You can convert units of length to units of length, and units of area to units of area. But it doesn't make sense to convert between units of length and units of area.
It doesn't make sense to convert that. You can convert units of length to units of length, for example; but you can't convert a pure number to a length.
It doesn't make sense to convert that. You can only convert units of the same kind, e.g., units of length to units of length; units of area to units of area; units of time to units of time, etc.
You can't convert that. You can only convert units that measure the same type of thing - for example, units of length to units of length, units of mass to units of mass, units of time to units of time, etc.
It depends on what you consider to be "customary" and what you wish to convert them to.
You can convert units of length to units of length, and units of area to units of area. But it doesn't make sense to convert between units of length and units of area.
It doesn't make sense to convert that. You can convert units of length to units of length, for example; but you can't convert a pure number to a length.
It doesn't make sense to convert that. You can only convert units of the same kind, e.g., units of length to units of length; units of area to units of area; units of time to units of time, etc.
It doesn't make sense to convert units that measure completely different things. You can only convert units of length to units of length, units of mass to units of mass, etc.
You can't convert that. You can only convert units that measure the same type of thing - for example, units of length to units of length, units of mass to units of mass, units of time to units of time, etc.
Units of weight do not convert into units of length. Weight and length are different things.
It depends on what you consider to be "customary" and what you wish to convert them to.
You cannot convert millilitres to metres. Millilitres are units of volume and metres are units of length or distance.
You can't convert between units of length and units of area.
You can't convert units of length to units of volume.
You can't convert units of area to units of length! You can, however, convert a hectare to square meters, since both are units of area.
It doesn't make sense to convert from units of mass to units of length.