Adding a percentage to a base amount and removing an included percentage from a total require different operations. The GST Calculator handles both using a rate you choose. It performs arithmetic; it does not determine the applicable tax rate, eligibility, invoicing requirements, or filing obligations.
Decide what the entered amount represents
Enter Amount and GST rate (%). In Calculation, choose Add GST if the amount excludes tax, or Remove included GST if it already includes tax. Then choose Calculate. The amount must be nonnegative and the rate must be between 0 and 100 percent.
The result always labels the three quantities separately: Before tax, GST, and Including tax. Use one currency for the input and interpretation. No currency conversion or jurisdiction lookup occurs, and no current tax rate is supplied by an external service.
Work through both directions
For an illustrative amount of 1,000 and a manually selected rate of 18%, choose Add GST. Tax is 1000 × 0.18 = 180, and the total is 1000 × 1.18 = 1180. The result is 1,000.00 before tax, 180.00 GST, and 1,180.00 including tax.
Now enter 1,180 with the same rate and choose Remove included GST. The base is 1180 / 1.18 = 1000, and the tax is 180. The two directions agree because both use the tax-exclusive base as the reference for the percentage.
Simply subtracting 18% of 1,180 would give 967.60, which is not the original base. That subtraction treats the larger tax-inclusive total as the percentage reference. For an included amount T and decimal rate r, the correct base in this model is T / (1 + r), and the tax is T − base.
Check the source document’s rounding rules
This tool applies one rate to one amount and rounds displayed values to two decimals. It does not split taxes into components, handle multiple rate categories, model input credits, or calculate a whole invoice line by line. A document that rounds each item separately can differ slightly from a single calculation on the combined amount.
Confirm the appropriate rate and required treatment independently before using the result in records. The example rate above is a mathematical assumption, not a statement about any product or location.
Inputs are processed in your browser without uploading a financial document. Use the Percentage Calculator for a simple share calculation and the Discount Calculator for a price reduction. If a document combines discount and tax, establish which base each operation uses before calculating them separately.