# Units and constants¶

The units of MDAnalysis trajectories are the Å (ångström) for length and ps (picosecond) for time. Regardless of how the original MD format stored the trajectory data, MDAnalysis converts it to MDAnalysis units when reading the data in, and converts back when writing the data out. Analysis classes generally also use these default units. Exceptions to the default units are always noted in the documentation; for example, mass densities can be given in $$g/cm^3$$.

Other base units are listed in the table Base units in MDAnalysis.

Base units in MDAnalysis

Quantity

Unit

SI units

length

Å

$$10^{-10}$$ m

time

ps

$$10^{-12}$$ s

energy

kJ/mol

$$1.66053892103219 \times 10^{-21}$$ J

charge

$$e$$

$$1.602176565 \times 10^{-19}$$ As

force

kJ/(mol·Å)

$$1.66053892103219 \times 10^{-11}$$ J/m

speed

Å/ps

$$100$$ m/s

mass

u

$$1.66053906660(50) \times 10^{-27}$$ kg

angle

degrees

$$\frac{\pi}{180}$$ rad

## Unit conversion¶

Quantities can be converted from units with convert(). convert() simply multiplies the initial quantity with a precomputed conversion factor, as obtained from get_conversion_factor().

The computed conversion factors for each quantity type are stored in MDAnalysis.units and shown below.

## Constants¶

### Length¶

Unit

Conversion factor

A

1

Angstrom

1

angstrom

1

femtometer

100000

fm

100000

nanometer

0.1

nm

0.1

picometer

100

pm

100

1

### Density¶

Unit

Conversion factor

A^{-3}

1

Angstrom^{-3}

1

Molar

1660.54

SPC

30.3718

TIP3P

29.8566

TIP4P

29.8864

nanometer^{-3}

1000

nm^{-3}

1000

water

30.0063

Å^{-3}

1

### Time¶

Unit

Conversion factor

AKMA

20.4548

femtosecond

1000

fs

1000

nanosecond

0.001

ns

0.001

picosecond

1

ps

1

s

1e-12

sec

1e-12

second

1e-12

### Charge¶

Unit

Conversion factor

Amber

18.2223

As

1.60218e-19

C

1.60218e-19

e

1

### Speed¶

Unit

Conversion factor

A/ps

1

Angstrom/AKMA

0.0488882

Angstrom/femtosecond

1000

Angstrom/fs

1000

Angstrom/picosecond

1

Angstrom/ps

1

angstrom/femtosecond

1000

angstrom/fs

1000

angstrom/picosecond

1

m/s

100

nanometer/picosecond

0.1

nanometer/ps

0.1

nm/ns

100

nm/ps

0.1

pm/ps

100

Å/ps

1

### Force¶

Unit

Conversion factor

J/m

1.66054e-11

N

1.66054e-11

Newton

1.66054e-11

kJ/(mol*A)

1

kJ/(mol*Angstrom)

1

kJ/(mol*nm)

10

kJ/(mol*Å)

1

kcal/(mol*Angstrom)

0.239006

### Energy¶

Unit

Conversion factor

J

1.66054e-21

eV

0.0103643

kJ/mol

1

kcal/mol

0.239006