GST Calculator NZ

Calculate GST (15%) and easily split an amount between business and personal use.

How to Calculate GST Manually

Here’s a comprehensive guide on how to calculate GST (Goods and Services Tax) in New Zealand, including methods for both removing GST (GST-inclusive) and adding GST (GST-exclusive) amounts, and how to handle amounts that are partly for business and partly for personal use.

Calculating GST:

1. Removing GST From a GST-Inclusive Amount:

A GST-inclusive amount is the total amount that includes GST. In New Zealand, the GST rate is 15%. However, when we remove GST from a GST-inclusive amount we need to use a special formula.

Formula: GST Amount = GST-Inclusive Price x 3 ÷ 23

Example: If your car insurance bill is $250 then 250 x 3 ÷ 23 = $32.61

So, the GST amount is $32.61, and the GST-exclusive amount is 250 – 32.61 = $217.39

2. Adding GST to a GST-Exclusive Amount:

A GST-exclusive amount is the amount before GST has been added.

Formula: GST Amount = GST-Exclusive Amount x 15%

Example: If you have a GST-exclusive amount of $100 and the GST rate is 15% then 100 x 0.15 = $15

So, the GST amount is $15, and the GST-inclusive amount is 100 + 15 = $115

Partial Business and Personal Use Calculation:

When an amount is partly for business use and partly for personal use, only the business use portion can be claimed for GST purposes.


  1. Determine total GST on the amount: Use the formulas above to calculate the total GST on the full amount.
  2. Determine business use percentage: Identify the percentage of the expense that is for business use.
  3. Calculate business portion of GST: Multiply the total GST amount by the business use percentage.

Example: Let’s say you have a GST-inclusive amount of $230, and 60% of this amount is for business use.

  1. Calculate the total GST: 230 x 3 ÷ 23 = $30
  2. Calculate the business portion of GST: 30 x 0.60 = $18

So, the amount of GST you can claim for business purposes is $18.

In summary, by following these steps and formulas, you can accurately calculate and apportion GST for both GST-inclusive and GST-exclusive amounts, as well as for amounts that are partly for business and partly for personal use.

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