Definitions

# French units of measurement

In France, before the decimalised metric system of 1799, a well-defined old system existed, however with some local variants. For instance, the lieue could vary from 3.268 km in Beauce to 5.849 km in Provence. Between 1812 and 1839, many of the traditional units continued in metrified adaptations as the mesures usuelles.

## Length

The French law for the definitive metre of 1799 states that one decimal metre is exactly 443.296 French lines, or 3 French feet, 0 French inches and 11.296 French lines. The French royal foot is exactly 9000/27706 metres, or about 0.3248 metres.

In Quebec, the surveys in French units were converted using the relationship 1 pied (of the French variety; the same word is used for English feet as well) = 12.789 English inches. This makes the Quebec pied very slightly smaller (about 4 parts in one million) than the pied used in France.

This article uses the Paris definitions, although the difference is not significant for the level of precision available at the time. In addition, the changing definitions of the metric units since 1799 means that even the precise relationship between French feet and metres may no longer be so accurate.

Table of length units
Unit Relative
value
SI
value
Imperial
value
Notes
point 1/12³ ~0.188 mm ~7.401 thou This unit is usually called the Truchet point in English.
ligne 1/12² ~2.256 mm ~88.81 thou This corresponds to the line, a traditional English unit.
pouce 1/12 ~27.07 mm ~1.066 in This corresponds to the inch, a traditional English unit.
pied du roi 1 ~32.48 cm ~1.066 ft Commonly abbreviated to 'Pied', this corresponds to the foot, a traditional English unit.
toise 6 ~1.949 m ~6.394 ft, or
~2.131 yd
This corresponds to the fathom, a traditional English unit. Unlike the fathom, it was used in both land and sea contexts.

Paris
perche d'arpent 22 ~7.146 m ~7.815 yd
arpent 220 ~71.46 m ~78.15 yd

lieue ancienne 10,000 ~3.248 km ~2.018 miles This is the old French league, defined as 10,000 (a myriad) feet. It was the official league until 1674.
lieue de Paris 12,000 ~3.898 km ~2.422 miles This league was defined in 1674 as exactly 2000 toises. After 1737, it was also called the "league of bridges and roads" (des Ponts et des Chaussées).
lieue des Postes 13,200 ~4.288 km ~2.664 miles This league is 2200 toises. It was created in 1737.
lieue tarifaire 14,400 ~4.678 km ~2.907 miles This league is 2400 toises. It was created in 1737.

North America
perche du roi 18 ~5.847 m ~6.394 yd This perch was used in Quebec and Louisiana
arpent 180 ~58.47 m ~63.94 yd

Local
perche ordinaire 20 ~6.497 m ~7.105 yd This perch was used locally.
arpent 200 ~64.97 m ~71.05 yd

• The French typographic point, the Didot point, was 1/72 French inches, i.e. two royal points. The French pica, called Cicéro, measured 12 Didot points.

## Area

Table of area units
Unit Relative
value
SI
value
Imperial
value
Notes
pied carré 1 ~1055 cm² ~1.136 sq ft This is the French square foot.
toise carrée 36 ~3.799 m² ~40.889 sq ft, or
~4.543 sq yd
This is the French square fathom.

Paris
perche d'arpent carrée 484 ~51.07 m² ~61.08 sq yd This was the main square perch in old French surveying. It is a square 22 feet on each side.
vergée 12,100 ~1277 m² ~1527 sq yd This is a square 5 perches on each side.
acre, or
arpent carré
48,400 ~5107 m² ~6108 sq yd, or
~1.262 acres
The French acre is a square 10 perches on each side.

North America
perche du roi carrée 324 ~34.19 m² ~40.89 sq yd This square perch was used in Quebec and Louisiana. It is a square 18 feet on each side.
vergée 8,100 ~854.7 m² ~1022 sq yd This is a square 5 perches on each side.
acre, or
arpent carré
32,400 ~3419 m² ~4089 sq yd, or
~0.8448 acres
This acre is a square 10 perches on each side. Certain U.S. states have their own official definitions for the (square) arpent, which vary slightly from this value.

Local
perche (ordinaire) carrée 400 ~42.21 m² ~50.48 sq yd This square perch was used locally. It is a square 20 feet on each side.
vergée 10,000 ~1055 m² ~1262 sq yd This is a square 5 perches on each side.
acre, or
arpent carré
40,000 ~4221 m² ~5048 sq yd, or
~1.043 acres
This acre is a square 10 perches on each side.

## Volume

### Liquid measures

Table of (liquid) volume units
Unit Relative
value
SI
value
U.S.
value
Imperial
value
Notes
roquille 1/32 ~29.75 ml
posson 1/8 ~119 ml
demiard 1/4 ~238 ml
chopine 1/2 ~476.1 ml
pinte 1 ~952.1 ml Although etymologically related to the English unit pint, the French pint is about twice as large. It was the main small unit in common use, and measured 1/36 of a cubic French foot.
velte 8 ~7.617 L
quartaut 72 ~68.55 L A quartaut is 9 veltes.
feuillette 144 ~137.1 L
muid 288 ~274.2 L The muid is defined as eight French cubic feet.

cubic
pouce cube 1/48 ~19.84 ml This is the French cubic inch.
pied cube 36 ~34.28 L This is the French cubic foot. In ancient times, a cubic foot was also known as an amphora when measuring liquid volume.

### Dry measures

Table of (dry) volume units
Unit Relative
value
SI
value
U.S.
value
Notes
litron 1/16 ~793.5 ml The SI unit litre is etymologically related to this unit.
quart 1/4 ~3.174 L
boisseau 1 ~12.7 L A boisseau was defined as 10/27 of a French cubic foot.
minot 3 ~38.09 L
mine 6 ~76.17 L
setier 12 ~152.3 L
muid 144 ~1828 L

cubic
pouce cube 1/640 ~19.84 ml This is the French cubic inch.
pied cube 2.7 ~34.28 L This is the French cubic foot.

## Mass

According to the law of 19 Frimaire An VIII (December 10, 1799),

• The kilogramme is equal to 18,827.15 grains. The kilogramme is, in addition, defined as the weight of 1 dm³ of distilled water at 4 degrees centigrade, i.e. at maximum density.

Traditionally, the French pound (livre) was defined as the mass of exactly of a French cubic foot of water. When the kilogramme was defined, knowledge that a pied du roi cube filled with water masses exactly 70 French pounds was apparently lost. According to the traditional (cubic foot) definition, one livre would have been about 489.675 grammes. According to the kilogramme definition, one livre was about 489.506 grammes. The difference is about 0.035%. However, a small difference in salinity (i.e. the difference between distilled water and very good quality drinking water) is enough to explain this difference.

The units in the following table are (except for the talent) calculated based on the kilogramme definition of the livre.

Table of mass units
Unit Relative
value
SI
value
Imperial
value
Notes
Poids de marc, mid 14th – late 18th century
prime 1/24³ once ~2.213 mg
grain 1/24² once ~53.11 mg ~0.8197 grains This is the French grain.
denier 1/24 once ~1.275 g ~19.67 grains
gros 1/8 once ~3.824 g ~2.158 dr
once 1/16 ~30.59 g ~1.079 oz This is the French ounce.
marc 1/2 ~244.8 g ~8.633 oz
livre 1 ~489.5 g ~1.079 lb This is the French pound.
quintal 100 ~48.95 kg ~107.9 lb This is the French hundredweight.

talent
talent ~70.02 ~34.28 kg ~75.57 lb This is the mass of one French cubic foot of water; this value is calculated based on the French cubic foot and an assumed water density of 1 g/cm³; other values in this table are based on the kilogramme definition.

bullion
felin 1/1280 ~382.4 mg ~5.902 grains
maille 1/640 ~764.9 mg ~11.8 grains
estelin 1/320 ~1.53 g ~23.61 grains