#calculator {
  width: 400px;
  height: 250px;
  margin: 30px auto;
  border-spacing: 0;
  border-collapse: collapse;
}

#calculator th {
  width: 100%;
  padding: 4px 8px;
  vertical-align: middle;
  border: 1px solid silver;
}

#calculator td {
  width: 25%;
  text-align: center;
  border: 1px solid silver;
}

#calculator td:hover {
  background-color: whitesmoke;
}

#result {
  width: 100%;
  padding: 0;
  text-align: right;
  border: 0;
}

#calculator {
  width: 400px;
  height: 250px;
  margin: 30px auto;
  border-spacing: 0;
  border-collapse: collapse;
}

#calculator th {
  width: 100%;
  padding: 4px 8px;
  vertical-align: middle;
  border: 1px solid silver;
}

#calculator td {
  width: 25%;
  text-align: center;
  border: 1px solid silver;
}

#calculator td:hover {
  background-color: whitesmoke;
}

#result {
  width: 100%;
  padding: 0;
  text-align: right;
  font-size: 1.5em;
  border: 0;
}

Enlarge the font-size of result
Enlarge the font-size of result
