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The bar is logarithmic. It has to be: at −20 °C a gear oil is five orders of magnitude thicker than petrol, and on a linear scale every other fluid would be an invisible sliver.
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Pressure drop, roughness and fittings live in the pipe flow calculator — this panel is only about which regime the fluid is in, because that is what viscosity decides.
Click a column to sort. Greyed rows are outside that fluid’s valid temperature range — frozen, boiling, or below its pour point — and the tool will not quote a number it does not believe.
| Fluid | ν cSt | μ cP | ρ kg/m³ | SG | ν40/ν100 | ISO VG |
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Every oil datasheet quotes kinematic viscosity at 40 °C and 100 °C. Type yours in and it joins the chart and the table.
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The petroleum industry’s own viscosity–temperature relation, log₁₀ log₁₀(ν + 0.7) = A − B·log₁₀T. Two datasheet points give A and B.
Water is the most anomalous common liquid. A Vogel-type correlation holds within 2% from 0 to 100°C; density uses Kell’s equation, which finds the maximum at 3.98°C.
Petrol, kerosene, ethanol, mercury and glycol coolant follow simple Arrhenius behaviour: within 1% for ethanol, 5% for mercury.
Freezing and boiling points, pour points and spec limits set each fluid’s range. Outside it the row greys out and the tool refuses the number.
Mercury is thirteen times denser than water and flows just as freely. Honey is barely denser than water and will not pour. Density is mass per volume; viscosity is internal friction. μ = ν·ρ is a conversion, not a relationship.
Every oil is roughly the same when hot: that is what the grade guarantees. SAE 30 and 5W-30 are identical at 100°C; at −20°C the monograde is about nine times thicker. Everything a multigrade is for happens at the cold end of the chart.
In laminar flow the rate goes as the fourth power of the bore: halve the diameter for the same pressure and you get a sixteenth. A slightly undersized hydraulic line is not a slightly slower one, and warming the oil is usually cheaper than a bigger pump.
“ISO VG 46” means 46 cSt at 40°C, ±10%. Quoting a viscosity without its temperature says almost nothing: over the range on this page the same oil varies by a factor of a thousand. If a spec gives one number and no temperature, it is not a spec.
A single exponential term cannot hold its shape across 170°C, and the failure is not subtle. D341 is also undefined below 0.3 cSt, where log₁₀(ν + 0.7) goes to zero and the outer logarithm diverges, so the thin fluids could not use it even if it fitted.
Fitted through the same two anchors, D341 and a single-term fit both come out 22–23% low for water at 0°C. Kell’s density reproduces the maximum at 3.98°C, not at freezing; a plain fifth-order polynomial tried here was 11 kg/m³ out at 100°C, and since μ = ν·ρ a wrong density silently corrupts every dynamic viscosity on the page.
ASTM D2270 needs tabulated L and H values for ν₁₀₀ below 70 cSt, and the published polynomial form is only valid above 70: it returns a negative L for an ISO VG 46. Rather than print a number dressed up as a standard it does not implement, the tool shows the raw ν₄₀/ν₁₀₀ ratio, which is the same information without the false authority.
Two-point fits, and a fit is only as good as its two points. No additives and no shear thinning: a multigrade’s VI improver behaves differently in a loaded bearing than in a viscometer. Everything is at 1 atm, and viscosity rises steeply with pressure. Honey, olive oil and heavy fuel oil vary enormously batch to batch, so read them as representative. Heavy fuel oil starts at 30°C because below its pour point it is a solid, and no correlation describes it.
Pump curves, pressure drop and Reynolds number want kinematic (ν, cSt). Force, torque and shear calculations want dynamic (μ, cP). They differ only by density: μ = ν·ρ. Both are shown here at whatever temperature you pick, because quoting either without a temperature is meaningless.
The number is the oil’s nominal kinematic viscosity in centistokes at 40 °C, ±10%. So an ISO VG 46 sits between about 41 and 51 cSt at 40 °C and says nothing directly about its viscosity anywhere else. SAE grades are a different scheme measured at different temperatures.
Because doing it properly needs data this tool does not have. ASTM D2270 requires tabulated L and H values for ν₁₀₀ below 70 cSt; the published polynomial form is only valid above 70 and returns a negative L for an ISO VG 46, which is nonsense. Rather than print a number dressed up as a standard, the tool shows the raw ν₄₀/ν₁₀₀ ratio — the same information, without the false authority.
Each fluid uses the relation that fits it. Oils use ASTM D341, the industry’s own, fitted through the two points every datasheet quotes. Water and seawater use a Vogel correlation, within 2% of published values from 0 to 100 °C. Thin liquids use Andrade, within 1% for ethanol and 5% for mercury. They are exact at 40 and 100 °C by construction and best near them.
Because the fluid does. Each one carries its own valid range — freezing and boiling points, pour points, spec limits — and outside it no correlation means anything. Water stops at 0 and 100 °C; heavy fuel oil does not start until 30 °C, because below that it is a solid rather than a slow liquid.
No. Use the actual product datasheet. These are representative values for common fluid types, and real products vary with additive package, batch and age — honey, olive oil and heavy fuel oil especially. This is for understanding and sizing intuition, not for signing anything off.
This page has changed in 157 archived releases, but each of those was a site-wide pass, so none is attributable to this tool on its own and none is listed here. That is not a claim that the tool never changed — a release that reworked many pages at once may well have altered this one too. The changelog has them.