 radian (rad)

a unit of angle measure. One radian is the angle at the center of a circle that
cuts off an arc of length equal to the radius, or 1/(2 pi)
of the circle, or approximately 57.295 779°.
 radiocarbon year (^{14}C year)

a unit used in stating the nominal ages of plant or animal remains dated by
radiocarbon testing. While a plant or animal is alive, the fraction of
radioactive carbon in its body remains equal to the fraction in the atmosphere
at that time. After a plant or animal dies, as the carbon14 trapped in its
body slowly decays, the age of the tissue can be measured by the fraction of
radioactive carbon remaining.
 rai

a traditional unit of land area in Thailand. The rai is now equal to 1600
square meters, which is 0.16 hectare or approximately 0.3954 acre. The rai is
divided into 4 ngan.
 Rankine

an absolute temperature scale, see degree Rankine.
 Réaumur

a temperature scale, see degree Réaumur.
 rebah

an ancient Hebrew unit of weight or mass equal to 1/4 shekel, means
"quarter" in Hebrew.
 register ton

a unit of cargo capacity equal to 100 cubic feet (about 2.832 cubic meters).
 ri

a Japanese unit of distance, called the Japanese league. The ri
equals 2160 ken or 12 960 shaku (the shaku is the Japanese foot). This is
about 3927 meters or 2.44 statute miles.
 right angle

a unit of angle measure equal to 1/4 circle, 90°, 100 grads, or
pi/2 radians.
 rod (rd)

a traditional unit of distance equal to 5.5 yards (16 feet 6 inches
or 5.0292 meters). The rod and the furlong were the basic distance units
used by the AngloSaxon residents of England before the Norman conquest of
1066. The Normans preferred to call it as a pole or a perch
(a word of French origin, meaning a pole; see perche). The relationship
between the rod and the other English distance units was confirmed again by the
Parliamentary statute of 1592, which defined the statute mile to be either 320
rods or 1760 yards, thus forcing the rod to equal exactly 5.5 yards or 16.5
feet.
 rood

an old unit of distance, another name for a rod in Dutch, but in old England
and Scotland the rood was often longer than a rod (16.5 feet),
sometimes it was 20 feet, 21 feet, or even 24 feet. In South Africa, the rood
was standardized to be equal to 12 Cape feet, which is 12.396 English feet or
3.7783 meters.

a traditional unit of area used to measure land. A rood is the area of a narrow
strip of land one furlong (40 rods, or 660 feet) long and one rod (16.5 feet)
wide, equal to 40 square rods (or perches), which equals 1210 square yards, or
10 890 square feet, or exactly 1/4 acre.
 rope

a traditional distance unit, another name for the rood.
 round

the basic unit of time in boxing, equals to 3 minutes.
 royal foot

the French pied de roi , called the Paris foot in
English.
 running foot

another name for a linear foot.
 rute

a traditional German unit of distance corresponding to the English rod. This
could be anywhere from about 3 meters to 4.5 meters.
