GST Calculator

GST (Goods and Services Tax) is a broad consumption tax charged on most goods and services. Australia, Canada, India, New Zealand and Singapore all run one, along with dozens of other countries. The rules around the edges differ; the arithmetic in the middle does not.

Adding GST to a pre-tax price: GST Amount = Pre-Tax Price × GST Rate Total Price = Pre-Tax Price × (1 + GST Rate)

Pulling GST out of a GST-inclusive price: GST Amount = Inclusive Price × (GST Rate ÷ (1 + GST Rate)) Pre-Tax Price = Inclusive Price ÷ (1 + GST Rate)

That second one is where people go wrong. Taking 10% off a $935 inclusive total gives $841.50, which is not the pre-tax price. Divide by 1.10 instead and you get $850. The difference is small on one invoice and painful across a year of them.

Rates:

India splits the tax in two. An 18% rate inside one state is 9% CGST to the central government and 9% SGST to the state. Sell across a state line and the same 18% goes out as a single IGST charge instead. The customer pays the same either way, but the return you file is different, and this calculator shows the split when you pick an Indian slab.

Input Tax Credit, which is the whole point of GST. A registered business collects GST on what it sells and claims back the GST it paid on what it bought. Only the difference goes to the tax office:

Net GST Payable = GST Collected on Sales − GST Paid on Purchases

Worked example, Australia at 10%:

Adding GST to an $850 web design invoice gives $85 of GST and a $935 total. Going the other way, a $935 total contains $935 × (0.10 ÷ 1.10) = $85 of GST and $850 of price.

Worked example, a quarterly return: A business sells $220,000 excluding GST and buys $85,000 excluding GST, both at 10%.

• GST collected: $22,000
• Input tax credit: $8,500
• Net payable: $13,500

Without that credit the tax would compound at every link in the supply chain, and a product passing through four businesses would carry tax on tax on tax. That is exactly what GST replaced in most countries that adopted it.