Surface Tension Converter
Same value in every unit
| Unit | Value |
|---|
The conversion most people come here for turns out to be a factor of 1
One dyne per centimetre is exactly one millinewton per metre, and one erg per square centimetre is exactly the same size again. A value of 72 dyne/cm copied out of a 1960s paper is 72 mN/m today. Nothing changes but the label, which is why the two most widely used surface tension units have never needed reconciling.
The conversions that involve real arithmetic are the decimal steps to newton per metre and newton per centimetre, plus the one awkward unit in the set: pound-force per foot, which is 14.5939 N/m (4.44822 N spread over 0.3048 m). That one turns up in older imperial engineering tables rather than in laboratory work — even oilfield practice normally reports dyne/cm.
Force per length, or energy per area
Surface tension can be written as N/m or as J/m² because those are dimensionally identical. The force-per-length view matches how it is measured: the pull on a Du Noüy ring or a Wilhelmy plate. The energy-per-area view matches what it does: the cost of creating new interface, which governs wetting, droplet break-up and adhesion. Erg per square centimetre is simply the CGS form of that second view.
Reference points worth keeping in your head
- Pure water at 20 °C: about 72 mN/m (72.8 in most tables) — very high for such a small molecule, because of hydrogen bonding
- Pure water at 100 °C: about 59 mN/m
- Mercury at room temperature: several hundred mN/m, roughly six to seven times water — approximate, and very sensitive to surface cleanliness
- Water carrying enough surfactant: roughly half the clean value or a little less (approximate)
- Any liquid at its critical temperature: zero
Water's high figure is why it beads on a waxed panel instead of spreading, and it is also why detergent works. The surfactant gathers at the interface, drops the tension, and lets water creep into fabric and around grease rather than pulling itself into droplets.
Record the temperature with the value
Surface tension falls as a liquid warms, so a figure quoted without a temperature is incomplete. Near room temperature, water loses roughly 0.15 mN/m for each degree Celsius. Contamination pushes the same way and harder: a trace of oil or a fingerprint on the glassware will drag a reading well below the textbook value. A low result is more often a dirty sample than a wrong one.
Share this tool with friends
Free to use, no sign-up, works on any phone.
Frequently Asked Questions
Yes, exactly, with no rounding involved. A dyne is 10⁻⁵ N and a centimetre is 10⁻² m, so one dyne per centimetre works out at 10⁻³ N/m, which is one millinewton per metre. Erg per square centimetre lands on the same value, so all three scales are interchangeable digit for digit — only the label changes.
About 72 mN/m at 20 °C — most tables give 72.8 — and that is the same number as 72 dyne/cm. It is unusually high for a molecule that small, because hydrogen bonding pulls surface molecules back into the bulk. Hot water is lower, around 59 mN/m at 100 °C, so always note the temperature a value was measured at.
Multiply by 1000, so 0.072 N/m is 72 dyne/cm, and divide by 1000 to go the other way. Newton per centimetre runs the opposite direction: 1 N/cm is 100 N/m, so water's 72 mN/m comes out as 0.00072 N/cm. That run of leading zeros is easy to misread, which is why liquid values are almost never reported in N/cm.
Surfactant molecules have a water-liking end and an oil-liking end, so they crowd into the air-water interface and break up the hydrogen bonding that holds the surface taut. The tension drops well below water's clean value of roughly 72 mN/m. That is the actual mechanism of washing: lowered tension lets water spread into fabric and around grease instead of beading up on it.
For a pure liquid they are the same quantity in different units: 1 N/m equals 1 J/m². That is precisely why erg per square centimetre sits beside dyne per centimetre on this converter, since 1 erg/cm² is 1 mN/m. For solids the two can come apart, because a solid surface can be stretched elastically rather than enlarged by adding fresh area.
mN/m is the safest default for instrument output and publication, and dyne/cm is equally acceptable since the number is identical — it still dominates petroleum literature. Quote the temperature next to the value, as the figure is incomplete without it. Pound-force per foot is worth converting before you share it; hardly anyone reads surface tension in that unit.