Bar to Millibar Converter
Common Conversions
| bar | mbar |
|---|---|
| 0.001 | 1 |
| 0.01 | 10 |
| 0.05 | 50 |
| 0.1 | 100 |
| 0.25 | 250 |
| 0.5 | 500 |
| 1 | 1000 |
| 2 | 2000 |
| 5 | 5000 |
| 10 | 10000 |
| 50 | 50000 |
| 100 | 100000 |
Why this conversion matters in chemistry
Solvent vapor-pressure tables and reactor ratings live in bar; rotary-evaporator controllers, freeze-dryer setpoints, and most vacuum gauges read in millibar. The factor between them is exactly 1000. A 0.150 bar setpoint for ethanol removal at 40 °C reads as 150 mbar on the rotavap controller — the unit with mbar resolution that's better matched to the 10–500 mbar working range of solvent distillation than whole-bar steps would be.
Formula
Where the factor comes from
The bar is not an SI unit, yet it takes SI prefixes exactly as SI units do, and that is the entirety of this conversion. Milli is defined as exactly 10⁻³, so a bar contains exactly 1000 millibars. No second unit definition enters, and no ratio between two independently fixed constants has to be worked out — the two names denote one quantity at two scales, which makes the multiplier an integer by construction. Routing through pascals confirms it without adding anything: a bar is exactly 10⁵ Pa and a millibar exactly 10² Pa, giving 10⁵ ÷ 10² = 1000. That a millibar coincides exactly with a hectopascal falls out of the same arithmetic, which is why barometric readings can be relabeled between the two with no calculation at all.
Precision and significant figures
Every digit survives the trip. The factor is a power of ten, so the decimal point moves three places and nothing else changes: 0.0250 bar and 25.0 mbar carry the same three figures. Worth saying, because the millibar form can look more precise than it is — writing 25 mbar rather than 25.0 quietly discards a figure the bar value was carrying. Where precision genuinely degrades is the display. A controller showing whole millibars quantizes in 1 mbar steps, a fraction of a percent at 500 mbar but a full ten percent at 10 mbar. Below roughly 20 mbar the readout's resolution, not the conversion, limits what you can honestly write.
Worked Examples
The IUPAC standard pressure expressed at the mbar scale most gauges actually read.
One standard atmosphere — the sea-level baseline for any barometric work.
Half-bar reduced pressure, in the working range of vacuum distillation for moderately volatile solvents.
About the elevated pressure inside a moderately pressurized reactor or autoclave.
Common mistakes
Negative bar readings are gauge, not absolute
Dials that show vacuum as a negative bar value are reporting gauge pressure. A reading of −0.9 bar converts arithmetically to −900 mbar, which is meaningless as an absolute pressure; against a 1013 mbar ambient the actual absolute pressure is about 113 mbar. The conversion is fine, but it has to be applied after the ambient reference is added back, not before.
Millibar and micron differ by 750
Vacuum manifolds often carry one gauge reading millibar and another reading microns, where a micron is a thousandth of a torr. One millibar is about 750 microns, not 1000. Converting bar to millibar and then comparing the result against a micron display as though the scales matched misjudges the pressure by a third, in whichever direction the comparison ran.
Exact arithmetic on a temperature-dependent value
A solvent's vapor pressure converted from bar to millibar is exact to as many figures as you like, but the underlying number was tabulated at one temperature. Choosing a setpoint from a value quoted at 20 °C and running the bath at 40 °C changes what actually distils. The precision of the conversion says nothing about whether the pressure is the right one.