Reverse GST Calculator

Extract the GST from a GST-inclusive amount. Free reverse GST calculator to find the base value and tax from a total at 5%, 12%, 18% or 28%.

How it works

When a price already includes GST, the base is total × 100 ÷ (100 + rate) — not simply the total minus the rate percent, which is a common mistake. This matters when a client quotes you an all-inclusive number and you need the pre-tax figure for your books.

Frequently asked

How do I remove GST from a total?: Divide the total by (1 + rate). For 18%, base = total ÷ 1.18. The GST is the total minus that base.
Why isn’t it just total minus 18%?: Because the 18% was added to the base, not the total. Subtracting 18% of the total over-removes the tax. The correct base is total × 100 ÷ 118.

On a Retainer invoice you enter the base and GST is added correctly — you never have to work backwards from a total. Try Retainer free