What a tariff stack is
A tariff stack is the total duty cost on an import, made up of multiple layers. The base duty is the standard rate from the HTS general column. Chapter 99 (9903) lines can add additional tariff on top of the base rate for specific products and origins. Section 301 additional duties apply to products from designated countries under unfair trade investigations. Section 232 adds surcharges for steel and aluminum products. Each layer may apply or may not apply depending on the product, origin, and current tariff actions. TariffCatalog estimates are preparation aids only — verify the applicable rates with the official USITC HTS database or a licensed customs broker before filing.
Step 1 — Identify the correct HTS code
Start with the 10-digit US HTS code for the product. The first 6 digits follow the international HS system; the 7-10 digits are US-specific duty rates. Search the USITC HTS database (hts.usitc.gov) using the product description, material, and function. Read the heading description, chapter notes, and any additional notes. Do not assume a code is correct without reading the official tariff wording. The HTS code determines which duty columns apply, which additional tariff lines exist, and which trade programs may be available. A candidate HTS code from TariffCatalog or any other tool must be verified in the official USITC HTS database before use.
Step 2 — Read the base duty column
The HTS general column 1 (General) rate is the base duty for most countries. This rate applies when no preferential trade program is claimed. Look up the product HTS code in the USITC HTS database and note the column 1 general rate. This is the starting point of the tariff calculation. Some products carry a zero base duty; others carry rates from 0% to 37.5% or more. The general rate is an estimate-only starting point — the final applied rate depends on the origin, the product classification, and any additional tariff columns.
Step 3 — Check for Chapter 99 / 9903 additional tariff lines
Under the product HTS code, look for a Chapter 99 subheading (beginning with 9903) that may apply. Chapter 99 lines are used to apply additional tariffs that layer on top of the base duty. A 9903 line may appear alongside the product HTS code in the tariff schedule — it is not a replacement for the product code, it is an additional layer. Check the applicable 9903 line for the product and origin. The 9903 reference may affect total duty for certain China-origin goods and other designated products. Chapter 99 rates change more frequently than base duty rates; always check the current schedule.
Step 4 — Determine whether Section 301 additional duties apply
Section 301 additional duties apply to products from countries subject to active Section 301 investigations, most notably China. They are typically imposed at the 9903 HTS chapter level and appear as an additional rate on top of the base duty. To determine whether Section 301 applies: check whether the product HTS code is on the applicable USTR Section 301 product list for the origin, and check the applicable 9903 rate for that code and origin. Section 301 rates are estimate-only — verify with the official USTR notices and the USITC HTS Chapter 99 schedule before filing. Section 301 additional duties may stack on top of the base duty when applicable.
Step 5 — Determine whether Section 232 applies
Section 232 duties apply to steel and aluminum products and certain derivatives, based on national security import measures. They are imposed per kilogram or as an ad valorem rate and apply in addition to base duty. Section 232 is a separate tariff authority from Section 301 — it covers different products and is based on a different legal justification. Check whether the product is classified in a Section 232-covered HTS range and whether the origin triggers the measure. Verify the current Section 232 rate with the Department of Commerce proclamations and the Federal Register.
Step 6 — Confirm customs value
Duty is calculated on the customs value of the shipment. The customs value is typically the transaction value — the price paid for the goods, including freight and insurance if shipped DDP. A higher customs value produces a higher duty bill. Check whether the customs value on the commercial invoice is complete and consistent with CBP requirements. Freight, insurance, and other costs may or may not be included in customs value depending on the incoterm used. Verify the correct customs value basis before calculating total duty.
Step 7 — Add the layers together
Total duty estimate = customs value x (base duty rate + Chapter 99 rate + Section 301 rate + Section 232 rate, where each applies). Express each rate as a decimal (e.g., 25% = 0.25). This estimate is preparation use only — it is not a guarantee of the duty that will be assessed. Carrier brokerage fees, duties taxes, and processing fees are charged separately and are not part of the tariff calculation.
Step 8 — Account for why carrier bills may differ from estimates
The carrier or broker may assess a different duty amount than your estimate. Common reasons include: a different HTS code was applied at filing, a 9903 line was added or the applicable rate was different, the customs value was adjusted, Section 301 or Section 232 rates changed between the estimate date and the entry date, or brokerage and disbursement fees were added. A difference between an estimate and the actual bill is not automatically an error. Review the carrier duty bill against the commercial invoice to identify the cause.
Evidence checklist
Before relying on a tariff estimate for a business decision, confirm each layer with the following: (1) USITC HTS printout for the 10-digit product code and the applicable 9903 line. (2) USTR Federal Register notice or product list confirming Section 301 applicability for the product and origin. (3) Department of Commerce proclamation or Federal Register notice confirming Section 232 applicability. (4) Commercial invoice with confirmed customs value and country of origin. (5) Any applicable trade program documentation if a preferential rate is claimed.
Official-source checklist
Official sources to verify each tariff layer: (1) USITC HTS database at hts.usitc.gov — base duty and 9903 lines. (2) USTR website and Federal Register — Section 301 product lists and rate notices. (3) Federal Register and Commerce Department — Section 232 proclamations and steel/aluminum notices. (4) CBP CSMS messages for current tariff alerts. TariffCatalog estimates are preparation aids only; they do not substitute for official tariff verification.
Source note
The USITC HTS database at hts.usitc.gov is the official US tariff schedule. Section 301 actions are published by the USTR on ustr.gov and in the Federal Register. Section 232 proclamations are published by the Department of Commerce and the Federal Register. TariffCatalog is not a licensed customs broker or legal advisor. Estimates are preparation aids only. Verify all applicable tariff rates in official sources and consult a licensed customs broker before filing.