Colour Temperature Mixing Calculator (Tunable White)
Blend two light sources of different correlated colour temperatures to find the resulting CCT from mixing ratios (mired-weighted), the ratio needed to hit a target CCT, the mired shift of gels and filters, and how tunable-white fixtures map warm and cool channels across the day.
How it works
mired = 1,000,000 ÷ K; mixed mired = Σ(lumens × mired) ÷ Σ lumens; CCT = 1,000,000 ÷ mixed mired; fraction of A for a target = (M_target − M_B) ÷ (M_A − M_B); gel: new mired = mired + shift
Colour temperature is not additive — half warm light and half cool light does not average the kelvin values — but its reciprocal, the mired, very nearly is, because equal steps in mired are roughly equal perceived colour steps. Weighting each source's mired by its lumen contribution gives the mixed value, which is how tunable-white fixtures, film-set lighting and photographers' correction gels are planned: a CTO gel adds a fixed +131 mireds whatever the source, turning daylight into tungsten. The kelvin scale's non-linearity shows up in practice: a 50/50 blend of 2700 K and 6500 K lands near 3800 K, well below the arithmetic midpoint, so a tunable fixture set to "50%" is warmer than intuition suggests.
Worked example
Equal 800-lumen outputs at 2700 K (370 mired) and 6500 K (154 mired) mix to 262 mired — about 3,815 K, neutral white, not 4,600 K. To hit 4,000 K the warm channel must supply 44% of the lumens; adding a full CTO (+131 mired) to the mix would drop it to about 2,540 K.
Frequently asked questions
Why mireds?
Because the eye's sensitivity to colour temperature differences is roughly constant in reciprocal kelvin: a 100 K change is obvious at 2700 K and invisible at 6500 K, but a 10-mired change looks similar everywhere. Filters and mixing therefore behave linearly in mireds.
What CCT should I use where?
Living rooms and bedrooms 2700–3000 K; kitchens and baths 3000–3500; offices 3500–4000; task, garages and retail 4000–5000; daylight matching and display 5000–6500. Circadian schemes run 2700 K evening to 5000 K midday.
Related calculators
Lumen-method and point-source arithmetic with typical IES recommended illuminance levels and generic fixture data. Manufacturer photometric files (IES), the applicable energy code (ASHRAE 90.1 / IECC / Title 24) and a lighting designer govern real installations.