Introduction
Viscosity tells us how much a fluid resists flowing. Honey has high viscosity because it pours slowly, while water has low viscosity because it moves fast and easy. Engineers, scientists, and technicians need to measure and convert viscosity values every day when working with oils, paints, fuels, and many other liquids. This Viscosity Calculator makes that work simple and fast.
This tool has three built-in calculators. The Unit Converter lets you switch between common dynamic viscosity units like Pascal-seconds (Pa·s), centipoise (cP), and Poise, as well as kinematic viscosity units like centistokes (cSt), Stokes, and Saybolt Universal Seconds (SSU). Just enter a value in any field, and all other units update right away. It also uses the fluid's density to convert between dynamic and kinematic viscosity, since kinematic viscosity equals dynamic viscosity divided by density.
The Blending Calculator uses the Refutas method (ASTM D7152) to find the resulting kinematic viscosity when you mix two or more fluids together. Enter each component's viscosity in centistokes and its volume fraction, and the tool calculates the blend's final viscosity with a clear bar chart showing the results.
The Efflux Cup Converter helps you work with flow cups like Ford, Zahn, ISO (DIN 53211), and Shell cups. These cups are widely used in the paint and coatings industry to measure viscosity by timing how long a fluid takes to drain through a small opening. Enter either the efflux time in seconds or the viscosity in centistokes, and the calculator converts one to the other using standard cup formulas. A built-in reference table lists every supported cup type, its formula, and its recommended time range.
How to Use Our Viscosity Calculator
This viscosity calculator has three tools: a unit converter for switching between viscosity units, a blending calculator for mixing fluids, and an efflux cup converter for paint and coating work. Enter your known values and the calculator does the rest.
Density (g/cm³): Enter the density of your fluid. This number is needed to convert between dynamic viscosity (resistance to flow under force) and kinematic viscosity (resistance to flow under gravity). The default is set to 1.0 g/cm³, which is the density of water. Change it to match your specific fluid.
Dynamic Viscosity Fields: Enter a value in any one dynamic viscosity unit — Pa·s, mPa·s (cP), Poise, kg·m⁻¹·h⁻¹, lb·ft⁻¹·s⁻¹, or lb·in⁻¹·s⁻¹ — and every other dynamic unit fills in automatically. If you have provided a valid density, the kinematic viscosity fields will also update.
Kinematic Viscosity Fields: Enter a value in any one kinematic viscosity unit — m²/s, Stokes, Centistokes (cSt), ft²/s, ft²/h, in²/s, or SSU (Saybolt Universal Seconds) — and every other kinematic unit fills in automatically. With a valid density, the dynamic viscosity fields update too.
Blending Calculator – Component Viscosity (cSt): Enter the kinematic viscosity in centistokes for each fluid you want to blend. You start with two components and can add more by clicking the "Add Component" button. The calculator uses the Refutas method (ASTM D7152) to find the resulting blend viscosity.
Blending Calculator – Volume Fraction: Enter the volume fraction for each component. This is the portion of the total blend that each fluid makes up. All fractions must add up to 1.0 (which equals 100%). For example, a 50/50 mix would be 0.5 and 0.5. You can use our Ratio Calculator to help determine volume fractions from mix ratios.
Efflux Cup Converter – Cup Type: Select the type of efflux cup you are using from the dropdown menu. Options include Ford Cups, Zahn Cups, ISO/DIN Cups, and Shell Cups. Each cup has its own formula and recommended time range, which are shown in the results area and the reference table below.
Efflux Cup Converter – Efflux Time (seconds): Enter the measured drain time in seconds. The calculator converts this to kinematic viscosity in centistokes using the formula for your selected cup. A warning appears if your time falls outside the cup's recommended range.
Efflux Cup Converter – Kinematic Viscosity (cSt): If you already know the viscosity and need the matching efflux time, enter the kinematic viscosity in centistokes here instead. The calculator works backward to give you the expected drain time in seconds for your chosen cup.
Understanding Viscosity
Viscosity is a measure of how much a fluid resists flowing. Think of it like this: honey pours slowly because it has high viscosity, while water pours quickly because it has low viscosity. Every liquid and gas has its own viscosity, and knowing this value is important in fields like engineering, manufacturing, medicine, and food science.
Dynamic vs. Kinematic Viscosity
There are two main types of viscosity. Dynamic viscosity (also called absolute viscosity) measures the internal friction inside a fluid when a force is applied to it. Its most common units are Pascal-seconds (Pa·s), centipoise (cP), and Poise (P). Kinematic viscosity measures how fast a fluid flows under gravity. It equals the dynamic viscosity divided by the fluid's density. Common kinematic units include centistokes (cSt), Stokes (St), and square meters per second (m²/s). Because one type depends on density to convert to the other, you always need to know the fluid's density when switching between dynamic and kinematic viscosity.
Why Viscosity Units Matter
Different industries and countries use different viscosity units. A paint manufacturer might work in centipoise, an oil refinery might use centistokes, and an aerospace engineer might need values in square feet per second. Saybolt Universal Seconds (SSU) is an older unit still found in petroleum standards. Being able to convert accurately between all of these units prevents costly mistakes in product formulation, equipment sizing, and quality control.
Viscosity Blending with the Refutas Method
When you mix two or more liquids together, the viscosity of the blend is not a simple average of the individual viscosities. A 50/50 mix of a 2 cSt oil and a 10 cSt oil does not give you exactly 6 cSt. The relationship is nonlinear. The Refutas method, described in ASTM D7152, is the standard way to predict blend viscosity. It works by converting each component's kinematic viscosity into a Viscosity Blending Number (VBN) using a double-logarithmic formula. The VBN values can be averaged by volume fraction. The weighted average VBN is then converted back into centistokes to give the final blend viscosity. This method is widely used in petroleum refining, lubricant manufacturing, and chemical processing. If you need to verify the underlying logarithmic conversions, our Log Calculator can help.
Efflux Cup Measurements
An efflux cup is a simple tool used to measure viscosity in workshops and factories. You fill the cup with liquid, let it drain through a small hole in the bottom, and time how many seconds it takes to empty. That time relates directly to the liquid's kinematic viscosity through a formula specific to each cup type. Different cups have different hole sizes and shapes, so each one covers a different viscosity range.
Ford Cups (#3 and #4) are common in the paint and coatings industry in North America. Zahn Cups (#1 through #5) are dip-style cups popular for quick checks on production lines. ISO/DIN Cups (#3 through #6) follow international standards and are widely used in Europe. Shell Cups (#1 through #4) come from the petroleum industry. Each cup uses a formula in the form ν = at − b/t, where t is the efflux time in seconds and a and b are constants unique to that cup. Results are only accurate when the measured time falls within the cup's recommended range.
Factors That Affect Viscosity
Temperature is the biggest factor. Almost all liquids become thinner (less viscous) as they get warmer. Motor oil at 100 °C flows much more easily than the same oil at 40 °C. This is why viscosity values should always be reported alongside the temperature at which they were measured. The relationship between temperature and molecular motion also ties into concepts like kinetic energy—as thermal energy increases, molecules move more freely and internal friction drops. Pressure also plays a role, though it usually matters only at very high pressures found deep underground or in hydraulic systems. For everyday conditions, pressure effects are small. Our Hydrostatic Pressure Calculator can help you evaluate pressure conditions in fluid columns. Composition matters too—dissolving sugar in water raises its viscosity, and blending a light solvent into a thick resin lowers it.
Common Viscosity Values for Reference
Water at 20 °C has a dynamic viscosity of about 1 cP (1 mPa·s) and a kinematic viscosity of about 1 cSt. Olive oil is roughly 80 cP, honey is around 2,000–10,000 cP, and peanut butter can exceed 200,000 cP. Motor oils are typically rated between 5 and 30 cSt at 100 °C. These reference points help you judge whether a calculated result makes sense for the fluid you are working with.
Viscosity is closely tied to other fluid mechanics properties. The Reynolds Number Calculator uses kinematic viscosity to determine whether a flow is laminar or turbulent—a critical distinction in pipe design, aerodynamics, and chemical engineering. If you are working with fluid systems, our Flow Rate Calculator and Pipe Flow Calculator can help you size pipes and predict flow behavior. For situations involving submerged objects, the Buoyancy Calculator is another useful companion tool. And when dealing with paint or epoxy applications where viscosity directly affects coverage, you may also find our Paint Calculator or Epoxy Calculator helpful for estimating material needs.