pressure at a point in a fluid at rest acts equally in all directions and does not follow the laws of vector addition; therefore, pressure is a scalar quantity .

• Coefficient of viscosity and Relative density are magnitudes/ratios and thus scalars.
• Surface tension is typically treated as a scalar (surface energy per unit area) in this context.