How US Tariffs Are Calculated: Formula and 2026 Examples

To calculate US import duty, multiply the customs value by the HTS base rate for the country of origin plus every Chapter 99 tariff in force on the entry date (after exemptions and stacking rules), then add the Merchandise Processing Fee and, for ocean freight, the Harbor Maintenance Fee. On October 6, 2026, $10,000 of steel articles from China owes $7,790 in duty plus $47.14 in fees. Here's the method, step by step, with four real examples.

The import duty formula

Every US import duty comes down to the same four steps:

1. Find the HTS code for the product.
2. Apply the base rate for its country of origin.
3. Add every Chapter 99 tariff that applies on the entry date, after exemptions and stacking rules.
4. Add the customs fees.

Total = customs value × (base rate + each Chapter 99 rate that applies)
      + Merchandise Processing Fee
      + Harbor Maintenance Fee (ocean only)

Customs value is usually the transaction value: the price actually paid or payable for the goods. Unlike many countries, the US leaves international freight and insurance out when they're shown separately from the price (19 U.S.C. 1401a). A few rates are specific instead, like cents per kilogram, and use the quantity rather than the value.

The arithmetic is easy. The work is in steps 1 and 3: getting the code right, and knowing exactly which Chapter 99 headings apply to that code, from that country, on that date.

Step 1: Find the HTS code

The HTS code decides the base rate and which additional tariffs cover the product. It's 10 digits, like 7326.90.86.88 (other articles of iron or steel). Classification follows the General Rules of Interpretation and the legal notes of each section and chapter, and CBP rulings are the best guide to how close calls get decided.

A wrong code is the most common reason a duty calculation is wrong. Two codes a few digits apart can sit on different Section 301 lists or Section 232 product lists.

HTS classification

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Your duty is only right if your HTS code is right. HTS Hero walks the tariff schedule with you and backs every decision with notes and CBP rulings.

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What you get

• Classifications built on the GRIs and legal notes
• Supporting CROSS rulings for every code
• A report you can hand to your broker or an auditor

Step 2: Apply the base rate

Each HTS line has three rate columns. Which one you use depends on the country of origin and whether you claim a trade preference:

• Column 2 if the country is Cuba, North Korea, Russia or Belarus.
• Special if the goods qualify for a free trade agreement or preference program on that line (USMCA, for example) and you claim it.
• General for everyone else.

Step 3: Add the Chapter 99 tariffs

This is where most of the duty comes from in 2026. For each Chapter 99 heading, three questions decide whether it applies:

• Does it cover the product? Most headings apply to a list of HTS codes in a U.S. note.
• Does it cover the country? Section 301 China only applies to China; Section 232 applies to every country, with deals for some.
• Is it in force on the entry date? Every heading has a start date, and many have end dates or rate changes.

Then exemptions and stacking rules decide what's actually charged. Some exemptions need a fact about your goods (does it contain steel? is the drug a generic?). Some tariffs switch off when another applies: for example, the Section 301 forced-labor duty doesn't apply to goods that pay a Section 232 tariff (exemption heading 9903.05.90). Our guide to US tariff stacking rules covers every combination, and the latest US tariff changes lists what moved in 2026.

Step 4: Add the customs fees (MPF and HMF)

• Merchandise Processing Fee (MPF): 0.3464% of the value on formal entries. For fiscal year 2027, starting October 1, 2026, the minimum is $34.58 and the maximum is $670.86. On a $10,000 entry it's $34.64.
• Harbor Maintenance Fee (HMF): 0.125% of the value, ocean shipments only. On a $10,000 entry it's $12.50.

Import duty calculation examples (October 2026)

Every example below was calculated with HTS Hero's tariff engine for an entry on October 6, 2026, with a customs value of $10,000, shipped by ocean, and no exemptions claimed.

A steel article from China

HTS 7326.90.86.88, other articles of iron or steel.

Section 301 China stacks on top of Section 232, but the forced-labor duty switches off because a Section 232 heading applies. If the importer could show the article contains no steel, aluminum or copper, the $5,000 Section 232 line would disappear and the 12.5% forced-labor duty would apply instead.

A cotton T-shirt from Vietnam

HTS 6109.10.00.12, men's cotton T-shirts.

From February 24 to July 23, 2026, the same shirt paid the 10% Section 122 surcharge instead of the 12.5% forced-labor duty: $2,650 in duty instead of $2,900. One date, a different answer (what changed on July 24).

A patented medicine from Germany, and from India

HTS 3004.90.92.60, medicaments in measured doses, on the Section 232 pharmaceutical list.

Same product, same value, an $8,500 difference. The EU, Japan, South Korea, Switzerland and Liechtenstein pay a 15% all-in rate under their deals, the UK pays no additional duty, and most other countries pay up to 100% on patented drugs. A generic version of the drug, confirmed as such, would pay no Section 232 duty from either country. More in our post on the Section 232 pharmaceutical tariff.

An upholstered wooden seat from Japan

HTS 9401.61.40.11, upholstered seats with wooden frames.

Common duty calculation mistakes

• Leaving out the base rate. Several free calculators show only the additional tariffs.
• Using last month's rules. A rate that was right in July may be wrong in October. The HTS was revised 20 times in 2026 through September.
• Adding freight to the value. US duty is charged on the price of the goods. International shipping and insurance shown separately on the invoice stay out of the customs value.
• Missing exemptions, or claiming ones that need proof you don't have.
• Stacking everything. Adding every tariff that mentions your country overstates the duty when a Section 232 tariff switches others off.

The safest way to check a number is to see every line with the Chapter 99 heading behind it, so you can follow the math yourself. That's what the HTS Hero duty calculator shows for any HTS code, country and entry date.