One imperial teaspoon is 5.9193905 millilitres, close to 5.92. It is almost twenty per cent larger than the metric and American teaspoons, which both sit at about 5 ml, and that gap is large enough to matter for anything strongly flavoured.
About the units
The imperial teaspoon is 1/24 of an imperial gill, which works out to 5/24 of an imperial fluid ounce of 28.4130625 ml. Like every imperial liquid measure it descends from the imperial gallon of 4.54609 litres, defined originally by the volume of ten pounds of water. The imperial tablespoon that goes with it is 17.758 ml — three imperial teaspoons, preserving the same three-to-one relationship the American and metric systems use. Modern British recipes use the metric 5 ml teaspoon, so the imperial value appears only in older cookbooks and in some Commonwealth sources that never converted.
The exact factor
The exact value comes from the imperial gill: 142.0653125 ÷ 24 = 5.91939 ml, wrongly rounded to 6 ml in some conversion tables, which adds another 1.4 per cent on top of an already large difference. Against the metric teaspoon of 5 ml the ratio is 1.1839, so an imperial teaspoon is a little under one and a fifth metric teaspoons. Across a recipe using three teaspoons of a spice, converting imperial to metric one-for-one loses 2.8 ml — well over half a teaspoon.
Where you meet this conversion
Older British, Irish and Commonwealth recipes, particularly for preserves, curries and baking where spice quantities are specified in teaspoons rather than by weight. The difficulty is identification: a recipe from 1965 says teaspoon and means the imperial one, a recipe from 1995 says teaspoon and means 5 ml, and nothing on the page distinguishes them. The date and origin of the source are the only reliable guides. Where a recipe seems underspiced when made with metric spoons, an imperial original is a likely explanation.