Units based on fundamental constants

Non-SI units whose values in SI units must be obtained experimentally

Name	Symbol	Quantity	Value in SI units
electronvolt	eV	energy	1.602 176 634 × 10−19 J
dalton, unified atomic mass unit	Da u	mass	1.660 539 040 (20) × 10−27 kg
astronomical unit	ua	length	149 597 870 700  m (exact)
Natural units	(n.u.)
n.u. of speed	c0	speed	299 792 458 m s-1 (exact)
n.u. of action	ħ	action	1.054 571 817 646 × 10−34 J s
n.u. of mass	me	mass	9.109 382 6 (16) × 10−31 kg
n.u. of time	ħ/(mec02)	time	1.288 088 667 7 (86) × 10−21 s
Atomic units	(a.u.)
a.u. of charge, (elementary charge)	e	charge	1.602 176 634 × 10−19 C
a.u. of mass, (electron mass)	me	mass	9.109 382 6 (16) × 10−31 kg
a.u. of action, (reduced Planck constant)	ħ	action	1.054 571 817 646 × 10−34 J s
a.u. of length, bohr, (Bohr radius)	a0	length	0.529 177 210 8 (18) × 10−10 m
a.u. of energy, hartree, (Hartree energy)	Eh	energy	4.359 744 17 (75) × 10−18 J
a.u. of time	ħ/Eh	time	2.418 884 326 505 (16) × 10−17 s