; symbol or abbreviation: ft
or, sometimes, ′
– the prime symbol
) is a non-SI
, in a number of different systems, including
, Imperial units
, and United States customary units
Its size can vary from system to system, but in each is around a
quarter to a third of a meter. The most commonly used foot today is
the international foot. There are three feet in a yard
and 12 inches
July 1, 1959 the length of the international yard in the United States and countries of the Commonwealth of Nations was defined
as 0.9144 meters.
the international foot is defined to be equal to 0.3048 meters
(equivalent to 304.8 millimeters
This was 2 ppm
the previous U.S definition and 1.7 ppm longer than the
previous British definition.
The international standard symbol for a foot is "ft" (see ISO 31-1
, Annex A). In some cases, the foot is
denoted by a prime
, which is often
approximated by an apostrophe
, and the
inch by a double prime
; for example,
2 feet 4 inches is sometimes denoted as 2′ 4″. This
use can cause confusion, because the prime and double prime are
also international standard symbols for arcminutes
time the international foot was defined in 1959, there was already
a huge amount of survey data which had been collected based on the
former definitions, especially in the United States and in India.
small difference between survey and international feet would not be
detectable on a survey of a small parcel, but becomes significant
for mapping, or when a state plane coordinate system
is used, because the origin of the system may be hundreds of miles
from the point of interest. Hence the previous definitions
continued in use for surveying in these two countries for many
years, and are denoted survey feet
them from the international foot. The United Kingdom was unaffected
by this problem, as the retriangulation of Great
(1936–62) was done in meters.
States survey foot is defined as exactly meters,
In 1986 the National Geodetic Survey
released the North American
of 1983, which underlies the state plane coordinate
systems and is entirely defined in meters. An NGS policy from 1991
has this to say about the units used with the new datum to define
the SPCS 83:
In preparation for the adjustment of the North American
Datum of 1983, 31 states enacted legislation for the State Plane
Coordinate System of 1983 (SPCS 83). All states defined
SPCS 83 with metric parameters. Within the legislation, the
U.S. Survey Foot was specified in 11 states and the
International Foot was specified in 6 states. In all other
states the meter is the only referenced unit of measure in the
SPCS 83 legislation. The remaining 19 states do not yet
have any legislation concerning SPCS 83.
Since that time, several states have abandoned the non-metric
versions of SPCS 83: seven states continue to keep location
data in survey feet as well as in meters, while an eighth keeps
data in international feet as well as in meters. State legislation
is also important for determining the conversion factor to be used
for everyday land surveying and real estate transactions, although
the difference (2 ppm
) is of
no practical significance given the precision of normal surveying
measurements over short distances (usually much less than a mile).
Twenty-four states have legislated that surveying measures should
be based on the U.S. survey foot, eight have legislated that they
be made on the basis of the international foot, and eighteen have
not specified the conversion factor from metric units.
The Indian survey foot is defined as exactly , presumably derived
from a measurement of the previous Indian standard of the yard. The
current National Topographic Database of the Survey of India
is based on the metric
which is also used by the Global Positioning System
In the United States, the foot was defined as 12 inches, with
the inch being defined by the Mendenhall Order
of 1893 by
39.37 inches = 1 m. In Imperial units
, the foot was defined as
yard, with the yard being realized as a physical standard
(separate from the standard meter). The yard standards of the
periodically compared with one another. The value of the United
Kingdom primary standard of the yard was determined in terms of the
meter by the National
in 1964 as , implying a pre-1959 foot in
the UK of .
Obsolete use in different countries
- 1 Amsterdam foot (voet) = .
- 1 Danish foot (after 1835) = .
- 1 French foot (pied du roi) = 12 pouces = .
- 1 Norwegian foot (after 1824) = .
- 1 Portuguese foot = .
- 1 Rotterdam foot = .
- 1 Russian foot (English foot borrowed by Peter Ι) = 12 inches = 1/7 Russian sazhens
- 1 Spanish foot (till 1752) (Pie (foot) de Ribera/de Rey) = 12
Pulgadas = .
- 1 Spanish foot (1752 to 1765) (Pie (foot) de Burgos/Castellano)
- 1 Spanish foot (after 1765) (Pie (foot) de Rey) = 12 Pulgadas =
- 1 Swedish foot (fot) = 12 inches (tum) = .
- 1 Venetian foot = .
- several different units by that name were in use in Poland:
- 1 Old Polish (or Warsaw) foot (stopa staropolska / stopa
warszawska, till 1819) =
- 1 Galician (or Lviv) foot (stopa galicyjska / stopa lwowska,
- 1 Wroclaw foot (stopa wrocławska, till 1816) =
- 1 Krakow foot (stopa krakowska, 1836-1857) =
The foot as a unit of measure was used in most Western cultures
and was usually divided
into 12 or sometimes 10 inches/thumbs, or into 16 fingers/digits.
known standard foot measure was from Sumer,
where a definition is given in a statue of Gudea of Lagash from around
2575 BC. Some metrologists
speculate that the imperial foot was adapted from an Egyptian measure by
the Greeks, with a subsequent larger
foot being adopted by the Romans.
The popular belief is that the original standard was the length of
a man's foot
. In rural regions and without
calibrated rulers, many units of measurement were in fact based on
the length of some part of body of the person measuring (or for
example the area that could be ploughed
day). In that sense, the human foot was no doubt the origin of the
measuring unit called a "foot" and was also for a long time the
definition of its length. To prevent discord and enable trade, many
towns decided on a standard length and displayed this publicly. In
order to enable simultaneous use of the different units of length
based on different parts of the human body and other "natural"
units of length, the different units were redefined as multiples of
each other, whereby their lengths no longer corresponded to the
original "natural" standards. This process of national standardization
began in Scotland in 1150 and
in England in 1303, but
many different regional standards had existed in both these
countries long before.
Some believe that the original measurement of the English foot was
from King Henry I
, who had a foot
12 inches long; he wished to standardize the unit of measurement in
England. However this is unlikely, because there are records of the
word being used approximately 70 years before his birth. This of
course does not exclude the possibility that this old standard was
redefined ("calibrated") according to the ruler's foot. In fact,
there is evidence that this sort of process was common before
standardization. A new, important ruler could try to impose a new
standard for an existing unit, but it is unlikely that any king's
foot was ever as long as the modern unit of measurement.
The average foot length is about 9.4 inches (240 mm) for
current Europeans. Approximately 99.6% of British men have a foot
that is less than 12 inches long. One attempt to "explain" the
"missing" inches is that the measure did not refer to a naked foot,
but to the length of footwear
, which could
theoretically add an inch or two to the naked foot's length. This
is consistent with the measure being convenient for practical uses
such as building sites. People almost always pace out lengths while
wearing shoes or boots, rather than removing them and pacing
There are however historical records of definitions of the inch
based on the width (not length) of a man's thumb that are very
precise for the standards of the time. One of these was based on an
average calculated using three men of different size, thereby
enabling surprising accuracy and uniformity throughout a country
even without calibrated rulers. It therefore seems likely that at
least since about the Twelfth century, the precise length of a foot
was in fact based on the inch, not the other way around. Since this
length was fairly close to the size of most feet, at least in
shoes, this enabled the above-mentioned use of one's shoes in
approximating lengths without measuring devices. This sort of
imprecise measuring that in addition excessively multiplied the
measuring error due to repeated use of a short "ruler
" (the foot) was never used in surveying
and in constructing more complicated
- BBC World Service
- National Physical Laboratory,
On what basis is one inch exactly equal to 25.4 mm?
Has the imperial inch been adjusted to give this exact fit and if
- A. V. Astin & H. Arnold Karo, (1959), Refinement of values for the yard and the
pound, Washington DC: National Bureau of Standards,
republished on National Geodetic Survey web site and the Federal
Register (Doc. 59-5442, Filed, June 30, 1959, 8:45 a.m.)
- National Geodetic Survey, (January
1991), " Policy of the National Geodetic Survey Concerning Units of
Measure for the State Plane Coordinate System of 1983.
- Schedule to the Standards of Weights and Measures Act,
of India, " National Map Policy – 2005".
- See, for example, Report on the Comparisons of the
Parliamentary Copies of the Imperial Standards with the Imperial
Standard Yard and the Imperial Standard Pound and with each other
during the Years 1947 to 1948 (H.M.S.O., London, 1950).
Report on the Comparisons of the Parliamentary Copies of the
Imperial Standards with each other during the Year 1957
(H.M.S.O., London, 1958).
- Laws Æthelstan