Solar cycle 18 was the eighteenth solar cycle since 1755, when extensive recording of solar sunspot activity began. The solar cycle lasted 10.2 years, beginning in February 1944 and ending in April 1954. The maximum smoothed sunspot number (monthly number of sunspots averaged over a twelve-month period) observed during the solar cycle was 151.8 (May 1947), and the minimum was 3.4. There were a total of 446 days with no sunspots during this cycle.
Cycle 18 was characterised by "giant" sunspots. The recording of the 10.7 cm (2800 MHz) solar radio flux began partway during this cycle, and values of the solar flux during this cycle turned out to be particularly high.