What units is atomic mass measured in?

is measured in atomic mass units or amu.
Atomic mass units were taken from one proton or neutron of carbon-12 which each weigh 1 amu. Carbon-12 weighs exactly 12 amu. On you can see that carbon (C) has a number underneath it that is very close to 12. This is called the atomic mass.
To put this in different words 1 amu is equal to 1/12 the mass of a carbon-12 atom which has a mass of exactly 12 amu. For another example a normal helium atom has 2 electrons 2 protons and 2 neutrons. Because each proton and each neutron weights one amu a normal helium atom weighs 4 amu.
However atomic masses are slightly different than atomic mass units. Notice that on the periodic table atoms don’t have full numbers underneath them but very precise decimals. This is because atomic masses of as a whole are calculated by taking a weighted average of the different (or variations in terms of neutrons) of an element.
For example carbon has two naturally occurring isotopes carbon-12 (with 12 neutrons) and carbon-13 (with 13 neutrons). Carbon-12 is the form of 98.9% of naturally occurring carbon (weighing 12 amu) and carbon-13 is only the remaining 1.1% (weighing 13.003355 amu). To find the weighted average we multiply the percentage by the amu of each isotope.
(0.989)(12.000000 amu) + (0.011)(13.003355 amu) = 11.8680 amu + 0.1430369 amu = 12.011 amu the atomic mass of carbon as an element. Hope this helped!