“按重量”常见的英文表达有 “by weight” 或 “according to weight”。
by weight：这是一个简洁常用的表达，例如：The freight is charged by weight.（运费按重量计算 。）
according to weight：表达相对更正式、书面一些，例如：Goods are priced according to weight.（商品按重量定价。 ）