Interest on Late GST Payment: How It Actually Gets Calculated

Miss a GST payment deadline and the cost isn't a one-time penalty — it's interest that accrues for every day the tax stays unpaid, counted from the original due date, not from whenever you eventually get around to filing.

How the interest period is counted

Interest is calculated from the day after the payment due date up to the actual date of payment — including weekends and holidays. There's no grace period; a payment made one day late still accrues interest for that day.

The calculation itself

Interest = (Outstanding tax × Interest rate × Number of days late) / (100 × 365)

The rate applies on an annual basis but is calculated daily, so the longer a payment stays outstanding, the more it costs — not linearly perceived, but the same simple daily rate compounding in effect as days accumulate.

Why the exact rate matters

The applicable interest rate depends on the specific circumstances of the delay (a straightforward late payment is treated differently than tax recovered following a department assessment), and rates are set by government notification rather than fixed permanently — always confirm the current applicable rate on the official GST portal before relying on a calculation for a real filing.

Why this catches businesses off guard

Because the interest is calculated daily rather than billed as a lump sum, it's easy to underestimate what a "just a few days late" payment actually costs — especially on a large outstanding tax amount where even a single-digit day count adds up to a meaningful number.

Work out what's owed

Our GST Interest Calculator works out the interest amount for a given outstanding tax, rate, and number of days late, so you know the real cost of a delay before you make it. To get the original tax figure right in the first place, the GST Calculator handles adding or removing GST from a price.