SGD 100 before GST
- Input
- Mode: add GST; pre-tax amount SGD 100
- Result
- GST SGD 9; total SGD 109
- What it means
- When a quote excludes tax, calculate 9% of the pre-tax amount and add it back.
GST calculator
Convert GST-inclusive and exclusive prices.
Adding GST and extracting GST use different formulas
You cannot find the tax inside a GST-inclusive SGD 109 by calculating 109 × 9%; extraction starts by dividing by 1.09.
SGD 109 including GST
- Input
- Mode: remove GST; inclusive amount SGD 109
- Result
- Pre-tax amount SGD 100; GST component SGD 9
- What it means
- Calculating 109 × 9% gives SGD 9.81 and wrongly taxes an amount that already contains tax.
A GST-inclusive SGD 250 bill
- Input
- Mode: remove GST; inclusive amount SGD 250
- Result
- Pre-tax amount about SGD 229.36; GST component about SGD 20.64
- What it means
- An invoice may round each item or the whole bill, so the final cents can differ slightly.
How should you use the calculated result?
“Add GST” is for a pre-tax price; “remove GST” is for a total that already includes tax. Both use 9%, but the base is different, so the formulas are not interchangeable.
When should you not use this result directly?
- Zero-rated, exempt and out-of-scope supplies, as well as some overseas services or import arrangements, may not be a simple 9% case.
- A non-GST-registered business generally cannot charge GST merely because this calculator produces an amount.
- Service charges, discounts, line-item rounding and classification affect invoices; use the supplier’s tax invoice as the formal amount.
Continue with the GST conversion guide
The guide separates tax-inclusive extraction from adding tax, then uses shopping and invoice examples to explain common errors and rounding.
Official source: Open authority guidance
Rules last checked: 7 Sep 2026. See the Sources page.