Add or remove GST at any rate — with the CGST + SGST (or IGST) split. Reverse GST from an inclusive price too.
Pick "Remove GST", enter the GST-inclusive amount and the rate — you get the base price and the GST included. Base = amount ÷ (1 + rate/100).
A sale within your state splits GST equally into CGST + SGST (half each). A sale to another state is one IGST at the full rate.
Completely free, no login — it runs in your browser.