Choose the right coordinate system in Sweden
WGS84, SWEREF 99 and RT90 are not interchangeable labels.
Start with the source
Find the coordinate reference system, projection zone, units and axis order in the source documentation. Do not guess solely from the number of digits. Keep the original values unchanged while testing a conversion.
WGS84 and axis order
WGS84 locations are commonly given as latitude and longitude in degrees. This converter's input fields use X = longitude and Y = latitude. A Stockholm example is X 18.0686, Y 59.3293. Reversing those values moves the point far away even though both numbers remain valid geographic values.
SWEREF 99 and local zones
SWEREF 99 TM is a Swedish projected system in metres. Local SWEREF 99 projection zones use different definitions and must be selected explicitly. In this form projected X is easting and Y is northing; a document using northing first must be reordered accordingly.
Older RT90 material
An older Swedish map may use RT90 rather than SWEREF. Select the documented variant, then inspect the converted location on the map. A conversion that runs without an error does not prove that the chosen source system was correct. Consult Lantmäteriet's reference-system information for authoritative definitions.