To / from British, American, imperial and metric weights here (https://www.thecalculatorsite.com/cooking/teaspoons-grams.php) (archived here). (https://archive.ph/wip/rEouz)
From which we can see that, in the opinion of that site at least, a teaspoon may vary between 4.929ml and 5.919ml. No crazy suggestions such as 2ml or 7.3ml.
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** Phil.