A standard atmosphere is exactly 101,325 pascals.
Atmospheres to Millibars Converter — atm to mbar
Convert Atmosphere (atm) to Millibar (mbar) using the exact conversion factor (1 atmosphere = 1,013.25 millibar). See the formula, worked examples, conversion table, and common uses.
Atmospheres to Millibars converter
Converter
Pressure Converter
Convert pascals, bars, PSI, atmospheres, mercury columns, water columns, and engineering pressure units.
Context values
Some unit families need one extra value for a physically accurate result.
A millibar is 100 pascals.
Result = input x 101325 / 100.
Pressure uses the pascal as the base unit.
Reserved above the conversion cards and below the converter.
About Converting Atmospheres to Millibars
Atmosphere and Millibar both show up on pressure gauges — psi is standard on US tire and tool specifications, while bar is the everyday pressure unit across most of the rest of the world. Atmosphere is most often used for chemistry (standard conditions are often defined at 1 atm).
Formula
millibars = atmospheres × 1013.25
This factor comes from each unit's defined relationship to the category's base unit: 1 atmosphere equals 101325 base units, and 1 millibar equals 100 base units, so dividing one by the other gives the direct atmosphere-to-millibar factor of 1013.25.
Simple example
1 atm × 1013.25 = 1,013.25 mbar
1 atmosphere = 1,013.25 millibars.
Real-world example
35 atm × 1013.25 = 35,463.75 mbar
35 atmospheres = 35,463.75 millibars.
Conversion table
| Atmosphere (atm) | Millibar (mbar) |
|---|---|
| 1 atm | 1,013.25 mbar |
| 5 atm | 5,066.25 mbar |
| 10 atm | 10,132.5 mbar |
| 25 atm | 25,331.25 mbar |
| 50 atm | 50,662.5 mbar |
| 100 atm | 101,325 mbar |
Reverse conversion: Millibar to Atmosphere
1 mbar × 0.0009869232667 = 0.0009869233 atm
1 millibar = 0.0009869233 atmospheres.
atmospheres = millibars × 0.000986923266716
For a page dedicated to this direction, see Millibar to Atmosphere.
Understanding the Atmosphere
Atmosphere (atm) measures pressure. Defined as exactly 101,325 pascals, originally based on the average atmospheric pressure at mean sea level. It is classified as a non-SI unit accepted for use with the SI.
Where it's used: Widely used internationally in chemistry, meteorology, and diving/aviation contexts alongside the pascal and bar
Uses of the Atmosphere today
- Chemistry (standard conditions are often defined at 1 atm)
- Diving and aviation pressure references
- General reference for everyday atmospheric pressure
Understanding the Millibar (mbar)
A millibar is 100 pascals.
Atmosphere — sources
- BIPM SI Brochure — Standard atmosphere listed among non-SI units with a fixed exact value in pascals.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many millibars are in 1 atmosphere?
1 atmosphere equals 1,013.25 millibars, using the exact defined conversion factor rather than an estimate.
How do I convert atmosphere to millibar?
Multiply the atmosphere value by 1013.25. The converter above does this instantly to whatever precision you set.
How do I convert millibar back to atmosphere?
Use the reverse factor: 1 millibar equals 0.0009869233 atmospheres. You can also use the swap control in the converter above to flip the direction instantly.
What is a atmosphere?
Atmosphere (atm) measures pressure. Defined as exactly 101,325 pascals, originally based on the average atmospheric pressure at mean sea level. It is classified as a non-SI unit accepted for use with the SI.
What is a millibar?
Millibar (mbar) is a unit of pressure.
Is the atmosphere to millibar conversion exact?
Yes. Defined as exactly 101,325 pascals, originally based on the average atmospheric pressure at mean sea level. That fixed definition is what the converter above uses, so any rounding you see is a display choice, not a limitation of the math.
Where is the atmosphere still used today?
Widely used internationally in chemistry, meteorology, and diving/aviation contexts alongside the pascal and bar