Even on a quiet day, the magnetic field has a smooth variation. This variation is disregarded when one computes the K indices.
Then the difference between the maximum and minimum (for the three-hour interval in question) is computed for both horizontal components (normally X and Y meaning (geographical) north and east, or H and D meaning magnetic north and east).
The largest of the two differences is then compared to a table, different for different observatories. For the Lovö observatory it looks like this (upper limit inclusive):
| (nT) | K |
|---|---|
| 0--6 | K=0 |
| 6--12 | K=1 |
| 12--24 | K=2 |
| 24--48 | K=3 |
| 48--84 | K=4 |
| 84--144 | K=5 |
| 144--240 | K=6 |
| 240--396 | K=7 |
| 396--600 | K=8 |
| 600-- | K=9 |
| (This is my first table in HTML.) |