I think it's written 'tonne'. And you should call it metric tonne if it's not clear from the context.

Wikipedia says:

The tonne is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton (United States customary units) and the long ton (British imperial units). The official SI unit is the megagram (symbol: Mg), a less common way to express the same amount.

https://en.wikipedia.org/wiki/Tonne

So yes, you can call it a megagramme and you'd be right. But we european people also sometimes do silly stuff and colloquially use wrong things. For example we also say it's 20 degrees celsius outside. And that's not the proper SI unit either. But that's kinda another topic.

No good reason, just historical inertia and resistance to change. People stick to what they're familiar with, either the imperial system or to common metric units. Making a "metric ton" similar in size to an "imperial ton" arguably helped make it easier for some people to transition to metric.

Megagram is a perfectly cromulent unit, just like "cromulent" is a perfectly cromulent word, but people still don't use it very often. That's just how language works. People use the words they prefer, and those words become common. Maybe if you start describing things in megagrams other people will also start doing it and it will become a common part of the language. Language is organic like that, there isn't anyone making decisions on its behalf, although some people and organizations try.

I'm all for megagram. If nothing, it will stop the senseless people that insists on using imperial unities from confusing everybody.

Two relevant details:

• The OG metric system (from the XVIII century) had no prefix for 10⁶. "Mega-" would be only formally acknowledged by the SI in 1960.
• The ton units (yup, plural) backtrack all the way to a volume unit from the Middle Ages, the amount of liquid that you'd be able to put in a big arse cask*

Based on those two things, I think that the ton was standardised to 10⁶g considerably before the name "megagram" had the chance to appear, to the point that it became the default name across languages.

*I don't know the English name for the cask [EDIT: "tun" acc. to @theplanlessman@feddit.uk ], but in Portuguese it's "tonel". From that "tonelada" (the unit). It used to be 800kg before the metric system though.

I brought a shit ton of tacos. Or I have supplied us with a faecal megagram of tacos. You be the judge.

Apparently megagram is the correct term! Someone else was just posting about another metric question and they posted some historical reasons for why megagram never took off.

That car weighs in at 6 megagrams.

Yes.

Mega pints are more fun.

Okay in all seriousness, though, the “ton” has been in use for far longer than the gram or the metric system .

More-common terms in any language tend to be shorter or at least less standard/more irregular.

Also, same issue as with MB and mb, you might confuse megagram with milligram

Although that's not really the reason, more like an argument to keep it this way

i do see Mg instead of t in publications sometimes

A really big grandmother

The official unit name is megagram. Ton is just used much more commonly.

I've never heard of gigameter or gigalitre

Why a feet and not a fivger ?