Color Temperature to Mireds Converter: mired shift and CTO/CTB gel finder
Convert Kelvin to mireds (and back), find the mired shift between two light sources, the closest CTO or CTB gel or gel stack to make the match, and the color temperature a gel gives on any source.
How it works
Mired = 1,000,000 ÷ Kelvin · Kelvin = 1,000,000 ÷ mired · Mired shift = target mired − source mired (positive = warming/CTO, negative = cooling/CTB) · Result after a gel = 1,000,000 ÷ (source mired + gel shift) · 1 decamired = 10 mireds
Color temperature in Kelvin is not evenly spaced for the eye: going from 3000 K to 3100 K is a visible change, while 9000 K to 9100 K is hardly noticeable. The mired (micro reciprocal degree, one million divided by the Kelvin value, written MK⁻¹) fixes this, because equal mired steps look like roughly equal color changes. That is why lighting gels and camera filters are rated in mireds. A gel adds the same mired shift whatever light it covers, so you subtract the source’s mired value from the target’s to find the gel you need, or add a gel’s shift to a source to see what you will get. A positive shift warms the light toward amber (CTO, or an 85-series camera filter) and a negative shift cools it toward blue (CTB, or 80-series). Stacked gels add their shifts. The gel finder uses LEE Filters’ color temperature series: Full CTO +159, 1/2 CTO +109, 1/4 CTO +64, 1/8 CTO +26, Full CTB −137, 1/2 CTB −78, 1/4 CTB −35 and 1/8 CTB −18 mireds. Other brands’ “full” gels differ (Rosco’s full CTO is about +167, GAM’s about +146), so check the maker’s value before a critical job. The same Kelvin-to-mired arithmetic also applies to LED fixtures and white balance settings, though gels and LEDs also shift green–magenta, which mireds do not describe.
Worked example
Matching daylight (5500 K, the default) to tungsten (3200 K): daylight is 1,000,000 ÷ 5500 = 181.8 mireds and tungsten is 1,000,000 ÷ 3200 = 312.5 mireds, so the shift is 312.5 − 181.8 = +130.7 mireds, a warming (CTO) correction. The closest single gel is 1/2 CTO (+109), which gives 1,000,000 ÷ (181.8 + 109) = 3,439 K, still a little cool; a Full CTO (+159) would overshoot to 2,934 K. Stacking 1/2 CTO + 1/8 CTO (+135) gives 3,156 K, within about 4 mireds of the target. An 85B camera filter (+131) is the classic exact match for this conversion.
Sources: Mired (Wikipedia): definition and light source values · TV Technology: The Art of the Mired Measurement (Full CTO by brand, worked examples) · Glazer’s: LEE 201 Full CTB, −137 mireds · Glazer’s: LEE 202 1/2 CTB, −78 mireds · Glazer’s: LEE 203 1/4 CTB, −35 mireds · ProGear: LEE 85B conversion filter, +131 mireds
Frequently asked questions
What is a mired?
A mired (micro reciprocal degree, MK⁻¹) is 1,000,000 divided by the color temperature in Kelvin. 5500 K is 181.8 mireds and 3200 K is 312.5 mireds. Equal mired steps look like roughly equal color changes, which Kelvin steps do not.
How many mireds is a full CTO or full CTB?
LEE’s Full CTO (204) is +159 mireds and its Full CTB (201) is −137 mireds, converting 3200 K tungsten to 5700 K. Rosco’s full CTO is about +167 and GAM’s about +146, so “full” is not the same across brands.
Why do filters use mired shift instead of Kelvin?
A filter adds a fixed mired shift, not a fixed Kelvin change. Full CTO turns 5600 K into about 2,960 K but 7000 K into about 3,300 K: very different Kelvin changes from the same +159 mireds. Mireds let you add gel values directly.
Which way is warming and which is cooling?
A positive mired shift lowers the color temperature toward amber (warming: CTO gels, 85-series filters). A negative shift raises it toward blue (cooling: CTB gels, 80-series filters).
Can I stack gels?
Yes. Mired shifts add, so 1/2 CTO (+109) plus 1/4 CTO (+64) gives +173 mireds. Each layer also costs light, so use the fewest sheets that get close enough.
What is a decamired?
Ten mireds. Some older camera filter systems are labelled in decamireds, so a +13 decamired filter is the same as +130 mireds.
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This calculator is for educational purposes. Double-check important results before acting on them.