GST conversion has two directions: adding tax to a pre-tax amount and extracting tax from an inclusive price. Different denominators make this a common source of error.
How the rule works
The current standard rate is 9%. Add 9% to a pre-tax amount; for an inclusive total, GST is 9/109 of that total.
Calculation method
Inclusive = exclusive × 1.09. Exclusive = inclusive ÷ 1.09. GST within an inclusive total = inclusive × 9/109.
Three practical examples
- SGD 100 before tax gives SGD 9 GST and SGD 109 inclusive.
- SGD 109 inclusive contains SGD 100 before tax and SGD 9 GST.
- SGD 250 inclusive contains about SGD 20.64 GST, not SGD 22.50.
Special cases and limits
- Zero-rated, exempt and out-of-scope supplies do not use the ordinary 9% conversion.
- Invoice rounding may cause cent-level differences.
- Only GST-registered businesses may charge GST under the rules.
Frequently asked questions
Why not multiply an inclusive price by 9%?
Because the inclusive price represents 109%, making tax 9/109.
Does every supply include GST?
No. Zero-rated, exempt and out-of-scope categories exist.
Sources and review
Written and reviewed by the QuickCalc editorial team. Last reviewed: 4 Sep 2026. To report a rule or calculation error, email contact@zevlis.com.