One Scandinavian square mil is exactly 100 square kilometres. Not ten — one hundred. This is the single most misread unit on this site, and the reason is a squaring step that is easy to skip: the Scandinavian mil is 10 km, and squaring 10 gives 100.
About the units
The mil is a Nordic distance unit that survived metrication by being redefined rather than abolished. Historically it varied — the Swedish mil was 10,688 metres before 1889 — but when Sweden went metric it was rounded to exactly 10 kilometres, and in that form it is still in everyday use. Swedes give driving distances in mil, not kilometres, and fuel economy is quoted in litres per mil. The square mil is its area counterpart: the area of a square measuring one mil on each side. It appears mostly in older Swedish and Norwegian land, forestry and county statistics, where it was a convenient size for describing large tracts. The square kilometre needs no introduction by comparison, but it is worth stating precisely: it is the area of a square one kilometre on each side, exactly 1,000,000 square metres.
The exact factor
The derivation is one line and worth writing out, because skipping it is exactly how the error happens. One mil is 10 km. A square mil is therefore (10 km)² = 10 km × 10 km = 100 km². The temptation is to carry the linear factor straight across and say 10, which understates every area by a factor of ten. Anyone converting historical Swedish land records should treat a stray factor of ten as the first thing to check. For scale: 100 km² is roughly a square 10 km on a side, about the land area of a mid-sized city, or about 1.4 times the area of Manhattan.
Where you meet this conversion
You meet this conversion when reading Nordic sources: Swedish and Norwegian county descriptions, older forestry surveys, historical statistics on farmland and lake surfaces, and land registry material predating the switch to square kilometres. It also turns up in translation work, where a figure given in kvadratmil has to be made comprehensible to an international reader — and where getting the factor wrong produces a number that is plausible enough to pass unnoticed.