How this instrument works
Both units were born together. French Revolutionary reformers assembling the metric system in 1795 defined the are as a square 10 metres on a side — 100 square metres — and simply prefixed 'hecto-' to name a hundred of them a hectare, 10,000 square metres. Nothing about the relationship needed a treaty or a later correction, the way many customary-to-metric factors did; a hectare has been exactly 100 ares since the day the name was coined.
Neither unit is officially part of the SI, yet both remain what the International System calls units 'accepted for use with SI,' specifically reserved for measuring land area rather than any general length or volume. That special status is why a hectare survives everywhere from EU farm-subsidy paperwork to national forestry inventories, while the smaller are quietly persists on European property deeds and in garden allotment sizes too modest to describe sensibly in hectares.
In everyday use the are functions almost like a fine-grained ruler for the hectare: a Swiss vineyard parcel, a German community garden plot, or a French village allotment is commonly quoted in ares precisely because a single hectare would be far too large a unit to describe them usefully, the way describing a city apartment in square kilometres would be technically correct but practically useless.
- Enter the land area into Ares (are); the field opens at 50, exactly half a hectare.
- Read the converted size from Hectares (ha) beneath it, recalculated instantly to six decimal places.
- Working from a hectare figure instead? Multiply it by 100 to get ares — the reverse direction is equally exact.
- Large farm parcels convert just as cleanly: enter 1000 ares for a 10-hectare field.
Worked example — a 50-are smallholding
A market gardener outside Lyon farms a plot the local land registry lists as 50 ares. She needs the figure in hectares to fill in a farm-subsidy application form that only accepts that unit. Enter 50 into Ares (are) and Hectares (ha) reads 0.5 exactly — half a hectare, precisely the size that a 50-are plot has always been, since the relationship between the two units has never needed rounding.
The same division scales cleanly at any size: a single are, 100 square metres, comes to exactly 0.01 hectares, while a modest 7.5-are allotment garden works out to 0.075 hectares. None of those figures is an approximation — every decimal shown terminates because dividing by 100 can never do anything else to a number already written in base ten.
Questions
Is one hectare always exactly 100 ares?
Yes, with no exceptions and no rounding involved, because that is literally how the two units were defined. When the metric system was formalised in 1795, the are was fixed at 100 square metres and the hectare was defined as a hundred ares by simple prefix — hecto- meaning a hundred. Unlike many customary-to-metric conversions, this pair never needed a later treaty or refinement to become exact; it was exact from the day both names were coined.
Why isn't the are used more widely if it's part of the metric system?
Because it occupies an awkward size for most practical purposes. A single are, 100 square metres, is too small to usefully describe a farm and too large to be the natural unit for a house plot, which is instead usually given in plain square metres. It survives mainly for garden allotments, vineyard parcels and similar mid-sized holdings across parts of Europe, while agriculture at scale almost always defaults straight to hectares.
Is a hectare part of the International System of Units?
Not strictly — neither the hectare nor the are belongs to the SI proper. Both are officially classed by the BIPM as non-SI units accepted for continued use alongside SI, a category reserved for a handful of units, including the litre and the tonne, that are too deeply embedded in practical measurement to discard. That accepted status is precisely why hectares remain the standard unit for farm and forest area almost everywhere outside the United States.
How big is a hectare compared to something familiar?
About the size of a large sports stadium footprint, or roughly 2.471 acres — a square 100 metres on each side, somewhat larger than a full-size football pitch including its surrounding margins. Picturing an are as 1/100 of that same square, a strip roughly 10 metres by 10 metres, gives a reasonably intuitive sense of scale for the smaller unit as well.
How do I convert ares or hectares into acres?
Multiply hectares by roughly 2.47105381467 to get acres, since one hectare equals that many acres exactly (a non-terminating decimal in practice, because the acre's own definition carries a factor of 11). Ares convert to acres the same way after first dividing by 100 to reach hectares — or by multiplying ares directly by 0.0247105381467, one hundredth of the hectare-to-acre figure.
Can this factor be applied to any size of land, or only round numbers?
Any size at all, since dividing by exactly 100 never introduces rounding regardless of how precise or oddly specific the starting figure is. A parcel of 137.4 ares converts to exactly 1.374 hectares, and a sprawling 12,450-are estate converts to exactly 124.5 hectares — the decimal point simply shifts two places, with every digit of the original number preserved.