Viscosity Calculator

Introduction

Viscosity is how thick or thin a fluid is when it flows. Think of it like this: honey flows slowly because it has high viscosity, while water flows fast because it has low viscosity. Viscosity tells us how much a fluid resists movement. This property matters a lot in science, engineering, and everyday life.

Our Viscosity Calculator helps you find the viscosity of a fluid quickly and easily. Just enter the values you know, such as the force applied, the area, the velocity, and the distance between layers of the fluid. The calculator does the math for you using the basic viscosity formula from fluid mechanics. Whether you are a student learning about fluid dynamics or someone who needs a fast answer, this tool saves you time and reduces errors.

How to Use Our Viscosity Calculator

Enter the values below to calculate the viscosity of a fluid. You will need information about the force applied, the area, the velocity, and the distance between layers. The calculator will give you the dynamic viscosity of the fluid.

Shear Force (F): Enter the force applied to the fluid in newtons (N). This is the push or pull that makes the fluid move along a surface. If you need help computing the net force on an object, our Force Calculator can assist you.

Area (A): Enter the area of the surface in contact with the fluid in square meters (m²). This is the size of the flat surface where the force is applied.

Velocity (v): Enter the speed of the moving fluid layer in meters per second (m/s). This is how fast the top layer of the fluid moves compared to the bottom layer.

Distance Between Layers (dy): Enter the gap between the two fluid layers in meters (m). This is the space between the moving layer and the layer that stays still.

Once you fill in all four values, the calculator uses Newton's law of viscosity to find the dynamic viscosity. The result is shown in pascal-seconds (Pa·s), which tells you how thick or resistant to flow the fluid is. A higher number means the fluid is thicker, like honey, while a lower number means it flows easily, like water.

What Is Viscosity?

Viscosity is a measure of how thick or resistant a fluid is to flowing. Think of it like this: honey has a high viscosity because it flows slowly, while water has a low viscosity because it flows easily. Viscosity tells us how much a fluid resists being pushed or poured.

Why Does Viscosity Matter?

Viscosity plays a big role in everyday life and in science. Engineers need to know the viscosity of oil to keep car engines running smoothly. Doctors look at blood viscosity to understand how well blood flows through the body. Even the food industry uses viscosity to make sure things like ketchup and syrup have the right thickness.

In fluid mechanics, viscosity is essential for calculating the Reynolds number, which determines whether a flow is smooth (laminar) or turbulent. It also factors into computations involving hydrostatic pressure and buoyancy in engineering design.

Dynamic Viscosity vs. Kinematic Viscosity

There are two main types of viscosity you should know about:

• Dynamic viscosity (also called absolute viscosity) measures the force needed to move one layer of fluid past another. It is measured in units called Pascal-seconds (Pa·s) or poise (P).
• Kinematic viscosity is the dynamic viscosity divided by the fluid's density. It describes how fast a fluid flows under gravity. It is measured in square meters per second (m²/s) or stokes (St).

The relationship between them is simple: kinematic viscosity = dynamic viscosity ÷ density. If you're working with gases, the Ideal Gas Law Calculator can help you determine density at a given temperature and pressure.

What Affects Viscosity?

Temperature is the biggest factor. When you heat a liquid, its viscosity goes down — it flows more easily. That is why warm honey pours faster than cold honey. For gases, the opposite happens: heating a gas makes its viscosity go up. Temperature changes can also cause materials to expand, which you can explore with our Thermal Expansion Calculator.

Pressure can also change viscosity, but usually only at very high levels. For most everyday situations, pressure has little effect on how a liquid flows.

Common Viscosity Values

Here are some examples to give you a sense of scale:

• Water at 20°C: about 0.001 Pa·s
• Motor oil: about 0.1 to 0.3 Pa·s
• Honey: about 2 to 10 Pa·s
• Air at 20°C: about 0.000018 Pa·s

Use the calculator above to quickly convert between viscosity units or solve for dynamic viscosity, kinematic viscosity, or fluid density when you know two of the three values. For related fluid mechanics problems, you may also find our Momentum Calculator and Acceleration Calculator helpful when analyzing forces and motion in fluid systems.

Frequently Asked Questions

What is the difference between Pa·s and cP (centipoise)?

Pa·s (Pascal-second) and cP (centipoise) both measure dynamic viscosity. The conversion is simple: 1 Pa·s = 1,000 cP. So 1 cP equals 0.001 Pa·s. Centipoise is also the same as mPa·s (millipascal-second). Water at 20°C has a viscosity of about 1 cP or 0.001 Pa·s.

How do I convert dynamic viscosity to kinematic viscosity?

Divide the dynamic viscosity by the fluid's density. The formula is: kinematic viscosity = dynamic viscosity ÷ density. In this calculator, enter the density in g/cm³ in the density box at the top. Then type a value in any dynamic viscosity field, and all kinematic viscosity fields will update automatically.

Why do I need to enter density?

Density connects dynamic viscosity and kinematic viscosity. Without it, the calculator can only convert within the same type (dynamic to dynamic, or kinematic to kinematic). When you enter a valid density, the calculator can convert between both types at the same time. If you only need conversions within one type, the density does not matter.

What is the Refutas method for blending viscosity?

The Refutas method (from ASTM D7152) is a standard way to predict the viscosity of a fluid blend. It works by converting each component's kinematic viscosity into a Viscosity Blending Number (VBN), averaging those numbers by volume fraction, and then converting back to viscosity. It is more accurate than simply averaging viscosities directly because viscosity does not blend in a straight line.

What are volume fractions in the blending calculator?

Volume fractions tell the calculator how much of each fluid is in the mix. They represent the portion of total volume each component takes up. For example, if you mix equal parts of two oils, each has a volume fraction of 0.5. All fractions must add up to 1.0 (which means 100% of the blend).

What is an efflux cup and how does it measure viscosity?

An efflux cup is a small cup with a hole at the bottom. You fill it with a fluid and time how long it takes to drain out. The drain time in seconds is then converted to kinematic viscosity using a formula specific to that cup type. Longer drain times mean higher viscosity. Efflux cups like Ford, Zahn, and ISO cups are widely used in the paint and coatings industry.

Which efflux cup type should I use?

It depends on the viscosity range of your fluid. Zahn #2 and Ford #4 are the most common for paints and coatings. Low-viscosity fluids work better with smaller cup numbers (like Zahn #1), while thicker fluids need larger cup numbers (like Zahn #5). Check the reference table in the Efflux Cup tab to see each cup's recommended time range.

What does SSU (Saybolt Universal Seconds) mean?

SSU stands for Saybolt Universal Seconds. It is an older unit for kinematic viscosity based on how long it takes oil to flow through a standard Saybolt viscometer. It is still used in some petroleum and industrial settings. The calculator converts SSU to centistokes (cSt) using the ASTM D2161 formulas. SSU values below 32 seconds are outside the valid range.

What is the difference between centistokes (cSt) and stokes (St)?

Both are units of kinematic viscosity. 1 Stokes (St) = 100 centistokes (cSt). Centistokes is also the same as mm²/s, while Stokes equals cm²/s. Most people use centistokes because it gives easier numbers for common fluids. For example, water at 20°C is about 1 cSt.

Can I convert between different efflux cup types?

Yes. First, use one cup type to convert your efflux time to kinematic viscosity in cSt. Then switch to a different cup type and enter that cSt value in the viscosity field. The calculator will give you the equivalent efflux time for the new cup. This is useful when comparing measurements made with different cup types.

What does the warning about time being outside the recommended range mean?

Each efflux cup has a range of drain times where its formula is accurate. If the time falls outside this range, the viscosity result may not be reliable. The warning does not stop the calculation, but it tells you the answer might not be precise. Try using a different cup size that better fits your fluid's viscosity.

How accurate is the blending calculator for mixing more than two fluids?

The Refutas method works well for blending any number of Newtonian fluids (fluids with constant viscosity at a given temperature). You can add more components by clicking the Add Component button. The method is an industry standard (ASTM D7152) and is reliable for petroleum products, lubricants, and similar fluids. It may be less accurate for non-Newtonian fluids like gels or polymers.

What units does the blending calculator use?

The blending calculator uses centistokes (cSt) for kinematic viscosity input and output. It also shows the result in mPa·s (cP) for dynamic viscosity, using the density value from the Unit Converter tab. Make sure to set the correct density if you need the dynamic viscosity result.

Why does my efflux cup result say 'Out of range'?

This happens when the formula gives a negative or zero viscosity for the time you entered. It means the drain time is too short for that cup type, and the formula cannot produce a valid result. Try entering a longer time or switch to a cup type designed for lower-viscosity fluids.

What is the difference between Poise and Pascal-second?

Both measure dynamic viscosity. 1 Poise (P) = 0.1 Pa·s. The Poise comes from the CGS unit system, while the Pascal-second is the SI unit. In practice, many people use centipoise (cP), where 1 cP = 1 mPa·s = 0.001 Pa·s.