ConstantID

Reduced Planck constant times c in MeV fm

UniversalUnitsExact

ℏc = 197.326 9804… MeV fm

Value
197.326 9804… MeV fm
Standard uncertainty
exact
Relative uncertainty
exact
Status
Exact since the 2019 SI redefinition
Symbol
ℏc
Source
CODATA 2022 via NIST · NIST entry

NIST prints this exact value cut to 10 digits. ConstantID recomputes it from the defining constants: 197.326 980 459 302 465 890 894 408 187…

Use in a calculation

# reduced Planck constant times c in MeV fm, CODATA 2022 (NIST)
reduced_planck_constant_times_c_in_mev_f = 1.973269804593024658908944081867211e2  # MeV fm, exact

# SciPy: scipy.constants.physical_constants['reduced Planck constant times c in MeV fm']

What it is

ħ times c, about 197.3 MeV·fm: the factor nuclear and particle physicists use to turn energies into lengths and back. Exact since 2019.

It is built only from constants that have exact values since 20 May 2019, so it is exact too; CODATA 2018 was the first adjustment to list it that way.

In other units

UnitValue
CGS (cm3 g s−2)3.161 526 773 × 10−17exact

Converted with ConstantID's unit table; digits are rounded to the precision of the value, and a long exact value is rounded only for display.

How the value changed, 1998–2022

Each point is one CODATA adjustment, plotted as its difference from the 2022 value; the bars show one standard uncertainty (an arrowhead means the bar runs off the chart). Diamonds mark adjustments in which the value was exact.

reduced Planck constant times c in MeV fm: CODATA values 1998–2022 relative to the 2022 value, in parts per billion, with one-standard-uncertainty bars−150−100−500+50+100+150parts per billion1998200220062010201420182022

Relative uncertainty

reduced Planck constant times c in MeV fm: relative standard uncertainty in each CODATA adjustment, logarithmic scale10⁻⁹10⁻⁸10⁻⁷199820022006201020142018exact2022exact
CODATAValue (uncertainty)Relative uncertaintyChange from previous
2022197.326 9804…exact exact
2018197.326 9804…exact +1.3 σ of 2014
2014197.326 9788(12)6.1 × 10−9 +1.6 σ of 2010
2010197.326 9718(44)2.2 × 10−8 +1.8 σ of 2006
2006197.326 9631(49)2.5 × 10−8 −0.3 σ of 2002
2002197.326 968(17)8.6 × 10−8

“Change from previous” is the shift in the value divided by the standard uncertainty of the previous adjustment. Earlier values are converted into the current unit where NIST changed it (for example J s to J Hz⁻¹).

Related constants

Universal exact ℏ Reduced Planck constant 1.054 571 817… × 10−34 J s