How to add and remove New Zealand GST: 15% and the 3/23 fraction

Establish the treatment before doing arithmetic

Exclusive means the price before GST. Inclusive means the total with GST already included. Decide which starting amount you have before adding or extracting tax.

IRD’s overview and charging guidance describe the GST context. The New Zealand GST calculator starts with its treatment blank. Selecting NZD does not decide registration, eligibility or the time of supply.

This guide assumes one ordinary monetary price already confirmed as standard-rated. It is not a GST return, registration service or product-classification engine.

Add GST to an exclusive price

For an amount already confirmed as subject to 15% GST, calculate the tax from the price before GST. The New Zealand engine rounds the GST component first under its disclosed provisional cent-display policy, then adds it to the original price.

GST = roundGST(exclusive × 15 ÷ 100)
Inclusive = exclusive + GST

IRD’s calculation examples reconcile NZD 100 exclusive, NZD 15 GST and NZD 115 inclusive. Those published figures do not resolve how every possible half-cent should be displayed.

Extract GST with the exact inclusive fraction

For that standard reference, included GST is the inclusive price multiplied by 3 and divided by 23. Do not use a truncated decimal substitute for the fraction, or subtract 15% directly from the total.

GST = roundGST(inclusive × 3 ÷ 23)
Exclusive = inclusive − GST

IRD separately publishes the reverse NZD 115 example. Its charging guidance also states NZD 13.04 GST inside NZD 100 inclusive; the corresponding exclusive amount in our table is derived by subtraction.

The fraction follows from the tax base: 15 parts GST divided by 115 parts inclusive price simplifies to 3/23. It is not 15/100 of the inclusive total. At NZD 115 including GST, subtracting 15% would leave NZD 97.75 rather than the NZD 100 before GST in IRD’s example.

Worked examples from the New Zealand engine

The last row is synthetic: NZD 0.10 exclusive produces exact GST of NZD 0.015, displayed by this tool as NZD 0.01. That provisional half-down boundary is intentionally different from the Australian and Irish engines’ half-up displays. The chart and table use the New Zealand engine, not the global manual calculator.

Why the rounding policy remains provisional

GST Act section 19O describes an optional approach for fractional cents in amounts of tax shown in taxable supply information or supply correction information. It distinguishes fractions no greater than half a cent from larger fractions; it also provides individual-amount and supply-total alternatives and requires consistency within a taxable period when relying on that approach.

The consolidated text viewed on 18 October 2026 is dated 30 June 2026 and carries an unincorporated-amendments notice. Do not treat the displayed version date or this one-amount implementation as a complete assessment of which requirements apply to your transaction or document. Amendment applicability and professional review remain incomplete; no universal mandatory half-down rule is claimed.

In this guide, roundGST means the calculator’s provisional GST-first, nearest-cent display with exact half-cents down. The tool neither aggregates invoice lines nor enforces period-wide consistency. Its arithmetic is not proof of invoice or filing compliance. Other tools may round a different component or use another convention, producing different boundary results.

Keep classifications and unsupported cases separate

Zero-rated supplies have conditions; a zero selection does not establish them. Exempt supplies are distinct. Exempt and outside-scope selections clear numerical results instead of being relabelled as zero-rated.

Mixed baskets, special supplies, cash payments, multi-line invoices, credit adjustments, discounts, imports, exports, GST credits and returns are outside this guide’s implemented scope. Confirm treatment independently before using arithmetic results.

Check your result before using it

Check the starting price, the independently established treatment and whether the price before GST plus the displayed tax equals the inclusive total. A calculator with tax can explain that arithmetic without deciding the legal treatment of the supply.

Keep mixed rates and individual invoice lines separate from a one-amount example. Seek appropriately qualified advice when eligibility, document requirements or filing obligations are uncertain. Cash-payment rounding and GST credits are not implemented here.

Sources and calculation limits

IRD supplies the independent addition and extraction examples and the distinction between zero-rated and exempt supplies. Section 19O provides the limited rounding context discussed above, not a universal compliance certification. The examples are calculations, not customer testimonials.

Use the New Zealand GST calculator, browse official New Zealand tax references or read our calculation methodology. This guide is informational and does not replace professional tax advice.