Duty stack
The ordinary HTS rate is only the starting point
A standard classification in Chapters 1 through 97 identifies the merchandise and its general rate. An official note may then direct the importer to a Chapter 99 heading. The heading can state that its duty applies in addition to the ordinary rate.
General HTS duty + supported Chapter 99 additional duty
Other applicable fees and measures remain separate.
Engine behavior
How TariffCalc treats a Chapter 99 reference
The same official schedule must contain a supported heading and an unambiguous additional percentage.
The complete legal description must be matched; the engine does not infer eligibility from an HTS number alone.
The automatic amount is withheld when the rule is not in the versioned registry or would overlap inside one stacking group.
Current release
What is automated today
The current release calculates supported China Section 301 headings in the 9903.88 and 9903.91 families when the ordinary HTS line explicitly points to the heading. It also includes one narrow Section 232 slice: China-origin, declarable HTS 7208 goods under 9903.82.02. Other Section 232 products or origins, Section 122, quotas, safeguards, AD/CVD and other Chapter 99 families remain review-only.
This boundary avoids a common tariff-calculator error: showing an apparently exact total after silently treating every unmodeled provision as zero.