Acres to hectares
One acre is 0.40468564224 hectares, and this shows the working rather than just the answer. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
The factor
One acre is 0.40468564224 hectares. That is the whole of the conversion: multiply by it, and nothing else happens.
And it is exact
Not rounded, not measured, not a convention that might be revised. The factor above is the definition, so an answer from it is as exact as the number you typed.
The acre is 4,840 square yards, which is exactly 4,046.8564224 m².
The hectare is 10,000 m², or a square 100 m on a side.
The number in your head
Most people use 0.4 for this, which is the factor to two significant figures. It is low by 1.1578% — 11.578 hectares too few in every thousand, and 115.78 in every ten thousand.
Going the other way
The reverse is not this sum backwards in your head. It is a different factor, 2.471053814672, and dividing by one is not the same as multiplying by a rounded version of the other.
Rounded, the way back is 2.47, which is low by 0.0426%. So the two roundings do not cancel: go out and back with both and you do not land where you started.
How many of those decimals mean anything
The box above will give you ten decimal places because it can. Whether they mean anything is decided by your measurement, not by the arithmetic.
If you measured to the nearest acre, your figure is already uncertain by half of one — so the answer is uncertain by about 0.202343 hectares, and every digit past that is decoration. Quoting 0.49960871 when you measured with a tape is claiming a precision the tape never had.
Where these two sit
Acres and hectares both measure area, which is why they can be converted at all. A conversion between families — a length into a weight — is not a conversion, it is a question with a missing quantity in it.