Although a few dictionaries advocate stadia as the plural of stadium, stadiums is more natural for English-speakers and far more common. In English, we’re not required to pluralize Latin-derived nouns in the Latin manner, though with some words we do so by convention. Stadia is not common enough to be considered standard. Besides, stadia has its own meanings unrelated to stadium (i.e., a telescopic instrument used to measure distances, plus several related definitions).
Examples
Stadia as the plural of stadiums seems most widespread in Indian-English publications—for example:
Regardless of this, pre-pubescent and pre-menopausal audiences will kill for tickets and standing-room at the stadia. [Times of India]
Director Sports Pargat Singh said there would be ten games for which matches would be held at various stadia across Punjab. [Indian Express]
It also appears in a few non-Indian publications—for example:
The truth is that you could say the same of pro football stadia anywhere. [NJ.com]
The Middle East country was too small, it has just one major city and no big football stadia. [Yorkshire Post]
But most reputable publications use stadiums:
For a while, NFL stadiums were sweet on suites. [Chicago Sun-Times]
In the shadow of one of the most iconic stadiums in football lies a depressing picture of dereliction. [Telegraph]
There are even more impressive efforts at energy-saving in sports stadiums around the world … [National Geographic]
She campaigned for female fans to be allowed to enter stadiums. [Guardian]
The issue of public subsidies for corporate-owned stadiums and arenas has been a source of contentious debate nationwide for years. [Los Angeles Times]
