Conversion Tables for Units
Chapter 1

3
Name Symbol Relation to SI
mass, m
kilogram (SI unit) kg
gram (cgs unit) g = 10
3
kg
electron mass (au) m
e
≈
9.109 39
×
10
31
kg
unified atomic mass u, Da = m
a
(
12
C)/12
≈
1.660 540
×
10
27
kg
unit, daltonS
gamma
γ
= µg
tonne t = Mg = 10
3
kg
pound (avoirdupois) lb = 0.453 592 37 kg
ounce (avoirdupois) oz
≈
28.3495 g
ounce (troy) oz (trou)
≈
31.1035 g
grain gr = 64.798 91 mg
time, t
second (SI, cgs unit) s
au of time h/E
h
≈
2.418 88
×
10
17
s
minute min = 60 s
hour h = 3600 s
day
1
d = 86 400 s
year
2
a
≈
31 556 952 s
svedberg Sv = 10
13
s
(1) Note that the day is not exactly in terms of the second since socalled leapseconds are
added or subtracted from the day semiannually in order to keep the annual average
occurrence of midnight at 24:00 on the clock.
(2) The year is not commensurable with the day and not a constant. Prior to 1967, when the
atomic standard was introduced, the tropical year 1900 served as the basis for the
definition of the second. For the epoch 1900.0. it amounted to 365.242 198 79 d
≈
31
556 925.975 s and it decreases by 0.530 seconds per century. The calendar years are
exactly defined in terms of the day:
Julian year = 365.25 d
Gregorian year = 365.2425 d.
The definition in the table corresponds to the Gregorian year. This is an average based
on a year of length 365 days, with leap years of 366 days; leap years are taken either
when the year is divisible by 4 but is not divisible by 100, or when the year is divisible
by 400. Whether the year 3200 should be a leap year is still open, but this does not have
to be resolved until sometime in the middle of the 32nd century.
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