CMYK to RGB: how to convert a print file for screens

How do you convert CMYK to RGB?

To convert CMYK to RGB, read the file’s CMYK numbers through its ICC profile, then write the colour out in sRGB. A CMYK number is an amount of ink, not a colour. The same four numbers print differently on each press and paper. The profile says which colour those inks make. The ICC’s FAQ explains the middle step. In the profile connection space, colour is specified “in terms of its appearance rather than in a device-dependent space such as CMYK”.

The ICC profile format sets the rules for this chain, and three tools run it for you. Photoshop has Edit > Convert to Profile and Image > Mode > RGB Color. An image converter that reads the embedded profile does it on upload. We tested how close each path gets, and two browsers too.

How we tested each way

Our test: one 1920 × 1280 photo of an iguana, taken to CMYK with the U.S. Web Coated (SWOP) v2 profile. We saved it as a CMYK JPEG at quality 90, once with the profile inside and once without. Then we turned it back into RGB 7 ways. Each result was compared, pixel by pixel, with an ICC conversion by LittleCMS 2.19. That reference uses relative colorimetric with black point compensation, as Adobe’s help recommends.

The score is ΔE, the CIEDE2000 colour difference, over all 2,457,600 pixels. A ΔE of 0 is the same colour. The bigger the number, the further apart two colours sit.

The CMYK to RGB formula, and how far it misses

The CMYK to RGB formula that calculator pages print turns each ink into a perfect filter. It works on fractions from 0 to 1:

Real ink is not a perfect filter. Process cyan reflects less light than the formula assumes. The formula makes C100 M0 Y0 K0 pure 0, 255, 255. The SWOP profile makes it 0, 174, 239, a ΔE 27.33 gap, the widest of our 10 builds. Green ink mixes miss by ΔE 24.14.

On the whole photo, the formula lands ΔE 5.43 from the reference on average. That is 11 times the engine’s ΔE 0.49. Use the formula for a rough guess at one flat colour. Use a profile for a brand colour or a photo.

What Blinkimg does with a CMYK JPEG

Blinkimg’s engine converts a CMYK JPEG to sRGB, whatever format you save. All 16 of our outputs, in 4 formats from both CMYK files, came back as sRGB with 3 channels. With the profile in the file, the engine landed ΔE 0.49 from the reference on average. At the 95th percentile it was ΔE 1.43. The engine used the profile’s perceptual table: against a perceptual LittleCMS conversion, the average gap was ΔE 0.00.

The output carries no profile by default, and a browser reads a file with no profile as sRGB. With Keep metadata on, the engine attaches a 480-byte sRGB profile. The CMYK file carried a 557,168-byte SWOP profile, 24% of its 2.32 MB.

The JPG at Auto scores ΔE 1.59 on average. The extra over ΔE 0.49 is the JPG’s own compression, not the colour. For a web page, the JPG to WebP converter gives the 214.5 KB file. To keep editing without further loss, the JPG to PNG converter writes a lossless RGB copy.

Why does a CMYK JPEG look washed out in a browser?

A CMYK JPEG can look washed out because the browser converts it badly, even with the profile inside. Headless Chromium 149 drew our file ΔE 9.70 from the reference on average. It lifted rich black to 53, 54, 58, a grey haze over every shadow. WebKit 26.5 drew the same file ΔE 0.52 away, near the engine’s ΔE 0.49.

These are Playwright’s headless builds on a Mac, not the browsers on your phone. They still show the risk: a CMYK JPEG on a web page looks different from one browser to the next. After conversion that risk is gone. Chromium drew the engine’s sRGB JPG ΔE 0.22 from the file’s own pixels, and WebKit ΔE 0.24.

What if the CMYK file has no profile?

A CMYK file with no profile gets a guess, and every program guesses differently. The ICC’s FAQ is blunt: “There is no way to tell any of the parameters you specify if you have a file with no profile.” Our copy without the profile went three ways:

• Blinkimg’s engine assumed libvips’s built-in “Chemical proof” profile and landed ΔE 2.95 from the truth.
• WebKit assumed macOS’s Generic CMYK Profile (within ΔE 0.84 of it) and landed ΔE 4.10 away.
• Chromium drew the formula, within ΔE 0.39 of it, and landed ΔE 5.54 away.

The fix is to name the profile before you convert. Ask the sender which profile made the file. In Photoshop, Edit > Assign Profile sets it. Adobe notes that assigning “doesn’t change the document’s color values”; it only tells Photoshop how to read them. Then convert to sRGB as usual.

How to convert CMYK to RGB in Photoshop

In Photoshop, Edit > Convert to Profile converts CMYK to RGB with the profile you choose. Adobe’s help describes both menus. Four steps:

1. 1

   Open the file and check its mode

   Photoshop shows the mode in the document tab, such as CMYK/8. If the file has no profile, Photoshop can ask which one to use when it opens.

2. 2

   Choose Edit > Convert to Profile

   Set Destination Space to sRGB IEC61966-2.1, the colour space browsers assume. Adobe recommends the Adobe (ACE) engine and Black Point Compensation checked.

3. 3

   Pick the intent

   Relative Colorimetric or Perceptual. On our photo, the two intents ended ΔE 0.49 apart on average.

4. 4

   Save a copy for the web

   Save the RGB copy as a JPG under a new name and keep the CMYK original for print. Or skip the save dialog: drop the CMYK JPEG on the JPG compressor, which converts and compresses in one step.

Image > Mode > RGB Color does the same in one click. It converts to the RGB working space named in Edit > Color Settings, so check that it says sRGB first. Adobe’s Color Settings page adds that the working spaces also “determine the appearance of colors in untagged documents”.

How to tell if a JPG is CMYK or RGB

To tell if a JPG is CMYK, look at the colour space your editor reports. Photoshop prints it in the document tab. On a Mac, the sips command in Terminal names the space and the profile. Our three files:

Read the second answer with care. The file without a profile shows “Generic CMYK Profile”: that is the profile macOS assumes, not one stored in the file. The engine’s JPG carries no profile either, and sips names sRGB IEC61966-2.1, the one it assumes for RGB.

CMYK vs RGB: what is the difference?

RGB mixes red, green and blue light; CMYK mixes cyan, magenta, yellow and black ink. Screens make colour with RGB, and printing presses with CMYK. A JPEG can hold either. Our CMYK JPEG has 4 channels; the engine’s sRGB JPG has 3.

Why is CMYK more dull than RGB?

CMYK looks dull because ink cannot make a screen’s brightest colours. We sent 6 pure screen colours to CMYK with the SWOP profile and back. Pure blue went to C88 M77 Y0 K0 and came back as 58, 83, 164, ΔE 17.06 away. Green lost ΔE 16.18. Red lost ΔE 5.35, and yellow only ΔE 4.27.

Can you convert CMYK to RGB without losing colour?

Converting CMYK to RGB loses almost nothing: the engine landed ΔE 0.49 from the reference. The loss happened earlier, on the way into CMYK. Our photo, taken to CMYK and back, sat ΔE 1.95 from the original on average, one JPEG save included. At the 95th percentile it was ΔE 4.47. No converter brings those colours back. If you still have the RGB original, use it.

RGB to CMYK: do you need it for print?

Convert RGB to CMYK only when your printer asks for CMYK and names its profile. The ICC’s FAQ notes that “Many printers only have a RGB interface.” It adds that converting to CMYK only helps when the printer uses CMYK data directly. Convert once, with the press’s profile, and keep the RGB master for screens.

Got a CMYK JPG for a web page or a social post? Drop it on the JPG compressor below. It comes back as an sRGB JPG, 89% smaller in our test.