When describing the exact size of an object in French, you use a specific formula linking the object, its measurement, and its dimension using the preposition de.
The dimension formula
To describe an object's physical dimensions, use this pattern: [Object] + de + [Measurement] + de + [Dimension Noun]
For example, to express "a wall three meters high", you literally say "a wall of three meters of height":
| He wants a wall three meters high. | Il veut un mur de trois mètres de hauteur. |
This pattern works with any dimension noun, such as diamètre (diameter), profondeur (depth), or l'épaisseur (thickness). Notice that de becomes d' before a vowel:
| I need a board two centimeters thick. | J'ai besoin d'une planche de deux centimètres d'épaisseur. |
| They bought a pipe ten centimeters in diameter. | Ils ont acheté un tuyau de dix centimètres de diamètre. |
| It is a well twenty meters deep. | C'est un puits de vingt mètres de profondeur. |
Using adjectives for dimensions
For length, width, and height, you can often replace the dimension noun with a masculine singular adjective: long (long), large (wide), or haut (high).
The structural formula remains the same:
| We need a beam four meters long. | Nous avons besoin d'une poutre de quatre mètres de long. |
| He dug a ditch two meters wide. | Il a creusé un fossé de deux mètres de large. |
| They are building a pillar five meters high. | Ils construisent un pilier de cinq mètres de haut. |
Discussing dimensions generally
When you aren't giving an exact measurement with de, you simply use these dimension words as normal nouns to talk about an object's size or superficie (surface area).
| The area of our garden is small. | La superficie de notre jardin est petite. |
| The height of this pillar is impressive. | La hauteur de ce pilier est impressionnante. |
