美国对等关税是怎么算出来的？一个简单粗暴的计算公式

2025年4月3日（星期三）是一个极其值得纪念的日子。这一天，美国总统特朗普正式打响贸易大战，对全球征收所谓的对等关税（reciprocal tariffs）。

特朗普宣布对美国贸易伙伴实施普遍关税，所有进口至美国的商品统一征收10%的基准关税。此外，对他认定为“不公平贸易行为最严重”的60个国家额外征收更高关税。其中，对欧盟和中国分别征收20%和34%的关税。对中国的额外关税是在特朗普此前实施的20%关税基础上追加，这样对中国进口商品的总关税将为54%。



特朗普称赞这是美国的“解放日”。这一宣布震惊了全世界，并引发了全球贸易战的担忧。股市应声下跌，关税的规模和范围令投资者感到措手不及。中国马上反击，次日宣布“对原产于美国的所有进口商品加征34%关税”。

同时，美国政府公布了额外关税的计算公式，如下：



这篇小文的目的是解释一下这个公式。至于这个公式能否行得通，那将是另外一个话题。

这个公式其实并不复杂，不过需要理解里面的几个概念。我把官方网站的原文放在后面，读者可以参考对照。

美国政府希望通过提高关税来提高进口产品的价格，从而降低进口量，最终达到进出口贸易额对等，即零顺差或逆差。

?τ_i 是新增加的额外关税率（change in tariff rate）。

【注：上面的 ？代表小三角。小三角符号文学城显示不出来】

ε 是进口产品的需求弹性（elasticity of imports）。需求弹性是经济学里的一个基本概念，其定义为：

需求弹性 = 需求量变动百分数/价格变动百分数。

这个参数有实证数据做参考。这里，美国政府取其值为4。也就是说，如果进口价格上升10%，那么进口量就会降低 40%。

φ 是所谓的 passthrough，即关税提高后有多少会反映在进口价格里。这里，美国政府取其值为 0.25，即25%。这意味着，如果关税提高10%，进口价格将增加 2.5%。

理解了这几个概念后，我们可以按部就班就把因果关系梳理一下：

目的：通过征收额外关税，把贸易逆差降至为零。

假若额外征收的关税率为 10%，进口价格就会增加 2.5% （=10%×25%）。

进口价格增加 2.5%，进口量就会减少10% (=4×2.5%)。

进口量减少 10%，进口额就会降低 10%*当前进口额。

所以，只要选择一个合适的 ?τ_i ，让  进口额降低量 = 当前的贸易差额 (x_i - m_i)，

贸易逆差神奇地就消失了。

这样，我们就可以得出上述那个简单粗暴的公式。感兴趣的读者不妨自己推演一下。

由于美国政府对 ε 和 φ 这两个参数的取值分别为 4 和 0.25，两者相乘恰好为1，所以那个公式就可以简化成

额外关税率 = （出口量 - 进口量）/ 进口量

一个简单粗暴的公式诞生了。

附：Reciprocal Tariff Calculations

Executive Summary

Reciprocal tariffs are calculated as the tariff rate necessary to balance bilateral trade deficits between the U.S. and each of our trading partners. This calculation assumes that persistent trade deficits are due to a combination of tariff and non-tariff factors that prevent trade from balancing. Tariffs work through direct reductions of imports.

Reciprocal tariff rates range from 0 percent to 99 percent, with unweighted and import-weighted averages of 20 percent and 41 percent.

Introduction

To conceptualize reciprocal tariffs, the tariff rates that would drive bilateral trade deficits to zero were computed. While models of international trade generally assume that trade will balance itself over time, the United States has run persistent current account deficits for five decades, indicating that the core premise of most trade models is incorrect.

The failure of trade deficits to balance has many causes, with tariff and non-tariff economic fundamentals as major contributors. Regulatory barriers to American products, environmental reviews, differences in consumption tax rates, compliance hurdles and costs, currency manipulation and undervaluation all serve to deter American goods and keep trade balances distorted.  As a result, U.S. consumer demand has been siphoned out of the U.S. economy into the global economy, leading to the closure of more than 90,000 American factories since 1997, and a decline in our manufacturing workforce of more than 6.6 million jobs, more than a third from its peak.

While individually computing the trade deficit effects of tens of thousands of tariff, regulatory, tax and other policies in each country is complex, if not impossible, their combined effects can be proxied by computing the tariff level consistent with driving bilateral trade deficits to zero. If trade deficits are persistent because of tariff and non-tariff policies and fundamentals, then the tariff rate consistent with offsetting these policies and fundamentals is reciprocal and fair.

Basic Approach

Consider an environment in which the U.S. levies a tariff of rate τ_i on country i and ?τ_i reflects the change in the tariff rate. Let ε0 represent the passthrough from tariffs to import prices, let m_i>0 represent total imports from country i, and let x_i>0 represent total exports. Then the decrease in imports due to a change in tariffs equals ?τ_i*ε*φ*m_i


Parameter Selection

To calculate reciprocal tariffs, import and export data from the U.S. Census Bureau for 2024. Parameter values for ε and φ were selected. The price elasticity of import demand, ε, was set at 4.

Recent evidence suggests the elasticity is near 2 in the long run (Boehm et al., 2023), but estimates of the elasticity vary. To be conservative, studies that find higher elasticities near 3-4 (e.g., Broda and Weinstein 2006; Simonovska and Waugh 2014; Soderbery 2018) were drawn on.  The elasticity of import prices with respect to tariffs, φ, is 0.25. The recent experience with U.S. tariffs on China has demonstrated that tariff passthrough to retail prices was low (Cavallo et al, 2021).

Findings

The reciprocal tariffs were left-censored at zero. Higher minimum rates might be necessary to limit heterogeneity in rates and reduce transshipment. Tariff rates range from 0 to 99 percent. The unweighted average across deficit countries is 50 percent, and the unweighted average across the entire globe is 20 percent. Weighted by imports, the average across deficit countries is 45 percent, and the average across the entire globe is 41 percent. Standard deviations range from 20.5 to 31.8 percentage points.

References

Boehm, Christoph E., Andrei A. Levchenko, and Nitya Panalai-Nayar (2023), “The long and short of (run) of trade elasticities, American Economic Review, 113(4), 861-905.

Broda, Christian and David E. Weinstein (2006). “Globalization and the gains from variety,” Quarterly Journal of Economics, 121(2), 541-585.

Pujolas, Pau and Jack Rossbach (2024). “Trade deficits with trade wars.” SSRN.

Simonovska, Ina and Michael E. Waugh (2014). “The elasticity of trade: Estimates and evidence,” Journal of International Economics, 92(1), 34-50.

Soderberry, Anson (2018). “Trade elasticities, heterogeneity, and optimal tariffs,” Journal of International Economics, 114, 44-62.

https://ustr.gov/issue-areas/reciprocal-tariff-calculations