How four inks make a colour on paper
A screen starts dark and adds light. Paper starts white and inks take light away. Cyan ink absorbs red light, magenta absorbs green and yellow absorbs blue, so what reaches your eye is whatever the inks left alone. Overlap cyan and magenta and only blue survives; magenta and yellow leave red; cyan and yellow leave green.
In theory all three at full strength absorb everything and give black. Real inks give a muddy dark brown and soak the paper, so presses carry a fourth ink, black. It is written K, for the key plate, and it stands in for the neutral part of a colour that would otherwise need all three inks.
The arithmetic behind this converter
The conversion is the textbook one. Each ink percentage is treated as the share of one channel it removes, and black scales all three: red is 255 times (1 minus C) times (1 minus K), with magenta governing green and yellow governing blue in the same way. For the default 86, 49, 0, 15 that gives 30, 111, 217, or #1E6FD9.
Going back, the converter takes K from the brightest channel and gives the remaining difference to cyan, magenta and yellow. At least one of the three is always left at zero, which is why 20, 60, 40, 0 comes back as 0, 50, 25, 20 and 100, 100, 100, 0 comes back as pure K. Each pair describes the same screen colour.
Why a screen colour shifts in print
Every device has a gamut, the set of colours it can physically produce. The sRGB gamut of a monitor reaches intense greens, blues and oranges that process inks cannot match, because ink only reflects part of the light that falls on it. Those colours are pulled inward to the nearest printable one and come out flatter.
Paper matters as much as ink. Uncoated stock absorbs ink and spreads each dot, which darkens and dulls the result compared with coated stock. The same four percentages therefore print differently on different presses and papers, and that is what people mean when they call CMYK values device-dependent.
An ICC profile is a measured description of one press, ink set and paper. Colour-managed software uses it to convert colours and to preview the result on screen, a step called soft proofing. This converter has no profile, so treat its CMYK row as a rough indication and leave the real conversion to your design application or your printer.
What to enter and how to read the result
Type four percentages from 0 to 100 separated by commas or spaces, or paste a cmyk() value with percent signs. A number above 100 is refused. All formats lists CMYK first, then HEX, RGB, HSL and HSV / HSB, with the reminder about ICC profiles underneath. HEX or RGB is what you copy when a printed brand colour has to appear on a website.
To estimate print values for a screen colour, give the field a hex code starting with #, an rgb() value or three bare numbers, or use the colour picker, which writes four percentages back. The Contrast card and the tint and shade strips describe the colour as a screen shows it, not as it will look on paper.
When the numbers go to a printer
For commercial printing, ask the printer which profile and total ink limit to use and convert in software that supports them. Large black areas are usually specified as a rich black, K plus some of the other inks. This converter shows every colour containing 100 percent K as #000000 and cannot tell the two apart.
Brand colours defined as spot inks come with official CMYK and RGB equivalents from the ink system's publisher. Use those, not a calculated value. For screen-only work the other pages are a better starting point: the HEX Color Converter for codes from a stylesheet and the RGB Color Converter for channel values.