Calc CSS

 html {
      font-size: 62.5%;
      box-sizing: border-box;
    }

    *,
    *::before,
    *::after {
      margin: 0;
      padding: 0;
      box-sizing: inherit;
    }

    .calculator {
      border: 1px solid #ccc;
      border-radius: 5px;
      position: absolute;
      top: 50%;
      left: 50%;
      transform: translate(-50%, -50%);
      width: 400px;
    }

    .calculator-screen {
      width: 100%;
      height: 80px;
      border: none;
      background-color: #252525;
      color: #fff;
      text-align: right;
      padding-right: 20px;
      padding-left: 10px;
      font-size: 4rem;
    }

    button {
      height: 60px;
      font-size: 2rem!important;
    }

    .equal-sign {
      height: 98%;
      grid-area: 2 / 4 / 6 / 5;
    }

    .calculator-keys {
      display: grid;
      grid-template-columns: repeat(4, 1fr);
      grid-gap: 20px;
      padding: 20px;
    }