Kelvin, mireds, light sources and correction gels.
Shows a color temperature: its tint (an on-screen approximation: a real light also depends on its spectrum), its value in mireds, and common light sources to compare with (orders of magnitude).
Type the temperature of your light and the one you want: you get the shift in mireds (1,000,000 ÷ K) and the closest Lee gel, alone or two of the same family, with the temperature it gives. Positive shift: warmer, an orange gel (CTO); negative: cooler, a blue gel (CTB). A full CTO turns daylight (≈ 6,500 K) into tungsten (3,200 K).
Mireds make it simple: the same gel always shifts by the same number of mireds, whatever the light. Values are the ones Lee Filters publishes; other brands differ slightly.
Free, no account, nothing is sent: it runs in your browser, even offline.