RGB to CMYK Converter

What is RGB to CMYK conversion?

RGB describes light: a screen adds red, green and blue together, and all three at full strength give white. CMYK describes ink: a press lays cyan, magenta, yellow and black onto paper, and each one subtracts from the white the paper already reflects. Converting between them is what you do before sending artwork to print.

Type a hex code or three RGB numbers above and you get the four ink percentages, exactly as any converter would produce them. What you also get here is the part the others leave out — which of several possible answers you were handed, and why.

The short version: there is no such thing as the CMYK value of a screen colour. There is only the CMYK value for a particular press, ink set and paper, and the formula on every converter including this one does not know any of those.

How to use the RGB to CMYK converter

1. Enter your colour as hex or RGB. Use the swatch to pick one, paste a hex code, or type the three channel numbers. Everything updates as you type, in your browser.
2. Choose how precisely to show the result. Whole percentages are what converters normally print. Switch to one decimal place and watch the round-trip line underneath change: whole numbers throw your colour away, one decimal keeps it.
3. Move the black slider. Every position produces the same colour on paper using a different amount of black. The total ink figure moves as you drag. The formula always sits at the far right, and never mentions the rest.

The formula loses nothing, and that is the problem

It is natural to assume converting to CMYK loses information, since it is a smaller gamut than your screen. The arithmetic says otherwise. Running every one of the 16,777,216 colours in sRGB through the standard formula and straight back gives zero failures and a worst error of 5.7 parts in a hundred trillion — pure floating-point noise. It is an exact, reversible mapping, and two lines of algebra show why: with m as the largest of the three channels, cyan reduces to 1 minus red over m, and the inverse hands red back untouched.

That should be unsettling rather than reassuring. A real CMYK separation depends on which press is running, which inks it holds and which paper is going through it. It is measured, encoded in an ICC profile, and it is emphatically not reversible. A transform that gives you back exactly what you put in has consulted none of those things. It is arithmetic on your screen colour wearing the vocabulary of print.

The give-away is in the numbers themselves. CMYK has four components, but the formula only ever uses three. For every colour in the entire cube, at least one of cyan, magenta and yellow comes out as exactly zero, and black is pinned to exactly one minus the largest channel. Look at any grey: the formula gives it as pure black with no coloured ink at all, which is a specific and often wrong way to print grey.

The choice it makes for you, and what it costs

Black is not determined by the colour. The inverse only requires that one minus black is at least as large as each channel, so any black value from zero up to one minus the largest channel reproduces your colour exactly. Across a sweep of two million colours there is a median of 21 whole-percent settings to choose from, and up to 101. The formula always takes the very top of that range: the most black, the least coloured ink. Printers call this grey component replacement, and how much to use is a decision made per job.

It is not a trivial decision. Take the dark grey rgb(30, 30, 30), which is what this page opens with. The standard formula prints it with 88.2 per cent total ink. Set black to zero and the same colour needs 264.7 per cent — three coloured inks doing the work black was doing. Both recipes put the identical colour on the page. Only the ink bill and the drying time differ.

Total ink matters because paper has a limit. Four inks at full strength is 400 per cent by definition, and no press runs anywhere near that. Web offset specifications sit around 300 per cent and uncoated stock lower still, because wet ink has to dry and a saturated sheet will set off onto the next one. So that 264.7 per cent figure is not an abstraction — on the wrong paper it is a job that cannot be printed.

The one genuine loss is something else entirely, and it is the display convention rather than the maths. Converters print whole percentages, and at that resolution only 12.73 per cent of sRGB survives a round trip — 2,136,545 colours out of 16,777,216, with the worst case off by two levels in a channel. Showing one decimal place brings every single colour back. If you have ever copied CMYK numbers out of a converter and found the colour had shifted, that is where it went.

Honest limits, and when to ignore this tool

This page cannot give you a print-accurate separation, and neither can any other free converter. Doing that properly means starting from an ICC profile for the actual press and paper, and letting a colour management system handle gamut mapping and black generation together. If you are sending a job to a printer, ask them for their profile and convert in your layout application using it. What you have here is the arithmetic, made honest about what it is.

It also cannot tell you whether your colour is printable at all. Plenty of bright screen colours — saturated oranges, vivid blues, anything neon — sit outside what four process inks can reproduce, and the formula will cheerfully hand you numbers for them anyway. Those numbers will print as the nearest thing the inks can manage, which may be noticeably duller than what you approved on screen.

Where it is genuinely useful: getting a workable starting point, understanding why two converters disagree, checking whether a dark colour is going to blow past an ink limit, and seeing what black generation actually does. The black slider is the part worth playing with, because it is the one decision every other tool makes silently.

Why is it free?

Because it costs us nothing. Converting RGB to CMYK is four divisions, and your browser does them as you type. Nothing is uploaded, nothing is stored, and there is no server involved to see your colours in the first place.

So there is no account, no sign-up and no limit on how many colours you run through it. The whole calculation, and the sweep of all 16.7 million colours behind the figures on this page, is a few kilobytes of code running on your own machine.