diff --git a/README.rst b/README.rst
index 335fb87..b163c47 100644
--- a/README.rst
+++ b/README.rst
@@ -55,6 +55,9 @@ It is largely based on the international system of units (`SI`_)
 
 but augments it with handy definitions, changing the basic length and time units.
 
+Note that many units are now *exact*, such as the speed of light in vacuum.
+The package is in agreement with the values in the 2020 Particle Data Group review.
+
 .. _CLHEP: http://proj-clhep.web.cern.ch/proj-clhep/
 .. _SI: http://www.physics.nist.gov/cuu/Units/index.html
 
diff --git a/src/hepunits/constants/constants.py b/src/hepunits/constants/constants.py
index 5c7ccdf..79a94d4 100644
--- a/src/hepunits/constants/constants.py
+++ b/src/hepunits/constants/constants.py
@@ -29,7 +29,7 @@
 # Physical constants
 # -----------------------------------------------------------------------------
 
-# Speed of light in vacuum
+# Speed of light in vacuum (exact value, taken from PDG 2020)
 c_light = 299792458 * m / s
 c_light_sq = c_light * c_light
 
@@ -37,12 +37,15 @@
 eminus = -eplus  # electron charge
 e_sq = eplus * eplus
 
-# Avogadro constant (taken from CODATA)
-Avogadro = 6.022140857e23 / mole
+# Avogadro constant (exact value, taken from PDG 2020)
+Avogadro = 6.02214076e23 / mole
+
+# Planck constant (exact value, taken from PDG 2020)
+h_Planck = 6.62607015e-34 * joule * s
 
-# Planck constant (taken from CODATA)
-h_Planck = 6.626070040e-34 * joule * s
 hbar_Planck = h_Planck / two_pi
 hbar = hbar_Planck
+
 hbarc = hbar_Planck * c_light
+
 hbarc_sq = hbarc * hbarc
diff --git a/src/hepunits/units/units.py b/src/hepunits/units/units.py
index 6bb7e33..88b7477 100644
--- a/src/hepunits/units/units.py
+++ b/src/hepunits/units/units.py
@@ -126,12 +126,14 @@
 attobarn = _pre.atto * barn
 
 mb = millibarn
+ub = microbarn
 nb = nanobarn
 pb = picobarn
 fb = femtobarn
 ab = attobarn
 
 invmb = 1.0 / millibarn
+invub = 1.0 / microbarn
 invnb = 1.0 / nanobarn
 invpb = 1.0 / picobarn
 invfb = 1.0 / femtobarn
@@ -155,6 +157,7 @@
 
 s = second
 ms = millisecond
+us = microsecond
 ps = picosecond
 fs = femtosecond
 # shortcut "as = attosecond" not possible since "as" is a reserved word ;-)
@@ -250,7 +253,8 @@
 # --------------------------------------------------------------------
 
 # Positron charge [Coulomb]
-e_SI = 1.6021766208e-19  # taken from CODATA
+e_SI = 1.602176634e-19  # exact value, taken from PDG 2020
+
 
 # Electric charge [Q]
 # --------------------------------------------------------------------
@@ -268,7 +272,7 @@
 
 # Energy [E]
 # ----------
-joule = electronvolt / e_SI  # joule = 6.24150 e+12 * MeV
+joule = electronvolt / e_SI  # joule = 6.24151e+12 * MeV
 
 J = joule
 
@@ -350,17 +354,43 @@
 # Activity [T^-1]
 becquerel = 1.0 / second
 
+kilobecquerel = _pre.kilo * becquerel
+megabecquerel = _pre.mega * becquerel
+gigabecquerel = _pre.giga * becquerel
+
 Bq = becquerel
 
+kBq = kilobecquerel
+MBq = megabecquerel
+GBq = gigabecquerel
+
 curie = 3.7e10 * becquerel
 
+millicurie = _pre.milli * curie
+microcurie = _pre.micro * curie
+nanocurie = _pre.nano * curie
+
 Ci = curie
 
+mCi = millicurie
+uCi = microcurie
+nCi = nanocurie
+
 # Absorbed dose [L^2][T^-2]
 gray = joule / kilogram
 
+megagray = _pre.mega * gray
+kilogray = _pre.kilo * gray
+milligray = _pre.milli * gray
+microgray = _pre.micro * gray
+
 Gy = gray
 
+MGy = megagray
+kGy = kilogray
+mGy = milligray
+uGy = microgray
+
 # Dose equivalent
 sievert = joule / kilogram
 
diff --git a/tests/constants/test_constants.py b/tests/constants/test_constants.py
index 1285763..9b78310 100644
--- a/tests/constants/test_constants.py
+++ b/tests/constants/test_constants.py
@@ -13,7 +13,7 @@
 
 def test_constants():
     assert pi_sq == two_pi * half_pi
-    assert Avogadro == 6.022140857e23
+    assert Avogadro == 6.02214076e23
     assert c_light / (m / s) == 299792458
     assert hbarc_sq / c_light_sq == approx((h_Planck / two_pi) ** 2)
     assert hbar / (eV * s) == hbar / 1.0e3