In short: water's surface tension at room temperature (20 °C) is about 72.7 mN/m (0.0727 N/m) — enough to support a paperclip or a water strider, and to pull water roughly 30 mm up a 1 mm-wide glass tube. It drops steadily as water warms, reaching about 58.9 mN/m at boiling (100 °C). Use the calculator above for any temperature or tube size.
These figures are planning aids based on published reference data for pure water. For lab, dosing or quality-control work, confirm with a calibrated tensiometer.
How to calculate the surface tension of water
Surface tension is the energy needed to stretch the surface of a liquid, caused by molecules at the surface pulling inward more strongly than they're pulled outward. For water, that pull comes from hydrogen bonding, and it weakens predictably as temperature rises. The calculator uses the correlation endorsed by IAPWS, the international body that maintains reference data for water's properties:
Worked example. At 20 °C, T = 293.15 K, so T/Tc = 0.4531 and (1 − T/Tc) = 0.5469. Raising 0.5469 to the power 1.256 gives 0.4686. The bracket term is 1 − 0.625 × 0.5469 = 0.6582. Multiplying: 0.2358 × 0.4686 × 0.6582 = 0.0727 N/m, or 72.7 mN/m — matching the calculator's default reading.
mN/m and dyn/cm are the same number. Surface tension is most often quoted in millinewtons per meter (mN/m), the SI-friendly form of N/m. The older CGS unit, dynes per centimeter (dyn/cm), converts at exactly 1:1 — so 72.7 mN/m is also 72.7 dyn/cm.
Why temperature changes surface tension
Water molecules at the surface are held in place by hydrogen bonds to their neighbors below and beside them, but have no water molecules above to bond with. That imbalance pulls the surface into the tightest possible shape — which is why droplets are round and why surface tension exists at all. Heat adds kinetic energy: molecules vibrate and move faster, weakening those hydrogen bonds and loosening the surface's grip. That is why surface tension falls smoothly and continuously as water warms, all the way to zero at the critical point (374 °C), where the distinction between liquid and gas disappears entirely.
This calculator uses the correlation fitted to the Vargaftik et al. (1983) reference measurements, the data set IAPWS endorses as the standard for water's surface tension. It is accurate to within about 0.1% across the 0–100 °C range shown here. Full detail is on our methodology page.
Surface tension of water by temperature
The values this calculator uses, from freezing to boiling. Note how the drop accelerates as water approaches its boiling point.
| Temperature | Surface tension (mN/m) | N/m | lbf/ft |
|---|---|---|---|
| 0 °C / 32 °F (freezing) | 75.65 | 0.07565 | 0.005184 |
| 4 °C / 39 °F | 75.08 | 0.07508 | 0.005145 |
| 10 °C / 50 °F | 74.22 | 0.07422 | 0.005086 |
| 20 °C / 68 °F (room) | 72.74 | 0.07274 | 0.004984 |
| 25 °C / 77 °F (lab standard) | 71.97 | 0.07197 | 0.004932 |
| 37 °C / 99 °F (body) | 70.08 | 0.07008 | 0.004802 |
| 60 °C / 140 °F | 66.24 | 0.06624 | 0.004539 |
| 80 °C / 176 °F | 62.67 | 0.06267 | 0.004294 |
| 100 °C / 212 °F (boiling) | 58.91 | 0.05891 | 0.004037 |
Capillary rise: what surface tension does in a tube
Surface tension is what pulls water up a narrow tube against gravity — the effect behind blotting paper, plant stems drawing water upward, and why a thin straw dipped in water shows a slight rise even before you sip. The height of that rise is:
Worked example. A clean glass tube 1 mm across (radius 0.0005 m) has a contact angle near 0°, so cos θ ≈ 1. At 20 °C, γ = 0.0727 N/m and water's density is 998.2 kg/m³. h = (2 × 0.0727 × 1) / (998.2 × 9.80665 × 0.0005) = 0.0297 m, or about 30 mm — matching the calculator's default reading. Halve the tube's diameter and the rise doubles; the effect is strongest in the finest tubes.
The contact angle matters as much as the tube width. On clean glass, water wets the surface completely (θ ≈ 0°) and climbs. On some plastics the angle is higher and the rise is smaller. Past 90° the surface is non-wetting — water is pushed down below the outside level instead of pulled up, which the calculator shows as a negative value.
Worked examples
Fine glass capillary (0.2 mm)
149 mm
A 0.2 mm-diameter clean glass tube at 20 °C pulls water up almost 15 cm — the kind of tube used in some lab capillary viscometers and wicking demonstrations.
Everyday drinking straw (6 mm)
4.9 mm
A standard 6 mm straw shows barely 5 mm of capillary rise — real, but small enough that you'd never notice it without looking closely.
Soapy water in the same 1 mm tube
12.3 mm
Dish soap cuts surface tension to roughly 30 mN/m, so the same 1 mm tube that pulls pure water up 30 mm only manages about 12 mm with soapy water.
What else affects surface tension
Surfactants (soap, detergent, oils). Anything that collects at the water's surface and disrupts hydrogen bonding lowers surface tension sharply — dish soap and laundry detergent can cut it from about 72 mN/m to 25–35 mN/m. This is deliberate: lower surface tension lets soapy water spread into fabric and grease instead of beading up.
Dissolved salts. Salts like sodium chloride raise surface tension slightly — seawater runs a percent or two above fresh water at the same temperature. The effect is small enough that this calculator, built for fresh water, is still a reasonable estimate for lightly mineralized water.
Pressure. Surface tension is essentially unaffected by pressure at the ranges found in household or industrial plumbing. It responds to temperature and surface contamination, not to how hard the water is pushed.
Tube cleanliness (for capillary rise specifically). Oil, dust or fingerprints on a tube's inner wall raise the effective contact angle and reduce the rise, which is why precision instruments that rely on capillary action specify scrupulously clean glass.
Frequently asked questions
What is the surface tension of water at room temperature?
At 20 °C (68 °F), pure water has a surface tension of about 72.7 mN/m (0.0727 N/m, or 72.7 dyn/cm). This is unusually high for a liquid — only mercury and a few molten metals are higher — because of hydrogen bonding between water molecules.
Does temperature affect surface tension?
Yes. Surface tension falls steadily as water warms, from about 75.6 mN/m at 0 °C to about 58.9 mN/m at 100 °C, a drop of roughly 22%. Warmer molecules move faster and hold together less tightly at the surface.
Why does soap reduce the surface tension of water?
Soap and other surfactants are molecules with a water-loving end and a water-repelling end. They collect at the surface and disrupt the hydrogen bonding between water molecules there, cutting surface tension from about 72 mN/m to roughly 25–35 mN/m, which is how soap helps water spread and lift away grease.
What is capillary rise and how is it calculated?
Capillary rise is how far a liquid climbs up a narrow tube against gravity, pulled by surface tension. It's calculated as h = 2γcosθ / (ρgr), where γ is surface tension, θ is the contact angle with the tube wall, ρ is the liquid's density, g is gravity, and r is the tube's inner radius. Narrower tubes produce a higher rise.
What units are used for surface tension?
The SI unit is newtons per meter (N/m), almost always expressed as millinewtons per meter (mN/m) because the values are small. The older CGS unit, dynes per centimeter (dyn/cm), is numerically identical to mN/m, so 72.7 mN/m equals 72.7 dyn/cm exactly.
How accurate is this calculator?
Surface tension is computed from the IAPWS-endorsed correlation fitted to the Vargaftik et al. (1983) reference data set, accurate to within about 0.1% across the calculator's range. Capillary rise adds an idealized, frictionless-tube assumption on top, so treat it as a planning estimate rather than a lab measurement. Full detail is on our methodology page.
Sources & references
- Vargaftik, N. B., Volkov, B. N., & Voljak, L. D. (1983). International tables of the surface tension of water. Journal of Physical and Chemical Reference Data, 12(3), 817–820.
- International Association for the Properties of Water and Steam (IAPWS). Release on Surface Tension of Ordinary Water Substance.
- Kell, G. S. (1975). Density, thermal expansivity, and compressibility of liquid water from 0° to 150 °C. Journal of Chemical & Engineering Data, 20(1), 97–105. (Used for the density term in capillary rise.)
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)