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GST Calculator

Add or remove GST with preset GST rates or a custom rate, and see original amount, GST amount, final amount, formula, and steps.

Enter the taxable amount for GST exclusive calculations or the GST-inclusive amount for reverse GST.

Use this only when Custom rate is selected.

Status: initial

Results

Awaiting calculation

Tax guide

Introduction

GST is added to or removed from invoices, bills, product prices, service fees, and business estimates every day. This calculator supports GST exclusive calculations for adding tax and GST inclusive calculations for removing tax from a total amount.

Purpose

Use this calculator to quickly find the original amount, GST amount, and final amount for common GST rates or a custom rate. It is designed for clear invoice checks, business estimates, and everyday tax understanding.

GST formula

For GST exclusive amounts, GST is added to the original amount. For GST inclusive amounts, the original amount is recovered by dividing the total by 1 plus the GST rate.

Variable explanations

Amount

The entered amount is either before GST for exclusive calculations or already including GST for inclusive calculations.

GST rate

Choose a common rate such as 0%, 3%, 5%, 12%, 18%, or 28%, or select a custom rate.

Calculation type

GST Exclusive adds GST to a base amount. GST Inclusive removes GST from a total amount.

Formula guide

GST Exclusive (add GST)

GST amount = original amount x GST rate / 100

  • Original amount is the price before GST.
  • GST rate is the selected percentage.
  • Final amount is original amount plus GST amount.

Use this when the entered amount does not include GST and you want the final GST-inclusive price.

GST Inclusive (remove GST)

Original amount = GST-inclusive amount / (1 + GST rate / 100)

  • GST-inclusive amount is the total price including GST.
  • GST rate is converted to a decimal.
  • GST amount is total amount minus original amount.

Use this when the entered amount already includes GST and you want to find the pre-GST amount.

Final amount

Final amount = original amount + GST amount

  • Final amount is the invoice or bill total after GST is applied.

For inclusive calculations, the entered amount is already the final amount.

Custom GST rate

Custom rate follows the same exclusive or inclusive formulas

  • Custom rate can be any value from 0% to 100%.

Use a custom rate for special cases, international GST/VAT-style checks, or temporary tax scenarios.

Worked, business, and invoice examples

Add 18% GST

  1. Choose GST Exclusive.
  2. Enter amount as 1,000.
  3. Choose 18%.
  4. GST is 180 and the final amount is 1,180.

Remove 18% GST

  1. Choose GST Inclusive.
  2. Enter amount as 1,180.
  3. Choose 18%.
  4. Original amount is 1,000 and GST is 180.

Zero-rated item

  1. Choose GST Exclusive.
  2. Enter the amount.
  3. Choose 0%.
  4. GST amount is 0 and final amount stays the same.

Custom rate

  1. Choose Custom rate.
  2. Enter the custom percentage.
  3. Calculate GST using the same inclusive or exclusive method.

Invoice check

  1. Use GST Inclusive when checking a total invoice amount.
  2. Enter the total amount shown on the invoice.
  3. The calculator separates the original amount and GST amount.

Quotation check

  1. Use GST Exclusive when preparing a quote before tax.
  2. Enter the base service or product amount.
  3. The calculator shows the GST and final quoted amount.

Common mistakes

Confusing inclusive and exclusive amounts

Adding GST to an amount that already includes GST will overstate the final price.

Using the wrong rate

GST rates vary by goods, services, and jurisdiction. Always confirm the applicable rate.

Subtracting the rate directly

To remove GST from an inclusive amount, divide by 1 plus the rate. Do not simply subtract the percentage from the total.

Rounding too early

Round final invoice values consistently and avoid rounding each intermediate step when checking larger invoices.

Treating estimates as tax advice

A calculator can help with arithmetic, but filing and compliance questions should follow official guidance.

FAQs

What is GST?
GST stands for Goods and Services Tax. It is a tax applied to many goods and services depending on the applicable rules and rate.
What is GST exclusive?
GST exclusive means the entered amount does not include GST. The calculator adds GST to find the final amount.
What is GST inclusive?
GST inclusive means the entered amount already includes GST. The calculator removes GST to find the original amount.
How do I add GST?
Multiply the original amount by the GST rate divided by 100, then add that GST amount to the original amount.
How do I remove GST from a total?
Divide the GST-inclusive amount by 1 plus the GST rate divided by 100. Subtract that original amount from the total to find GST.
What GST rates are supported?
The calculator supports 0%, 3%, 5%, 12%, 18%, 28%, and a custom rate from 0% to 100%.
Can I use decimals?
Yes. Amounts support decimal values up to two decimal places, and custom GST rates can use decimals.
What is the formula for GST exclusive calculation?
GST amount equals original amount multiplied by GST rate divided by 100. Final amount equals original amount plus GST amount.
What is the formula for GST inclusive calculation?
Original amount equals GST-inclusive amount divided by 1 plus GST rate divided by 100. GST amount equals total minus original amount.
Why is reverse GST not just subtracting the percentage?
The percentage is applied to the original pre-GST value, not the GST-inclusive total, so reverse GST requires division.
Does the calculator split CGST and SGST?
This version focuses on original amount, GST amount, and final amount. CGST, SGST, and IGST breakdowns can be added later.
Can businesses use this for invoices?
It can help check arithmetic for invoices and quotes, but official invoices should follow the applicable tax rules and accounting process.
Does 0% GST work?
Yes. A 0% rate leaves the original amount and final amount the same, with GST amount shown as zero.
What is a custom GST rate for?
Use a custom rate when checking a rate not listed in the preset options or when working with another GST/VAT-style rate.
Are formulas calculated in the page component?
No. The page renders a registered calculator specification and the Formula Engine performs the calculation.

References

Related calculators

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Last updated and version history

Last updated: 2026-07-03

  • 1.0.0 (2026-07-03): Initial CAL-0004 GST Calculator implementation using platform engines.