<div class="container">
  <label for="miles">Miles
    <input id="miles" type="number">
  </label>
  <p>=</p>
  <label for="kilometers">Kilometers
    <input id="kilometers" type="number">
  </label>
</div>

body {
  font-family: Arial, "Helvetica Neue", Helvetica, sans-serif;
  height: 100vh;
  width: 100vw;
  display: flex;
  justify-content: center;
  align-items: center;
  background-color: lightskyblue;
}

.container {
  border: 1px solid #efefef;
  border-radius: 4px;
  padding: 2rem;
  display: flex;
  flex-direction: column;
  justify-content: space-evenly;
  align-items: flex-start;
  background-color: white;
}

label {
  text-transform: uppercase;
  font-size: .8rem;
  color: #666;
}

p {
  margin: 0;
}


@media (min-width: 500px) {
  .container {
    width: 500px;
    flex-direction: row;
    align-items: center;
  }
  
  p {
  margin: 0 1rem;
  }
}

const miles = document.querySelector('#miles');
const km = document.querySelector('#kilometers');

miles.addEventListener('keyup', e => {
  e.preventDefault();
  km.value = Number(e.target.value * 1.60934).toFixed(2);
});

km.addEventListener('keyup', e => {
  e.preventDefault();
  miles.value = Number(e.target.value / 1.60934).toFixed(2);
});