SWS is charged on the aggregate, not on Basic Duty alone

SWS is charged on the aggregate of customs duties, not on Basic Duty alone. If your spreadsheet does the latter, every import you have costed is wrong — and always in the same direction.

What the rule actually says

Social Welfare Surcharge is levied at 10% of the aggregate of duties of customs. The word doing the work is aggregate. That means Basic Customs Duty plus AIDC plus any applicable cess — not Basic Duty on its own.

Most spreadsheets get this right when AIDC and cess are nil, because then the aggregate and the Basic Duty are the same number. The error only appears on tariff lines where AIDC or Health Cess actually applies, which is precisely where the amounts are large enough to hurt.

A worked comparison

₹10,000 looks survivable until you remember IGST is charged on assessable value plus all duties. The understated SWS drags the IGST base down with it, and the error compounds through to your selling price.

The aggregate is not always the aggregate

One caveat the rule-of-thumb version leaves out: the base is not fixed. Specific customs notifications exempt AIDC on named tariff lines, and where that applies the surcharge is no longer charged on a sum that includes it. That is why the calculator lets you choose the surcharge base — the three duties together, basic duty plus health cess, or basic duty alone — instead of hardcoding one. Read the notification for your own tariff line and set the base to match it; a rule applied blind is the same error in the other direction.

Why it matters more than the rupees

An understated landed cost does not just cost you the difference. It makes you quote a selling price that looked profitable and was not. You find out at the end of the quarter, across every shipment on that tariff line, all at once.