Surface tension lets insects walk on water, pulls droplets into spheres and makes small bubbles push harder than large ones. Water’s surface tension is about 72 mN/m at room temperature.
How to use the Surface Tension Calculator
- Method
- Force on the film
- Length of the wire
- Pressure difference ΔP
- Radius r
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose the method and enter the values.
Common mistakes to avoid
- Mixing units between fields.
- Using a rule outside the range where it applies.
- Expecting fun estimates to be exact predictions.
Inputs explained
| Field | What to enter |
|---|---|
| Method | Choose one: Film on a wire frame, Liquid droplet, Soap bubble, Liquid jet (cylinder) |
| Force on the film | Units: mN, N, µN |
| Length of the wire | Units: cm, mm, m, in, ft |
| Pressure difference ΔP | Units: Pa, kPa, mbar |
| Radius r | Units: mm, cm, m, in, µm |
Example values
How the main result changes when Force on the film changes and everything else stays at the example values above:
| Force on the film (mN) | Surface tension γ (mN/m) |
|---|---|
| 1.45 | 72 |
| 2.18 | 72 |
| 2.9 | 72 |
| 3.63 | 72 |
| 4.35 | 72 |
| 5.8 | 72 |
Formula
Film: γ = F/(2L) · Droplet: γ = ΔP·r/2 · Soap bubble (two surfaces): γ = ΔP·r/4 · Cylindrical jet: γ = ΔP·r
Worked example
Inputs — Method: Liquid droplet; Pressure difference ΔP: 144 Pa; Radius r: 1 mm.
Results — Surface tension γ = 72 mN/m; Comparable to = Water.
Key terms
- Laplace pressure
- Pressure difference across a curved liquid surface.
Frequently asked questions
Why do small bubbles have higher pressure?
Laplace pressure is inversely proportional to radius.
Why does a soap film count twice?
It has two surfaces.
What lowers surface tension?
Soap, detergents, alcohol and heat.
What is water's surface tension?
About 72 mN/m at 25 °C.