Viscosity is a measure of a fluid’s internal resistance to flow. It represents the frictional force that resists motion between adjacent layers of a liquid. The Ostwald viscometer operates on the principle of Poiseuille’s Law, which states that the rate of flow of a liquid through a capillary tube is directly proportional to the pressure and the fourth power of the radius, and inversely proportional to the viscosity and length of the tube.

η₁ / η₂ = (ρ₁ t₁) / (ρ₂ t₂)
Where:
η₁ = Viscosity of the test liquid
η₂ = Viscosity of water
ρ₁ = Density of the test liquid
ρ₂ = Density of water
t₁ = Flow time of the test liquid
t₂ = Flow time of water

If the density of both liquids is known, the absolute viscosity (η₁) of the given liquid can be determined using:
η₁ = η₂ × (ρ₁ t₁) / (ρ₂ t₂)

The viscosity of water at 25°C is approximately 0.89 cP. Temperature plays a critical role — hence the viscometer must be immersed in a constant-temperature water bath to maintain accuracy.