Ink Viscosity Converter

Ink viscosity measures how thick or thin a fluid is. Lower viscosity means thinner and more runny, while higher viscosity means thicker and more resistant to flow.

Common viscosity units for inks and coatings:

• Centistokes (cSt): Kinematic viscosity, which is what any cup viscometer actually measures. Identical to mm²/s. Water is about 1 cSt at 20C.
• Centipoise (cP): The standard scientific unit for dynamic viscosity. Water is about 1 cP at 20C.
• Zahn Cup #2: A simple dip cup with a calibrated hole in the bottom. You dip it in the fluid, lift it out, and time how many seconds the fluid takes to drain completely. This is the most common cup for printing inks.
• Ford Cup #4: A similar concept but a different standard cup size used more commonly in the paint and coatings industry. Also measured in seconds.

Cup conversion formulas, and the range each one is good over:

There is no single cup. Zahn cups come in five sizes with progressively larger orifices, so each covers a different band of thickness. Pick the cup whose drain time lands somewhere near the middle of its range; a reading at either extreme is where these approximations fall apart.

Centistokes are not centipoise. This is the part that catches people out, so here it is plainly. A cup viscometer works by letting a fluid fall under its own weight, so what it measures is kinematic viscosity, in centistokes. Centipoise is dynamic viscosity, and the two are related by density: cP = cSt x specific gravity. For a water-thin fluid the two numbers are close enough that nobody notices, which is exactly why the mistake survives. For a solvent ink at 0.85 specific gravity, or a pigment-loaded coating at 1.3, the gap is large enough to matter. Put the specific gravity in above and the calculator shows both.

Typical ink viscosities by application:

Offset ink is the outlier there, and the reason its number looks absurd is that it is not really a liquid. Litho ink is a stiff paste; you spread it with a knife, not pour it. No dip cup will measure it at all, and the trade uses a falling-rod or inkometer test instead. If a figure in the tens of thousands of centipoise comes back from this page, treat it as arithmetic rather than a measurement you could have taken.

Practical tips:

• Zahn cups work best for thin to medium viscosity inks such as flexographic and gravure.
• Ford cups are better for paints, varnishes, and medium-viscosity coatings. The #4 is the common one, though the family runs from #1 to #5 like the Zahn set.
• Temperature significantly affects viscosity measurements, so always test at a consistent temperature, typically 25C (77F).
• When thinning ink with solvent, add small amounts and re-measure after each addition to avoid over-thinning.