Surface Tension Force Calculator
Surface tension is the energy stored in a liquid surface, and it produces a measurable force along any line where the surface meets a solid edge. That force is what a du Nouy ring tensiometer measures and what lets a needle float on water. This calculator multiplies the surface tension by the wetted contact length and by the number of liquid surfaces pulling on the line, returning the total force in newtons and millinewtons. Enter the surface tension for your liquid and temperature, the contact length and the surface count for your geometry.
Surface tension force formula
F = gamma * L * n
gamma = surface tension (N/m)
L = length of contact line (m)
n = number of liquid surfaces (1 or 2)
Surface tension is defined as force per unit length of contact line. Multiplying by the wetted length gives force per surface; multiplying by the number of surfaces gives the total pull on the object.
Surface tension context
- Surface tension of pure water at 20 degrees Celsius is approximately 0.0728 newtons per metre.
- A free thin film has two surfaces, so use n = 2 for soap films and ring tensiometers.
- For a single open surface, such as a plate edge breaking the top of a liquid, use n = 1.
- Adding surfactant lowers surface tension, which is why soapy water wets and spreads more easily.
- Surface tension falls as temperature rises and reaches zero at the critical point of the liquid.
Surface tension force: frequently asked questions
What is surface tension force?
Surface tension force is the pull a liquid surface exerts along a line of contact, such as the edge of a wire dipped in water. It equals the surface tension of the liquid multiplied by the length of the contact line, multiplied by the number of liquid surfaces touching that line.
What is the formula for surface tension force?
The force is F = gamma * L * n, where gamma is the surface tension in newtons per metre, L is the length of the contact line in metres, and n is the number of liquid surfaces meeting the line. A thin film has two surfaces (front and back), so n = 2 for a wire lifting a soap film.
Why does the number of surfaces matter?
A free liquid film has two faces, an inner and an outer surface, each pulling on the contact line. A wire pulling up a soap film therefore feels twice the force of a single surface. For a single free surface, such as the rim of a floating object, n is 1. Set n to match your geometry.
What surface tension value should I use?
Surface tension is a temperature-dependent property of the liquid. Water at 20 degrees Celsius is about 0.0728 newtons per metre; soap solutions are lower, around 0.025. Because it is empirical, it ships here as a user-editable input so you can enter the correct value for your liquid and temperature.
How is this used to measure surface tension?
Instruments like the du Nouy ring and Wilhelmy plate measure the maximum force needed to detach a wetted object from a liquid surface, then divide by the contact length and surface count to find surface tension. This calculator runs that relationship in either direction once you supply the values.
Official sources
- U.S. National Institute of Standards and Technology: Physical Measurement Laboratory.
- USGS Water Science School: Surface tension and water.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.