How to Calculate GST: Adding It, Removing It, and Splitting It

GST arithmetic is two operations in opposite directions, and nearly every mistake comes from using the wrong one. Adding GST is a multiplication. Removing it is a division. Treating the second as a subtraction of the same percentage gives an answer that is close enough to look right and is always wrong.

Rates differ by goods, by service and by country, and they change. Every rate below is an example used to show the method — look up the current rate for whatever you are actually invoicing.

Adding GST to a base price

GST amount = base × rate, and total = base × (1 + rate).

A base price of 1,00,000 at an example rate of 18%:

1. GST = 1,00,000 × 0.18 = 18,000
2. Invoice total = 1,00,000 + 18,000 = 1,18,000

Equivalently, 1,00,000 × 1.18 = 1,18,000 in one step. This is the direction businesses quote in, because the base is the number they decided on.

Removing GST from an inclusive price

Here the total is known and the base is not. Divide by (1 + rate):

base = inclusive ÷ (1 + rate)

Take 1,18,000 inclusive of 18%:

1. base = 1,18,000 ÷ 1.18 = 1,00,000
2. GST = 1,18,000 − 1,00,000 = 18,000

The wrong method — multiplying by 0.82 because "18% is included" — gives 96,760, understating the base by 3,240. The reason is that the 18% was charged on 1,00,000, not on 1,18,000, so it is not 18% of the figure you are holding.

A second example with awkward numbers. A retail price of 2,499 inclusive of an example 12% rate:

• base = 2,499 ÷ 1.12 = 2,231.25
• GST = 2,499 − 2,231.25 = 267.75

Check it forward: 2,231.25 × 1.12 = 2,499. ✓

The share of an inclusive price

The tax fraction of a GST-inclusive total is rate ÷ (100 + rate), which is always smaller than the rate itself:

So 18% GST is 15.25% of the price on the shelf. That single line resolves most arguments about whether a tax was "already included".

Setting a round inclusive price

Retailers usually want the customer-facing number to be tidy, which means solving backwards for an untidy base. To land on a shelf price of exactly 1,500 at an example 18%:

base = 1,500 ÷ 1.18 = 1,271.19, GST = 228.81

Check: 1,271.19 × 1.18 = 1,500.00. ✓

The same approach at an example 5% rate for a 999 price point gives a base of 999 ÷ 1.05 = 951.43 and GST of 47.57. Build the price list from these derived bases rather than rounding the base to 951 and letting the total drift to 998.55, which is the version customers notice at the till.

CGST, SGST and IGST

The rate does not change; only its destination does.

• Intra-state supply — buyer and seller in the same state — splits the rate

in half between CGST (central) and SGST (state). An 18% charge appears on the invoice as 9% CGST + 9% SGST. On a 1,00,000 base that is 9,000 + 9,000.

• Inter-state supply is charged as a single IGST at the full 18%, which

is 18,000 on the same base.

The customer pays 18,000 either way. The split exists so that the destination state receives its share, which is also why the place of supply on the invoice matters more than where the seller happens to be registered.

An invoice that shows CGST and SGST for an inter-state sale, or IGST for a local one, is incorrect even when the total is right.

Input tax credit

A business does not pay GST on its whole selling price. It pays GST on the value it added, by offsetting the tax it already paid on purchases against the tax it collects on sales.

Worked through at an example 18%:

14,400 − 9,000 = 5,400, which is exactly 18% of the 30,000 of value added. The credit is why GST is described as a tax on consumption rather than on turnover, and why it does not pile up at each stage of a supply chain the way a simple sales tax does.

Two practical consequences: unregistered buyers cannot claim the credit, so the tax is a real cost to them; and a credit generally depends on the supplier having actually reported the sale, which is why invoice detail matters.

Note also that credit on an inter-state purchase arrives as IGST while an intra-state sale generates CGST and SGST, so the offsetting happens across different buckets with rules about which can be set against which. The total liability is unaffected; the bookkeeping is not.

Common mistakes

• Subtracting the rate from an inclusive price. Divide by 1 + rate instead.
• Rounding the GST before adding it. Compute the full figure, then round the

total once — rounding twice drifts by a few paise per line and can fail reconciliation on long invoices.

• Applying one rate to a mixed invoice. Different line items can carry

different rates; compute each line separately and sum.

• Treating a discount as post-tax. A trade discount shown on the invoice

reduces the taxable value, so GST is charged on the discounted figure, not the list price.

• Assuming a rate is permanent. Slabs and classifications are revised; this

is a method, not tax advice, and the current rate for your supply should be confirmed before you invoice.

Run the numbers both ways with the GST inclusive price calculator and the GST exclusive price calculator. The identical arithmetic applies to any value-added tax — see the VAT calculator.