A fluid's headline properties are its density (mass per volume) and its viscosity (resistance to
flow). Viscosity comes in two flavours: dynamic (μ, in mPa·s or centipoise) and kinematic
(ν, in mm²/s or centistokes) — related by density, ν = μ/ρ. Every liquid thins as it warms, so a
single viscosity number only means something with a temperature attached. This tool models that curve from two
reference points and lets you compare fluids on one chart.
Hydraulic ISO VG 46 oil is defined as 46 cSt at 40°C — that's what the grade number means. Heat it to 100°C and it thins to about 6.8 cSt, over six times runnier. Water at 40°C is only 0.66 cSt, so the oil is roughly 70× thicker.
The viscosity index captures how flat that curve is: a multigrade 10W-40 keeps its thickness across a wider temperature range than a monograde SAE 30, even when both start near 100 cSt at 40°C.
Pump and pressure-drop calculations usually want kinematic (ν); force and shear calculations want
dynamic (μ). This tool shows both at your chosen temperature. Convert with μ = ν·ρ.
The number is the oil's nominal kinematic viscosity in centistokes at 40°C, ±10%. So ISO VG 46 sits between roughly 41 and 51 cSt at 40°C. SAE grades work differently and are measured at different temperatures.
It's fitted through two reference points with a standard viscosity–temperature relation, so it's good between and near them and rougher at the extremes. It ignores additives, shear thinning and phase changes. Use it for intuition and quick estimates, not for specification.
Specific gravity (density relative to water) tells you whether a fluid floats or sinks and helps convert between mass and volume. Most oils and fuels are lighter than water (SG < 1); glycerine and mercury are heavier.
Heat water from freezing to boiling and its density falls about 4%. Its viscosity falls by a factor of six. Both are plotted below on their own scales, which is the only way to show them together — on a shared axis the density line would look flat, because next to viscosity it is.
So when a flow calculation changes with temperature, it is almost always viscosity doing it. The last column is the Reynolds number you would get in the same pipe at the same speed, relative to 20 °C: hot water is 3.4× more turbulent than room-temperature water moving at exactly the same rate, and cold water is markedly less.
| °C | Density kg/m³ | Viscosity mPa·s | Density vs 20° | Reynolds vs 20° |
|---|---|---|---|---|
| 0 | 999.8 | 1.753 | 1.00 | 0.57 |
| 10 | 999.7 | 1.300 | 1.00 | 0.77 |
| 20 | 998.2 | 1.002 | 1.00 | 1.00 |
| 30 | 995.6 | 0.797 | 1.00 | 1.25 |
| 40 | 992.2 | 0.651 | 0.99 | 1.53 |
| 50 | 988.0 | 0.544 | 0.99 | 1.82 |
| 60 | 983.2 | 0.463 | 0.98 | 2.13 |
| 80 | 971.8 | 0.351 | 0.97 | 2.78 |
| 100 | 958.4 | 0.279 | 0.96 | 3.45 |
Density from the standard polynomial fit; viscosity from a Vogel fit. Both checked against published values at 0, 20, 50 and 100 °C. Fresh water at atmospheric pressure — dissolved salts and pressure both shift these, and neither is modelled.
No changes to this tool’s own behaviour since the earliest archived release (v3.66). The full history for the site is in the changelog.