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SFI to Sunspot Number Converter

Convert the 10.7 cm solar flux index (F10.7) to the sunspot number and back. Type a value in either box.

Solar flux index (SFI, sfu)

Sunspot number (SSN)

How the conversion works

The tool uses the empirical relationship of Holland and Vaughan: SFI = 63.7 + 0.728 × R + 0.00089 × R², where R is the sunspot number. It is an average relationship: on any single day the real values can differ noticeably, because sunspots and radio flux respond differently to short-lived active regions. The solar flux never falls below about 63.7 sfu in this model, which is why a flux of 65 to 70 means almost no sunspots.

Reference table

Sunspot number	Solar flux (sfu)	What to expect on HF
0	64	Upper bands closed
25	82	Upper bands mostly closed
50	102	20 and 17 m good by day
75	123	15 m opens on good paths
100	145	15 and 12 m open regularly
150	193	10 m widely open
200	245	Worldwide 10 m, possible 6 m F2
250	301	Exceptional solar maximum

Learn more in the guides to the solar flux index and the sunspot number.

Data: NOAA Space Weather Prediction Center, NASA SDO. Band conditions are estimates computed by Map For HAM and are indicative only. Methodology and credits · Data feeds · Widgets