你说得对，用介词 by 表示单位时，意为“以……计”，其后通常接表示计量单位的单数名词，且名词前要用定冠词，如 by the hour, by the day, by the month, by the pound, by the dozen等。如：

They let out cars by the day. 他们按天出租汽车。

The cloth is sold by the yard. 这种布是按码卖的。

They are paid by the hour. 他们所得报酬按小时计。

Do you sell eggs by the kilo or by the dozen? 鸡蛋怎么卖，论公斤，还是论打?

Is gasolinesold by the gallon in America?在美国，汽油是以加仑出售的吗?

但 weight 并不是“单位”，所以它与上面的用法不同。这里的 weight 是一个不可数名词，意思是“重量”，by weight 的意思是“按重量”。如：

Do you charge carriage by weight? 你们按重量收运费吗?

Bananas are usually sold by weight. 香蕉通常按重量卖。

Are bananas sold by weight or at how much a piece? 香蕉是按重量出售, 还是按多少钱一根出售?

类似的表达还有 by size（按大小）、by volume（按体积）等。如：

The potatoes are graded by size (=according to size). 马铃薯按大小分等级。

The apples are washed and then graded by size. 苹果洗好后按照大小分类。

They sell screws and nails by volume. 他们按体积出售螺丝钉和钉子。