Charging GST incorrectly can cost more than just tax – it can impact your business’s reputation and even lead to penalties imposed by IRAS. Mistakes in GST are more common than any businesses realise, and the consequences can be significant.
GST is not just a formality. It directly affects how you price, how much you claim, and what you owe. Getting it wrong can lead to compliance issues and unnecessary financial exposure. This guide will walk you through how to calculate GST step by step.
Want to keep your business GST-compliant and financially sound? Let PLCO assist you with your bookkeeping, tax advisory, GST registration, and more. Speak to our consultant via WhatsApp, email or our online form for a free consultation.
Read more: Is GST Applicable on a Proforma Invoice? Here’s Why It Doesn’t Apply
What Exactly Is GST and Who Needs to Charge It?
Let us begin with a quick refresher before we tackle the GST calculations.
Goods and Services Tax (GST) in Singapore is a value-added tax charged on the supply of goods and services as well as on the import of goods into Singapore. The current GST rate stands at 9% as of 1 January 2024, up from 8% the year before.
If your taxable turnover exceeds S$1 million over a 12-month period, or is expected to exceed that in the next 12 months, you must register for GST.
Once registered, you are legally required to charge GST on your goods or services and submit regular GST returns to the Inland Revenue Authority of Singapore (IRAS).
Note: Even if registration is not compulsory, some businesses choose to do so voluntarily to boost credibility or to claim input tax on business expenses.
Explore why GST registration could benefit your business.
The Two Common GST Calculation Methods You Must Know
1. Charging GST: When You Are the Supplier
If you are selling goods or services and are GST-registered, you must charge GST on top of your selling price.
Here is how to calculate it:
GST Amount = Selling Price × 9% (0.09)
Total Invoice Amount = Selling Price + GST Amount
Example:
Selling price = S$600
GST = 600 × 0.09 = S$54
Total = S$654
This is known as GST-exclusive pricing, where GST is added on top of the listed price. It is commonly used in business-to-business (B2B) transactions, as companies need the GST amount to be shown clearly so they can claim it back as input tax from IRAS.
If your business quotes prices without GST included, this is the method to use. It is commonly applied in invoices, quotations, and supplier agreements where the GST component must be clearly separated for accounting and tax purposes.
2. Extracting GST: When the Price Already Includes It
Some businesses prefer to display GST-inclusive prices, especially for retail or consumer-facing sales.
To calculate how much GST is included in a GST-inclusive price, use the following formulas:
GST Amount = (GST-inclusive price × 9) ÷ 109
Net Price = GST-inclusive price − GST Amount
Example:
GST-inclusive price = S$654
GST = (654 × 9) ÷ 109 = S$54
Net = 654 − 54 = S$600
This formula ensures that the correct GST portion is extracted, avoiding underpayment or overpayment to the IRAS.
Use this approach when your prices are advertised as inclusive of GST. This is typical in business-to-consumer (B2C) transactions, such as retail, e-commerce, and F&B, where customers expect to see a final all-inclusive price.
If your business uses a point-of-sale (POS) system or issues receipts showing a single total amount, this method ensures accuracy GST reporting and compliance.
Do You Qualify to Claim Input Tax? Here Is How It Works
GST-registered businesses can claim the GST they have paid on business purchases. This is known as input tax. However, not all expenses qualify. You must meet certain conditions:
- The goods or services must be used for your business.
- You must have valid tax invoices or import permits.
- The supplier must be GST-registered.
- The claim must be made within five years from the date of the invoice.
Some expenses such as club membership fees, medical costs, and benefits provided to employees’ family members are non-claimable, even if GST was charged.
Your net GST payable to IRAS is calculated as:
Output GST (you charge) − Input GST (you pay) = Net GST Payable or Refundable
Example:
Your GST-registered company runs a design agency.
Output GST (you charge to customers):
- You invoice a client S$5,000 for services
- GST charged at 9% = S$5,000 × 0.09 = S$450
Total Output GST = S$450
Input GST (you paid to suppliers):
You had the following GST-inclusive expenses this quarter:
- Software subscription: S$218 (GST = 218 × 9 / 109 = S$18)
- Office furniture: S$1,000 (GST = 1,000 × 9 / 109 = S$82.57)
Total Input GST = S$18 + S$82.57 = S$100.57
Net GST Calculation:
Output GST (S$450) − Input GST (S$100.57) = Net GST Payable: S$349.43
This means you need to pay S$349.43 to IRAS for this filing period.
Note: If your input GST is more than your output GST, the equation works the same way. But, you will get a refund from IRAS instead of paying them.
Read more: Determining Supplies Applicable for Output Tax
Mistakes That Could Cost You: Avoid These GST Calculation Errors
Many businesses encounter common GST mistakes that can lead to penalties or compliance issues with IRAS:
- Using the wrong GST rate for different time periods.
- Forgetting to include GST in quoted prices.
- Misclassifying zero-rated, exempt, or out-of-scope supplies.
- Claiming input tax on non-qualifying expenses.
- Missing deadlines for return submissions.
Mistakes can lead to penalties, fines, or audits. IRAS may impose penalties of up to two times the amount of tax undercharged, in addition to requiring payment of the outstanding tax. Always cross-check calculations and keep proper records to support your filings.
Not All GST Rates Are the Same – Here’s What You Need to Know
There are three main GST treatment types in Singapore:
- 9% GST (Standard-rated) – Applies to most goods and services supplied locally.
- Zero-rated (0%) – Applies mainly to exported goods and international services. Although GST is charged at 0%, these are still considered taxable supplies, so you can claim input tax.
- Exempt – Includes financial services, sale or lease of residential properties, and investment-grade precious metals. These supplies are not subject to GST, and input tax cannot be claimed.
Understanding the differences is important, as incorrect classification may lead to errors in GST reporting and compliance issues with IRAS.
Planning Ahead: How to Stay GST-Compliant Without the Headache
Once you are GST-registered, you are required to submit quarterly or monthly GST returns via IRAS’ myTax Portal. Each return is due one month after the end of each accounting period, so timely submission is essential to avoid penalties
Use accounting software that is GST-compliant or accredited by IRAS under the Accounting Software Register (ASR). These systems automate GST calculations and help reduce manual errors.
It is important to maintain proper records for at least five years, including invoices, credit notes, receipts, and contracts. For digital businesses, keeping secure backups is equally crucial.
Outsourcing to a tax agent or accounting firm if managing GST becomes overwhelming. GST compliance can be handled efficiently and with minimal stress with the right systems, or the right support from a qualified tax professional.
Final Word: Calculating GST Right Is Good Business Sense
Calculating GST may seem like a technical task, but getting it right gives you clarity, accuracy, and better control over your finances.
It shields your business from financial missteps, potential penalties, and unnecessary confusion. Whether you are just starting out or already GST-registered, understanding how GST works gives you the confidence to make sound financial decisions.
GST is not just about compliance. It is about building a business that operates smoothly, supported by transparent and accurate tax practices.
Need help with your GST or tax matters? Reach out to us via WhatsApp, email, or our online form to get a free consultation.
