function calculateGST() {
var amount = parseFloat(document.getElementById('gstAmount').value);
var rate = parseFloat(document.getElementById('gstRate').value);
var mode = document.getElementById('calcMode').value;
var resultsDiv = document.getElementById('results');
if (isNaN(amount) || isNaN(rate) || amount < 0 || rate < 0) {
alert("Please enter valid positive numbers for Amount and GST Rate.");
return;
}
var netAmount = 0;
var taxAmount = 0;
var grossAmount = 0;
if (mode === 'add') {
// Input is Net, we add tax
netAmount = amount;
taxAmount = amount * (rate / 100);
grossAmount = netAmount + taxAmount;
} else {
// Input is Gross, we remove tax
grossAmount = amount;
// Formula: Net = Gross / (1 + Rate/100)
netAmount = amount / (1 + (rate / 100));
taxAmount = grossAmount – netAmount;
}
document.getElementById('resNet').innerText = netAmount.toFixed(2);
document.getElementById('resTax').innerText = taxAmount.toFixed(2);
document.getElementById('resGross').innerText = grossAmount.toFixed(2);
resultsDiv.style.display = 'block';
}
How to Calculate GST Rate
Goods and Services Tax (GST) is a value-added tax levied on most goods and services sold for domestic consumption. Understanding how to calculate GST is essential for business owners, accountants, and consumers alike. Whether you need to add GST to a net price or strip GST from a total price to find the pre-tax value, the math is straightforward once you know the formulas.
1. Adding GST to a Price (Exclusive to Inclusive)
If you have a product's net price (excluding tax) and need to calculate the final price by adding the GST rate, use the following logic:
GST Amount = Net Price × (GST Rate / 100) Total Price = Net Price + GST Amount
Example: You have a product costing 200 with a GST rate of 10%.
GST Amount = 200 × (10 / 100) = 20
Total Price = 200 + 20 = 220
2. Removing GST from a Price (Inclusive to Exclusive)
If you have the total price that already includes GST and you want to find the original net price and the tax component, the calculation changes slightly. You cannot simply subtract the percentage from the total.
Net Price = Total Price / (1 + (GST Rate / 100)) GST Amount = Total Price – Net Price
Example: You paid 220 total for an item, and the GST rate is 10%.
Net Price = 220 / (1 + 0.10) = 220 / 1.10 = 200
GST Amount = 220 – 200 = 20
How to Calculate the GST Rate Percentage
Sometimes you might know the pre-tax price and the post-tax price, but you need to determine what percentage rate was applied. The formula to find the GST Rate is:
GST Rate (%) = ((Total Price – Net Price) / Net Price) × 100
Example: An item costs 100 before tax and 115 after tax.
Difference (Tax Paid) = 115 – 100 = 15
Rate = (15 / 100) × 100 = 15%
Common GST Calculation Mistakes
The most common error when calculating GST is applying the subtraction method incorrectly. If an item costs 110 including 10% tax, subtracting 10% (11) results in 99, which is incorrect. The correct pre-tax price is 100. This happens because the tax is calculated on the net price, not the gross price. Always use the division formula when working backwards from a total amount.