Home / Guides
How to read a duty stack
Understand the base rate, additional duties, exceptions and fees before reviewing an import estimate.
Source review: September 28, 2026. This explains a method, not a complete or currently certified rate schedule.
The one idea
An import estimate can contain several layers: a base rate, additional duties and separate fees. Each has its own coverage and timing. Before adding percentages, identify which rules apply and whether they interact. A similar-looking product is not necessarily classified or treated the same way.
The layers, bottom to top
1. MFN base rate — the foundation
Start with classification and the relevant tariff treatment in the official USITC Harmonized Tariff Schedule. Do not stop at one search-result percentage: read the applicable headings, notes, special treatment and additional provisions. The HTS includes more than base rates. Preserve the schedule edition and classification evidence used for the estimate.
2. Section 301 — unfair-trade actions
Section 301 actions have separate country, product and exclusion rules. CBP's July 23, 2026 guidance, CSMS 69326983, specifies a July 24 effective date for its forced-labor action. It does not use one universal net-of-MFN formula: China is listed with an additional 12.5%, while the EU uses a 10% combined column-one/Section 301 threshold and Japan uses a 12.5% threshold, subject to stated exceptions. Check the exact country heading and exclusions; do not apply one country's arithmetic to another. Other Section 301 actions also require separate coverage review.
3. Section 232 — national-security actions
Check product coverage, derivative-product rules, origin treatment and applicable effective dates for each Section 232 action. A product label such as “steel part” is not enough to establish coverage. The portal's selectable research scenarios are not a verified list of all applicable Section 232 rates; product-specific rates and exclusions remain a broker-review item.
4. Other authorities
Safeguards and other trade measures may need separate checks; absence from this calculator does not establish an exemption. For a historical example, the February 20, 2026 Section 122 proclamation specified a February 24 start and a July 24 endpoint, with exceptions and provisions for changes. Announcement day was not the start date. For IEEPA entries, review the separate refund guide; a court ruling is not automatic proof of individual refund eligibility.
5. Fees — MPF and HMF
Keep Merchandise Processing Fee and Harbor Maintenance Fee calculations distinct from duty. Confirm the applicable entry type, transport mode, exemptions and current fee schedule, including any minimum and maximum. The stored prototype fee values have not been certified as current and should not be copied into a shipment quote without review.
How the layers combine
For a simple hypothetical additive scenario, a 5% base rate and an applicable additional 10% duty on the same $1,000 customs-value basis produce $150 before fees. That is arithmetic, not a tariff recommendation. Combined-rate thresholds, specific or compound duties, different valuation bases and exclusions can change the calculation. Confirm valuation rather than treating the invoice alone as conclusive. Antidumping and countervailing duties require their own case-specific review and are not fully modeled here.
Reading a stack like a pro
- Check the "as of" date. A stack computed from September data is wrong if a new action took effect in October. Freshness is the whole game.
- Verify coverage, not just rates. A 50% §232 rate means nothing if your HTS isn't a covered product. "Applies to covered products" is doing real work in every line.
- Check the legal timing condition. Entry for consumption, warehouse withdrawal, transit exceptions and exact cutoff times can matter. The Section 122 proclamation's stated start was February 24, not February 20.
- Review assumptions with a broker. Bring the source, effective date, HTS code, origin evidence and unanswered coverage questions. A portal result is not a filing instruction.
Also in this series: De minimis changes: a DTC planning guide · The IEEPA ruling and your refund