How to create a percentage calculator using JavaScript

In this tutorial we’ll create 3 different types percentage calculators using JavaScript.

• X is what % of Y?
• What is X% of Y?
• What is the % increase or decrease between X & Y?

Each calculator has almost identical HTML markup and the only difference in the JavaScript is the maths used to calculate the solution. If you just looking for a copy and paste solution a working HTML file with the 3 calculators can be downloaded from here.

1 – X is what % of Y?

Example – 50 is 20% of 250 || (50 / 250) * 100 = 20%

<form id="calc1">  
  <input id="calc1-num-x" type="number" /> is what percent of
  <input id="calc1-num-y" type="number" /> ?
  <input id="calc1-submit" type="submit" value="Calculate" />
  <input id="calc1-solution" type="number" readonly="readonly" />
</form>

document.getElementById("calc1-submit")
  .addEventListener("click", function (e) {
    e.preventDefault();
    const numX = document.getElementById("calc1-num-x").value;
    const numY = document.getElementById("calc1-num-y").value;
    const percentage = (numX / numY) * 100;
    document.getElementById("calc1-solution").value = percentage;
});

2 – What is X% of Y?

Example – 50% of 250 = 125 || (50 / 100) * 250 = 125

<form id="calc2">
  What is <input id="calc2-num-x" type="number" /> % of
  <input id="calc2-num-y" type="number" /> ?
  <input id="calc2-submit" type="submit" value="Calculate" />
  <input id="calc2-solution" type="number" readonly="readonly" />
</form>  

document.getElementById("calc2-submit")
  .addEventListener("click", function (e) {
    e.preventDefault();
    const numX = document.getElementById("calc2-num-x").value;
    const numY = document.getElementById("calc2-num-y").value;
    const percentage = (numX / 100) * numY;
    document.getElementById("calc2-solution").value = percentage;
});

3 – What is the % increase or decrease between X & Y?

Example – 50 to 75 = 50% || (75 / 50) / 50 * 100 = 50%

<form id="calc3">
  What is the percentage increase or decrease from
  <input id="calc3-num-x" type="number" /> to
  <input id="calc3-num-y" type="number" /> ?
  <input id="calc3-submit" type="submit" value="Calculate" />
  <input id="calc3-solution" type="number" readonly="readonly" />
</form>

document.getElementById("calc3-submit")
  .addEventListener("click", function (e) {
    e.preventDefault();
    const numX = document.getElementById("calc3-num-x").value;
    const numY = document.getElementById("calc3-num-y").value;
    const percentage = (numY - numX) / numX * 100;
    document.getElementById("calc3-solution").value = percentage;
});

For each of the calculators we’ve added a click event listener to the submit buttons. When these buttons are clicked the 2 numbers entered into the form are saved as variables. We then perform the percentage calculation on these numbers and output the value into the solution input field.