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The Toise is an historical French unit of length, used extensively before the adoption of the metric system. It was legally defined as 6 pieds du roi (French royal feet).

Based on the standard established for the pied du roi, the Toise equates to:

• Exactly 1.94903631 meters (m) (based on the 1799 definition of the meter relative to the standard Toise bar)
• Approximately 6.395 English feet (ft)
• Approximately 76.73 English inches (in)

The Toise was further subdivided:

• 1 Toise = 6 Pieds (French feet)
• 1 Toise = 72 Pouces (French inches)
• 1 Toise = 864 Lignes (French lines)

The Toise was a fundamental unit of length in France for centuries, used in various fields from the Middle Ages through the Ancien Régime and into the early post-revolutionary period. Its name derives from the Latin tensa, meaning "stretched (arms)", suggesting an origin related to fathom.

Standardization was crucial. The primary standard became the Toise du Châtelet, an iron bar embedded in the wall of the Grand Châtelet fortress in Paris. In 1668, this standard was physically copied by Jean Picard and used for geodetic surveys. Later, a copy known as the Toise du Pérou (or Toise de l'Académie) became the official reference standard for French length measurements. This specific bar was physically used in the famous French Geodesic Missions to Peru (1735–1744) and Lapland (1736–1737), which aimed to measure the length of a degree of latitude near the equator and the Arctic Circle, respectively.

These measurements, based on the Toise, were instrumental in determining the shape of the Earth and ultimately provided the basis for the definition of the meter. The meter was initially defined in the 1790s as one ten-millionth of the distance from the North Pole to the Equator along the meridian passing through Paris, a distance calculated from the geodetic surveys performed using the Toise standard.

The Toise was officially replaced by the meter following the French Revolution and the establishment of the metric system in France, formally adopted in 1799.

The Toise is now obsolete but was historically essential for:

• Land Surveying: Measuring land areas, property boundaries, and distances for cartography.
• Architecture and Civil Engineering: Designing and constructing buildings, bridges, canals, roads, and fortifications (notably by Vauban). Dimensions of major historical French structures were often specified in Toise.
• Military Engineering: Laying out fortifications, measuring artillery ranges.
• Geodesy and Scientific Measurement: Serving as the standard unit for precise scientific measurements, particularly in astronomy and the crucial 18th-century geodetic surveys that measured the Earth.
• Craftsmanship: Used in various trades requiring length measurements.