Dynamic viscosity, written μ, measures a fluid’s resistance to shearing flow. This compact table gives selected water and nitrogen values at 20°C from a NIST reference report. It is intentionally a narrow comparison rather than a universal fluid chart. Viscosity depends on state and fluid identity, so the temperature and pressure travel with every value. A room-temperature reference can help check units or a calculation, but it is not enough to size a pump, predict a pressure drop, or select a lubricant without the operating conditions.

Fluid Dynamic Viscosity Reference

Read the units correctly

The SI unit of dynamic viscosity is pascal-second (Pa·s). One millipascal-second equals 0.001 Pa·s, and one centipoise equals one millipascal-second. The table shows both Pa·s and mPa·s so small values are easier to compare. For water, the rounded NIST value is about 0.001 Pa·s, or 1 mPa·s. Nitrogen’s value is much smaller, about 0.0000178 Pa·s. The displayed figures are rounded references, not claims of precision beyond the source.

Dynamic is not kinematic viscosity

Kinematic viscosity ν is dynamic viscosity divided by density: ν=μ/ρ. Its SI unit is m²/s; a common engineering unit is mm²/s, also called centistokes. You cannot convert μ to ν by changing prefixes alone. Obtain density for the same fluid at the same temperature, pressure, and phase, then divide with consistent units. For a gas, density changes substantially with pressure, so a kinematic value should not be carried from one state to another without checking the density model.

How temperature changes the answer

For many liquids, dynamic viscosity falls as temperature rises; gases commonly show the opposite trend over ordinary ranges. The exact response depends on fluid chemistry and state. Water at 20°C is only a reference point: a hot-water process needs the temperature-dependent value, and a pressurized system may need pressure-dependent data as well. NIST’s fluid property interface lets the user select the species, property, units, and state range. Use that tool or another authoritative property source for the actual design point rather than interpolating two unrelated reference values.

Worked unit check

Take the table’s water value, 1.0×10⁻³ Pa·s. Multiplying by 1,000 gives 1.0 mPa·s. If the matching density were 998 kg/m³, the kinematic viscosity would be approximately 1.0×10⁻⁶ m²/s, or 1.0 mm²/s. This last step is illustrative and uses a density value that must match the same state; it is not an extra measured row in this dataset. Unit-check the result because 1 Pa·s is 1 kg/(m·s), while density is kg/m³.

Common mistakes and sources

Do not confuse dynamic viscosity with ISO viscosity grade. An oil grade is a classification framework, while μ is a property at a specified state. Do not compare values with different temperature, phase, composition, or shear-rate assumptions. The NIST WebBook lists viscosity among its thermophysical properties; the linked NIST reports provide the water measurement and rounded water/nitrogen reference values used here. For non-Newtonian materials such as many slurries and polymer melts, viscosity depends on shear rate, so a single number can mislead. When using a simulator or property package, record the model, units, state inputs, and version so another engineer can reproduce the output. A value reported at saturation conditions should not silently stand in for a compressed liquid or gas at a different pressure. If an application spans a broad temperature range, use a property curve or table at enough points to represent that range rather than carrying this 20°C reference through the whole calculation.

References

• NIST Chemistry WebBook, Thermophysical Properties of Fluid Systems
• NIST Journal of Research: Absolute Viscosity of Water at 20°C
• NISTIR 6395, Useful Constants

This reference is limited to the conditions named in the data and sources. Confirm the governing specification, complete geometry, material, operating environment, and service loads before using any value for design, procurement, inspection, or safety-critical decisions.