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Millibar to Atmospheres Converter
↔ Convert atm to mbar instead

Common Conversions

mbar atm
0.1 0.0000987
1 0.000987
10 0.00987
50 0.04935
100 0.0987
200 0.1974
500 0.4935
750 0.7402
1000 0.9869
1013.25 1
2000 1.974
5000 4.935

Why this conversion matters in chemistry

Millibar is the native unit on most modern vacuum-pump gauges and rotary evaporators, partly because it keeps the numbers in a readable range (a moderate rotovap vacuum sits around 100 mbar, a deep oil-pump vacuum around 10⁻² mbar). Atm is the unit most gas-law calculations still default to. Converting is a divide by 1013.25 — so 100 mbar is 0.099 atm, 500 mbar is 0.494 atm. The factor is slightly awkward, but the math is uneventful. Most of the time the question isn't how to convert but whether to — some calculations are cleaner in mbar than atm, and pressure ratios don't care which unit you pick as long as you're consistent.

Formula

atm = mbar / 1013.25

Worked Examples

1013.25 mbar = 1 atm

The defining anchor. Sea-level atmospheric pressure in both units.

1 mbar = 0.000987 atm

A deep rough vacuum — the kind of pressure a good oil-free diaphragm pump with multiple stages can reach. Well below what water aspirators can manage (those bottom out near the water's vapor pressure, around 20 mbar at room temperature).

100 mbar = 0.0987 atm

Typical rotary-evaporator working pressure for moderate-boiling solvents. Enough vacuum to pull ethanol or water off at benchtop temperatures.

500 mbar = 0.4935 atm

Half an atmosphere. Roughly the ambient pressure at 5500 m elevation — relevant when thinking about boiling-point depression for high-altitude work.

Frequently Asked Questions

How do I convert mbar to atm?
Divide by 1013.25. So 500 mbar is 0.494 atm, 100 mbar is 0.099 atm. The factor isn't clean — it's a historical legacy of how the atmosphere was originally defined — so this is one of the conversions where a calculator is usually quicker than mental math.
What is a millibar?
One thousandth of a bar, or 100 Pa, or 1 hectopascal. Sea-level atmospheric pressure is 1013.25 mbar. The unit is common in meteorology (weather reports are usually in hPa, which is the same number as mbar) and on most modern vacuum-pump displays.
Where do chemists encounter millibar?
On vacuum equipment, mostly. Rotary vane pumps, rotary evaporators, freeze dryers, vacuum ovens — all typically show their working pressure in mbar. Lyophilization for proteins and pharmaceuticals runs at roughly 0.05 to 0.5 mbar depending on the formulation and collapse temperature.
How does mbar relate to other pressure units?
1 mbar is 100 Pa, 0.1 kPa, 0.75 torr, and about 0.001 atm. It's convenient for vacuum work precisely because it avoids the tiny-decimal territory you end up in if you try to describe the same pressures in atm.