Wheel of Fortune with CSS

A "Wheel of Fortune" component just popped up in my feed. I always spin, but never win! Anyway, this type of component is often built with <canvas>, so I thought I'd write a tutorial on how to make it in CSS. For the interactivity, you still have to use JavaScript.

Here's what we'll be building:

The markup

For the wedges, we'll be using a simple list:

<ul class="wheel-of-fortune">
  <li>$1000</li>
  <li>$2000</li>
  <li>$3000</li>
  <li>$4000</li>
  <li>$5000</li>
  <li>$6000</li>
  <li>$7000</li>
  <li>$8000</li>
  <li>$9000</li>
  <li>$10000</li>
  <li>$11000</li>
  <li>$12000</li>
</ul>

OK, so we have a list of numbers. Now, let's set some initial styles:

:where(.ui-wheel-of-fortune) {
  --_items: 12;
  all: unset;
  aspect-ratio: 1 / 1;
  background: crimson;
  container-type: inline-size;
  direction: ltr;
  display: grid;
  place-content: center start;
}

First is a variable we'll be using to control the amount of items. As the list has 12 items, we set --_items: 12;.

I set the container-type so we can use container-query units (more on that later), then a grid with content placed "left center". This gives us:

OK, doesn't look like much, let's look into the wedges:

li {
  align-content: center;
  background: deepskyblue;
  display: grid;
  font-size: 5cqi;
  grid-area: 1 / -1;
  list-style: none;
  padding-left: 1ch;
  transform-origin: center right;
  width: 50cqi;
}

Instead of position: absolute we "stack" all the <li> in the same place in the grid using grid-area: 1 / -1. We set the transform-origin to center right, meaning we'll rotate the wedge around that axis.

So, now we have:

Because all the elements are stacked, we can only see the last.

Let's do something about that. First, we'll add an index variable to each wedge:

li {
  &:nth-of-type(1) { --_idx: 1; }
  &:nth-of-type(2) { --_idx: 2; }
  &:nth-of-type(3) { --_idx: 3; }
  &:nth-of-type(4) { --_idx: 4; }
  &:nth-of-type(5) { --_idx: 5; }
  /* etc. */
}

With that we only need to add one more line of CSS:

li {
  rotate: calc(360deg / var(--_items) * calc(var(--_idx) - 1));
}

Getting there! Let's use the same variables to create some color variations:

li {
  background: hsl(calc(360deg / var(--_items) *
  calc(var(--_idx))), 100%, 75%);
}

---

A Slice of π

For the height of the wedges we need the circumference of the circle divided by the amount of items. As you might recall from school, the circumference of a circle is:

C=2πr

Because we're using container-units, the radius is 50cqi, so the formula we need in CSS is:

li {
  height: calc((2 * pi * 50cqi) / var(--_items));
}

Isn't it just cool that we have pi in CSS now?!

Now, let's add a simple clip-path to each wedge. We'll start at the top left corner, move to the right center, then back to left bottom:

li {
  clip-path: polygon(0% 0%, 100% 50%, 0% 100%);
}

Let's deduct a little from the edges:

li {
  clip-path: polygon(0% -2%, 100% 50%, 0% 102%);
}

Not sure, if there's a mathematical correct way to do this?

Anyway, now we just need to add border-radius: 50% to the wrapper:

Hmm, not good. Let's use a clip-path instead, with inset and round:

.wheel-of-fortune {
  clip-path: inset(0 0 0 0 round 50%);
}

Much better:

And because we used container-units for the wedges and the font-size, it's fully responsive!

---

Make it spin

Now, let's add a spin-<button> (see CSS in code-example below) and trigger a spin using JavaScript:

function wheelOfFortune(selector) {
  const node = document.querySelector(selector);
  if (!node) return;

  const spin = node.querySelector('button');
  const wheel = node.querySelector('ul');
  let animation;
  let previousEndDegree = 0;

  spin.addEventListener('click', () => {
    if (animation) {
      animation.cancel(); // Reset the animation if it already exists
    }

    const randomAdditionalDegrees = Math.random() * 360 + 1800;
    const newEndDegree = previousEndDegree + randomAdditionalDegrees;

    animation = wheel.animate([
      { transform: `rotate(${previousEndDegree}deg)` },
      { transform: `rotate(${newEndDegree}deg)` }
    ], {
      duration: 4000,
      direction: 'normal',
      easing: 'cubic-bezier(0.440, -0.205, 0.000, 1.130)',
      fill: 'forwards',
      iterations: 1
    });

    previousEndDegree = newEndDegree;
  });
}

Instead of adding and removing a css-class and updating a @property with a new rotation-angle, I opted for the simplest solution: The Web Animations API!

Full code is here:

UPDATE: The shape-master, Temani Atif, has provided a much more elegant way to create the wedges using tan and aspect-ratio (see comments below).

---

More ideas

I encourage you to play around with other styles! Maybe add a dotted border?

Comments

[Temani Afif]

The height calculation is actually not correct. You need to consider the polygon shape around the circle to find the correct height (the circumscribed polygon)

It's equal to height: calc(2*50cqi*tan(180deg/var(--_items))); that you can simplify by setting the ratio aspect-ratio: 1/calc(2*tan(180deg/var(--_items))); to avoid using the width value twice.

With this you won't have issue when you apply the clip-path

[Mads Stoumann]

AH, perfect — thank you! You truly are the master of CSS Shapes. I've added an update, and will use your input for the follow-up article with spinning.

[Vladyslav Dev]

I really like your implementation because all libs create a wheel using canvas but i need the one to be highly customizable. Please help me to add logic to spin the wheel to the predefined (from backend or mocked) sector!

[Mads Stoumann]

You just need to set newEndDegree manually, matching your predefined destination.

[Martin Prihoda]

Has anyone solved the return value of the winning field? For example index for li tag. I have something, but the results are +- one position.

[Martin Prihoda]

I think it’s resolved ( Link on Codepen.io ).

// --------------------------------------------
// append code for getting index of wheel spim
// --------------------------------------------
const rangeAngles = [
  {index:  0, from: 345, to: 360},
  {index:  0, from:   0, to:  15},
  {index:  1, from:  15, to:  45},
  {index:  2, from:  45, to:  75},
  {index:  3, from:  75, to: 105},
  {index:  4, from: 105, to: 135},
  {index:  5, from: 135, to: 165},
  {index:  6, from: 165, to: 195},
  {index:  7, from: 195, to: 225},
  {index:  8, from: 225, to: 255},
  {index:  9, from: 255, to: 285},
  {index: 10, from: 285, to: 315},
  {index: 11, from: 315, to: 345},
];

// position at zero level
const calculateZeroAngle = (finalAngle) => {
  let zeroAngle = 360 - finalAngle + 90;
  zeroAngle = ((zeroAngle % 360) + 360) % 360;
  zeroAngle = Math.round(zeroAngle * 10) / 10;
  return zeroAngle;
};

// Save the final degree to determine the winning segment
animation.onfinish = async () => {
  const finalAngle = ((newEndDegree % 360) + 360) % 360;
  const zeroAngle = calculateZeroAngle( finalAngle );
  const indexfOfWinner = rangeAngles.find( a => a.from < zeroAngle && a.to >= zeroAngle ).index
  alert( `Index of Winner: ${indexfOfWinner}` )
};

[Mads Stoumann]

const normalizeAngle = (finalAngle) => {
  return (360 - finalAngle + 90) % 360;
};

const items = wheel.children.length;
const segment = 360 / items;
const offset = 15;

animation.onfinish = () => {
  const finalAngle = newEndDegree % 360;
  const normalizedAngle = normalizeAngle(finalAngle);
  const winner = Math.floor(((normalizedAngle + offset) % 360) / segment);
  console.log(wheel.children[winner].textContent);
};

[Mads Stoumann]

Cool!

[Mads Stoumann]

Please share — I need to think about it too!

[Jocom Vag]

Always here for an interesting CSS project. Kudos.

[Mads Stoumann]

Thanks!

[Debajyati Dey]

wholesome wheel! :)
Beautiful project!

[Mads Stoumann]

Thank you!

[Troy Forsyth]

So I found this posting a few days ago. I used the code pen and am trying to make this into a dynamically loading setup. Everything is fine with the wedges until you get below 4 slices. At 3 and less it breaks. I feel like it's something to do with the aspect ration of the LI tags.

Any thoughts?

[Eckehard]

I suppose this would be easier done in pure Javasript using CSS only where It's appropriate (e.g. the animation). Is it really worth the effort doing anything in CSS?

[LcsGa]

I don't think that'd be easier in javascript. Whenever you have something visual, the right tool for that is CSS. When it's easier in pure JS, this usually mean that you lack knowledge with CSS.

[Eckehard]

Oh, have fun to rebuild the [solarsystem][dev.to/cookiemonsterdev/solar-syst...] with pure CSS. There are good reasons the S in
CSS comes from "style", not from graphics...