Today I came across an interesting problem. I had to draw a React component in the browser and it had to be a specific width and height in centimeters.

So I naively thought that this would be as simple as setting the width and the height in css using the cm unit. I quite quickly found out that this would not work. I found this answer on stack overflow, so follow the link if you are interested in finding out why it doesn't work.

My Initial Code

Lets start out by looking at my initial attempt:

const SomeComponent = styled.div`
  background-color: black;
  width: 8.4cm;
  height: 2.2cm;
`;

function App() {
  return <SomeComponent />;
}

As previously stated this doesn't work. So let's try to fix it!

Some Prerequisites

I found a solution that worked for me. It is not perfect but it got the job done. If anyone have a better solution I would love to hear about it!

I will assume that you are familiar with styled-components; if not, you should probably read up a bit and come back later!

A prerequisite for my solution is the fact that I knew what device the code will run on. I started out by measuring the actual screen I had. In this example I will be using a 13.3" MacBook; I was actually able to find the measurements online so no ruler was needed (11.25" x 7").

The Solution

The first step I took was to define a variable that maps between vw and centimeters. I decided to define it in a styled-components theme since this will allow me to easily access it from anywhere in the component tree:

function App() {
  return (
    <ThemeProvider theme={{ oneCmInVw: 100 / 28.575 }}>
      <SomeComponent />
    </ThemeProvider>
  );
}

So my screen is 28.575cm (11.25") wide. If I divide 100 by 28.575 I get how many vw goes into one centimeter (given that the page is full screen on my 13.3" MacBook). I can now access this variable and set the actual width and height for SomeComponent in centimeters.

const SomeComponent = styled.div`
  background-color: black;
  width: ${props => props.theme.oneCmInVw * 8.4}vw;
  height: ${props => props.theme.oneCmInVw * 2.2}vw;
`;

function App() {
  return (
    <ThemeProvider theme={{ oneCmInVw: 100 / 28.575 }}>
      <SomeComponent />
    </ThemeProvider>
  );
}

The end result is a black box that is 84 x 22 millimeters.