When obtaining a cadastral statement or a notarial deed, the land area is almost always expressed in hectares, ares, and centiares. The problem is that real estate listing portals, closing quotes, and architectural plans work in square meters. This leads to juggling between two systems, sometimes on the corner of a table, at the moment of signing a compromise.
The conversion itself is simple once you understand the relationship between each unit. What complicates things in practice is reading the triplet ha/a/ca written on official documents and recomposing it into a single value in m².
Reading the triplet ha, a, ca correctly on a notarial deed
On an authentic deed, the area often appears in the form “02 ha 14 a 55 ca”. These are not three distinct areas; it is a single area divided into three ranks. Each rank corresponds to a factor of 100 relative to the next.
The centiare (ca) equals 1 m². The are (a) equals 100 m². The hectare (ha) equals 10,000 m². The logic is the same as for hours, minutes, and seconds: you do not add them directly; you first convert them into the same unit.
To know how to convert hectares, ares, and centiares into square meters, you apply the following formula: (ha × 10,000) + (a × 100) + ca. For our example, this gives (2 × 10,000) + (14 × 100) + 55 = 21,455 m².
The most common mistake is to place the numbers side by side like a postal code. With “02 ha 14 a 55 ca”, some mechanically write 21,455 and happen to be correct by chance. With “3 ha 7 a 9 ca”, the same method would yield 379 instead of 30,709. Always multiply each rank by its coefficient before adding.
Conversion table method to square meters
On the ground, when dealing with multiple plots in a land file, mental calculation quickly reaches its limits. The conversion table then becomes the most reliable tool.
Building the table in two minutes
Draw seven columns corresponding to the area units of the metric system: km², hm² (hectare), dam² (are), m² (centiare), dm², cm², mm². Each column occupies two number columns, except for km², which only takes one in practice for common areas.
Place the number of centiares in the m² column, the number of ares in the dam² column, and the number of hectares in the hm² column. Fill the empty boxes with zeros. Reading from left to right gives the value in m².
Concrete example with an agricultural plot
Let’s take a plot noted “0 ha 85 a 30 ca”:
- Hectares (hm²): 0, so no number in this column
- Ares (dam²): 85, place 8 then 5 in the two dam² boxes
- Centiares (m²): 30, place 3 then 0 in the two m² boxes
Reading: 8,530 m². We can verify by calculation: (0 × 10,000) + (85 × 100) + 30 = 8,530 m². The two methods agree.
Why real estate portals require m²
According to the Ministry of Ecological Transition, the median area of land purchased for building a single-family home has decreased by about 18% over ten years, reaching 670 m² during the period 2019-2022. Therefore, we are increasingly working on plots where the hectare has no visual meaning for the buyer.
The search filters of the main listing portals operate exclusively in m². A plot of 12 a 50 ca (1,250 m²) that would be listed in ares simply will not appear in the results of a buyer filtering between 1,000 and 1,500 m². Providing the area in square meters is not just a presentation detail; it is a condition for the visibility of the listing.
For real estate professionals, the recommendation is clear: always convert the notarial data into m² before publication, even if the source document uses the ha/a/ca format.
Inverse conversion: going from m² to hectares, ares, and centiares
This situation arises when measuring a plot with an online mapping tool (digital cadastre, area measurement tool) and needing to enter the result in an administrative form in the ha/a/ca format.
The method relies on successive divisions:
- Divide the area in m² by 10,000 to obtain hectares. The whole quotient gives the ha
- Take the remainder and divide by 100 to obtain ares. The whole quotient gives the a
- The final remainder corresponds to centiares
With 21,455 m²: 21,455 ÷ 10,000 = 2 (remainder 1,455). 1,455 ÷ 100 = 14 (remainder 55). Result: 2 ha 14 a 55 ca. We arrive back at the starting number.
Common trap with decimals
When the area from a GIS shows 3,427.6 m², we first round to the nearest whole m² (3,428 m²) before decomposing. Since the centiare is already equal to 1 m², any decimal below the m² has no equivalent in the ha/a/ca system. Returns on this point vary among notaries, but the dominant practice is to round to the nearest centiare.
Check your conversion with the online cadastre
The cadastre website (cadastre.gouv.fr) displays the cadastral area of each plot in ha/a/ca. Meanwhile, the Google Maps measurement tool allows you to trace the outline of a plot and obtain an area in m². Comparing the two results is a good verification reflex, especially before a transaction.
Discrepancies between cadastral area and actual measurement exist. The cadastral area is a fiscal data, not a surveyor’s report. For older plots, differences of several dozen m² are possible. Only a boundary survey by a certified surveyor is valid in case of a dispute over property boundaries.
Maintaining the reflex to systematically convert, verify through two different paths, and distinguish between cadastral area and actual area helps avoid most errors that delay sales compromises.