How to calculate import duty (with a worked example)

To calculate duty on a US import, classify the product to its 10-digit HTS code, add up every rate that applies to that code and origin (the general rate, any Section 301 list rate and the 2026 layer under Section 301), multiply by the customs value, then add the merchandise processing fee and, for ocean freight, the harbor maintenance fee. Duty is not a mystery fee. It is a set of rates, attached to a code, applied to a value. This guide walks the full math on a real shipment.

The three inputs

1. The tariff code. Every duty rate in the world is attached to a code line, not to a product name. Classify the product first; everything downstream inherits from this step. If the code is new to you, start with the plain-English guide to Harmonized System codes.
2. The duty rates for your origin. In 2026 a US import rarely pays one rate. Start with the general (column 1) rate for the code, use a preferential rate instead if a trade agreement covers the lane and the goods qualify, then add the Chapter 99 layers that apply to that origin: the original Section 301 lists for goods of China, the Section 232 metals, lumber and auto duties for covered articles, and since July 24, 2026 a Section 301 layer of 10 or 12.5 percent on goods of most investigated economies.
3. The customs value. Usually the transaction value, what you actually paid for the goods. The US values goods without international freight and insurance, which is cheaper than the CIF basis the EU uses, but assists, royalties and some commissions still belong in the value.

The formula

Duty = customs value × (general rate + every Chapter 99 rate that applies).
MPF = customs value × 0.3464%, at least $34.58 and at most $670 on a formal entry (fiscal 2027).
HMF = customs value × 0.125%, ocean shipments only.
Cost of entry = value + duty + MPF + HMF.

The US charges no federal import VAT, so these four lines are the whole government bill at entry. Each Chapter 99 layer is added to the general rate, not multiplied by it, and each is applied to the same customs value.

A worked example

Say you import 1,000 men's knitted cotton t-shirts from Türkiye into the United States by sea, at $6 a unit.

• Step 1, classify. Knitted cotton t-shirts sit at 6109.10.00. Knitted keeps them in Chapter 61 (the woven twin would be Chapter 62), cotton keeps them in the .10 subheading (polyester would drop to 6109.90 at a much higher rate).
• Step 2, the rates. The US general rate for 6109.10.00 is 16.5 percent. Türkiye has no US free-trade agreement, so the general rate stands. Türkiye is one of the economies under the July 2026 Section 301 action at 12.5 percent, so the stack is 16.5 plus 12.5, or 29 percent, unless the line appears in that economy's exemption annex.
• Step 3, the value. 1,000 × $6 = $6,000 customs value.
• Step 4, the duty. $6,000 × 16.5% = $990, plus $6,000 × 12.5% = $750, for $1,740 of duty.
• Step 5, the fees. MPF at 0.3464 percent would be $20.78, below the floor, so it is $34.58. HMF on an ocean entry is $7.50. The total paid to CBP is $1,782.08, about 29.7 percent of the goods value.

Now the point of the whole exercise: misclassify the same shirts as polyester, and the rate jumps to 32 percent general plus 12.5, so the duty is $2,670 instead of $1,740, an overpayment of $930 on one modest shipment, repeating on every entry until someone rereads the code. Misclassification the other way, underpaying, surfaces later as back duties plus penalties under 19 USC 1592.

The same calculation for a whole catalog

One shipment is a back-of-envelope job. A catalog is not, because every SKU has its own code, every code its own list status, and every supplier its own origin. A brand with 400 products from three countries is running 400 versions of the five steps above, and a single wrong code repeats on every reorder.

On the paid plans you upload a CSV with a description, material and origin per row and get the 10-digit code, the reasoning and every duty layer by origin for each line, ready to drop into a landed cost sheet. Plan details are on the HS code classification pricing page, and brands deciding whether to keep paying a broker per line can compare the two routes in customs broker vs classification software.

Where the calculation goes wrong in practice

• Wrong code. The dominant error, and the quietest. The rate belongs to the code, so every downstream number inherits the mistake.
• Right code, wrong origin treatment. Claiming a preferential rate without meeting the origin rules, or missing a layer that applies to your origin. In 2026 the July 24 Section 301 layer differs by economy (10 percent for some, 12.5 percent for others, a top-up to 10 percent for the EU and Taiwan), and goods covered by Section 232 are outside it.
• Specific and compound rates. Not every line is a simple percentage: some are cents per kilogram, or a percentage plus a specific component. Read the rate column, not just its headline number.
• Valuation misses. Assists, royalties and certain commissions belong in the customs value; getting this wrong understates the duty base.
• Stopping at duty. Duty is one bucket of what the shipment truly costs. Freight, insurance and fees belong in the landed cost formula.

Do it in seconds instead

The import duty calculator runs this entire chain from a plain-English product description: the AI classifies the product, reads every rate that applies to your origin, adds MPF and HMF, and shows each line of the math separately, with the classification rationale attached. It is a best-effort estimate for guidance, not a binding ruling, and it turns the verify-before-filing conversation with your broker into a five-minute check instead of a from-scratch lookup.