GST has two different operations: add tax to a before-GST price, or extract tax from an inclusive total. Writing both as “amount × 9%” makes the inclusive calculation wrong.
How do you add GST?
GST = before-tax price × 9%; inclusive price = before-tax price × 1.09. On SGD 250 before GST, tax is SGD 22.50 and the final amount is SGD 272.50.
How do you extract GST from an inclusive price?
An inclusive price represents 109 parts: 100 parts value and nine parts GST. Before-GST price = inclusive price × 100/109; GST = inclusive price × 9/109.
Why is SGD 109 not multiplied by 9%?
SGD 109 already contains SGD 9 tax. Multiplying by 9% gives SGD 9.81 because it taxes the tax again. The correct extraction is SGD 109 × 9/109 = SGD 9.
Inclusive examples at SGD 10, 100 and 250
| Inclusive total | Before GST | GST inside total |
|---|---|---|
| SGD 10.00 | About SGD 9.17 | About SGD 0.83 |
| SGD 100.00 | About SGD 91.74 | About SGD 8.26 |
| SGD 250.00 | About SGD 229.36 | About SGD 20.64 |
Line-by-line rounding and invoice-total rounding can differ by a cent or two. Use the tax invoice as the formal record.
Add service charge before GST on a restaurant bill
On a SGD 100 menu price with 10% service charge, the common GST base is SGD 110. GST is SGD 9.90 and the final total is SGD 119.90. Compare final payable prices rather than menu prices alone.
Which situations are not standard-rated at 9%?
- Zero-rated, exempt and out-of-scope supplies.
- A non-GST-registered business cannot label an ordinary charge as standard-rate GST.
- Bare residential rent, furniture and fittings can have different classifications.
- Imported low-value goods and cross-border digital services need supplier and place-of-supply checks.
Receipt and invoice questions
Does dividing by 1.09 give the GST?
No. It gives the before-GST price; subtract that from the total to get GST.
Why can the calculator differ from a receipt by one cent?
Rounding each line versus the full invoice can produce a small difference.
Should a displayed consumer price already include GST?
IRAS generally requires GST-registered businesses to display or quote an inclusive price clearly to the public, with specific display treatment for hotels and F&B businesses charging service charge.
Convert a price or read IRAS
Use the GST calculator · IRAS inclusive-price guidance
Written and reviewed by the QuickCalc editorial team. Last reviewed: 8 Sep 2026. Report a rule or calculation issue at w56975828@gmail.com.