Surface Tension & Permeability Converter
Convert two properties that matter in fluid and materials engineering. Surface tension (γ) is the energy holding a liquid surface together, in newtons per metre — it drives capillary rise, droplet shape and wetting. Permeability (κ) is how readily a porous material lets fluid flow through it, in square metres or the practical darcy used in petroleum and groundwater work. Pick a property and convert across its common units.
How to use this tool
- Choose the property: surface tension or permeability.
- Enter the value.
- Pick the unit you have and the unit you want; the menus update per property.
- Read the result and the full equivalents list.
- Use the context note to sanity-check against familiar reference values.
The formula
Surface tension is the energy per unit area of a liquid surface, in newtons per metre. Permeability measures how readily a porous material lets fluid pass, in square metres — or the practical darcy.
surface tension γ: N/m (= J/m²)
permeability κ: m² (1 darcy ≈ 9.869×10⁻¹³ m²)- γ
- Surface tension, N/m
- κ
- Intrinsic permeability, m² or darcy
Water’s surface tension is about 72 mN/m at room temperature. One darcy is roughly the permeability of a coarse, freely draining sand.
Worked examples
Water surface tension
- Given
- 72 dyn/cm
- Result
- 72 mN/m
The dyne per centimetre and millinewton per metre are numerically identical, so water’s ~72 dyn/cm is 72 mN/m.
Reservoir rock
- Given
- 500 millidarcy
- Result
- ≈ 4.93×10⁻¹³ m²
Half a darcy is a moderately permeable reservoir rock; in SI it is about 5×10⁻¹³ square metres.
SI surface tension
- Given
- 0.072 N/m
- Result
- 72 mN/m
Expressed in base SI, water’s surface tension is 0.072 newtons per metre.
Frequently asked questions
It is the tendency of a liquid surface to shrink to the smallest area, because molecules at the surface are pulled inward. It is what lets some insects walk on water, pulls water up narrow tubes, and rounds small droplets into beads.
A dyne is 10⁻⁵ newtons and a centimetre is 10⁻² metres, so a dyne per centimetre works out to exactly one millinewton per metre. The two units are numerically identical, which is why surface-tension tables move between them freely.
The darcy is a practical unit of permeability, roughly the permeability that lets a fluid of 1 centipoise flow at 1 cm³/s through a 1 cm² area under a set pressure gradient. It equals about 9.87×10⁻¹³ square metres and is standard in petroleum and groundwater engineering.
Soaps and detergents are surfactants that lower water’s surface tension, letting it spread into fabric and around grease rather than beading up. That improved wetting is a large part of how cleaning works.
Yes — surface tension falls as temperature rises, which is why hot water wets and cleans better than cold. This tool converts a given value between units; it does not model the temperature dependence itself.