How duties per kilogram, liter or proof liter are calculated

Many lines of the US tariff do not charge a percentage of the value. They charge an amount for each kilogram, liter, dozen or proof liter, sometimes on top of a percentage. For these goods the quantity matters as much as the price, and the quantity the duty counts is not always the one you count. Getting it wrong is the most common way a duty estimate goes astray.

Three forms of duty rate

Ad valorem: A share of the customs value, such as "16.5%". It follows the price.
Specific: An amount per unit, in cents or dollars: per kilogram ("¢/kg"), per liter, per dozen ("/doz."), per proof liter ("/pf. liter"), per square meter. It ignores the price: a cheaper product pays the same per unit, so a larger share of its value.
Compound: Both at once, a specific part plus a percentage, such as "…¢/kg + …%". Each part is calculated on its own quantity and the two are added.

Which quantity the duty counts

The rate names its unit. Each line also has a "Unit of quantity" column for trade statistics, sometimes with two units, such as dozens and kilograms for garments. The quantity a duty is charged on is the one its rate names, which is not always the one you count your goods in: you buy coats by the piece, and their rate may charge part of the duty by the kilogram.

Example: a compound rate on garments

Take a coat whose line, to keep the numbers simple, has a compound rate of 50¢/kg + 15% (an illustrative rate, not that of any line today). You import 500 coats at $40 each, weighing 1.2 kg each:

the value is 500 × $40 = $20,000, and the ad valorem part 15% × $20,000 = $3,000;
the weight is 500 × 1.2 kg = 600 kg, and the specific part 600 × $0.50 = $300;
the duty is $3,000 + $300 = $3,300.

Counting the 500 pieces as 500 kilograms would give a specific part of $250 instead of $300: the error is in the quantity, not in the rate.

Example: spirits by the proof liter

Spirits and some other alcoholic products are charged by the proof liter. A proof liter is a liter at 100 proof, that is, 50% alcohol by volume, and the duty of a product at another strength is in proportion to its proof (additional U.S. notes 5 and 6 to chapter 22 of the tariff). So the proof liters of a shipment are its liters times its alcohol strength, divided by 50%:

1,000 bottles of 70 cl hold 700 liters;
at 40% alcohol, they make 700 × 40 ÷ 50 = 560 proof liters;
at a rate of 20¢ per proof liter (illustrative again), the duty is 560 × $0.20 = $112.

Counting each bottle as a proof liter would give 1,000 proof liters and $200, 79% too much. The duty is also separate from the federal excise tax on alcohol, which is not a customs duty and which TariffTrust does not calculate.

Dozens, pairs and particular weights

Dozens and gross. A rate per dozen counts the pieces by twelve: 1,200 pieces make 100 dozen. A gross is 144 pieces.
A particular weight. Some rates say which weight they count: the drained weight of goods packed in a liquid, the gross weight with their packing, the clean weight of wool, or the weight of the sugar, copper or lead a product contains. Read the rate's own words, and the notes of its chapter, before you weigh.
Volume. A rate per liter counts the liquid, not the containers.

How TariffTrust asks for quantities

In the calculator, "Per unit" asks how many you import, in the unit the line is counted in, and the price of one: the value is their product.
When the duty counts another quantity, the calculator asks for it apart, by its name: the total weight of garments counted in pieces, the proof liters of bottles, with how to work them out, or the drained weight.
A quantity it can convert is not asked twice: pieces become dozens or gross, kilograms become metric tons.
Without the quantity a specific rate needs, the estimate gives no amount for that duty and asks for it ("Needs information"); it never assumes one. With a quantity, the result also says what one unit owes.

Updated October 30, 2026. General information as of that date, not legal or customs advice: the official texts prevail, and a licensed customs broker can confirm what applies to your goods.

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