One Swedish tesked is exactly 5 millilitres. The value is definitional rather than approximate, which distinguishes it from the US teaspoon at 4.929 ml and the imperial teaspoon at 5.919 ml.
About the units
The tesked is one of four standardised Swedish kitchen measures, sitting between the kryddmått at 1 ml and the matsked at 15 ml. All four were set as exact millilitre values, so the Swedish kitchen never inherited the fractional awkwardness of the American system. Three teskedar make one matsked, and five kryddmått make one tesked. The same 5 ml value is used across the Nordic countries and matches the metric teaspoon used in most of Europe and in Australia. Only the imperial teaspoon, at 5.919 ml, and the American teaspoon, at 4.929 ml, sit outside it — and the American one is close enough that the difference rarely shows.
The exact factor
The factor is exactly 5. Two teskedar is 10 ml, ten teskedar is 50 ml. The relationship to the American teaspoon is 5 ÷ 4.92892 = 1.0144, so a tesked is 1.4 per cent larger — a difference that disappears into measuring error for anything except leavening agents in precise baking. Against the imperial teaspoon the gap is the other way and larger: 5.919 ml is 18 per cent more than a tesked, which matters when working from an older British recipe.
Where you meet this conversion
Spices, salt, baking powder, bicarbonate, vanilla and yeast in Swedish recipes. Because 5 ml is also the standard medical dosing volume, Swedish measuring spoons and pharmacy dosing spoons agree, which is convenient but should not be relied on: pharmacies supply marked syringes for a reason, and a kitchen tesked filled by eye varies more than the stated volume suggests. In baking, the tesked is the smallest measure most recipes use, with the kryddmått reserved for strong spices.