Conversion from US Dry pints to Cubic inches

Information about the US Dry pint (pt)

The United States dry pint is a unit of volume equal to 550.610471 millilitres, or 33.6 cubic inches. Its symbol is pt. It belongs to a separate American measuring system used for produce and grain rather than for liquids, and it is about sixteen per cent larger than the United States liquid pint of 473 millilitres.

Two parallel chains exist because two kinds of goods were once measured differently. Liquids descend from the wine gallon of 231 cubic inches, while dry goods descend from the Winchester bushel of 2150.42 cubic inches, a corn measure standardised in medieval England. Dividing the bushel by four gives a peck, by thirty-two a dry quart, and by sixty-four a dry pint, and none of those figures has any relation to the liquid chain.

Berries are where the unit is still met. Blueberries, raspberries and strawberries are sold in American markets in pint and quart baskets, and the basket is a dry measure. A pint of blueberries is therefore 551 millilitres of fruit, not 473, and a shopper comparing that basket with a pint of cream is comparing two different quantities under one name.

Nothing on a label resolves the ambiguity. Both are written pt, both are called a pint, and the word gives no clue as to which chain is meant. In practice the contents decide: if it pours, it is liquid measure, and if it is heaped, it is dry. That rule is reliable but it is nowhere written down, which is why converted American recipes so often disagree with one another.

Its existence is a survival from selling by bulk. Before cheap accurate scales, grain, apples, potatoes and shellfish were sold by filling a stamped measure and levelling it off, and the whole dry chain from bushel down to pint served that trade. Almost all of it has now been replaced by weight, and United States produce regulations increasingly specify pounds instead.

For conversion, the safest approach is to abandon the name entirely. A dry pint is 551 millilitres, a dry quart 1101, a peck 8.81 litres and a bushel 35.24. Writing the metric figure removes an ambiguity that the English word carries and cannot shed.

One United States dry pint equals 550.61 millilitres, 33.6 cubic inches, half a US dry quart, or about 1.164 US liquid pints.

Information about the Cubic inch (in³)

The cubic inch is a unit of volume equal to 16.387064 cubic centimetres exactly. Its symbol is cu in, sometimes written in³. It is the volume of a cube one inch on each side, and 1728 of them make a cubic foot. Its most consequential role is legal rather than descriptive: the United States gallon is defined as exactly 231 cubic inches.

That definition is the reason American liquid measure is the shape it is. The figure of 231 comes from the English wine gallon standardised under Queen Anne in 1707, which the United States kept when Britain replaced its own gallons with a single imperial gallon in 1824. Every American fluid ounce, pint and quart descends from that one number, and it is why the two gallons differ by a fifth.

Engines made the unit famous. American car engines were quoted in cubic inches until the 1980s, and the numbers became names in their own right: a 350 is 5.7 litres, a 426 is 7.0, and a 302 is 4.9. Motorcycles still carry both figures, and the conversion is worth knowing, since 61 cubic inches make a litre.

Machinery uses it for swept volume. Pump and compressor displacement per revolution, hydraulic cylinder volume and metering pump strokes are all specified in cubic inches in American engineering, because the bores and strokes that produce them are already in inches and squaring or cubing an inch keeps the calculation in one system.

Freight pricing uses it in an unexpected way. American parcel carriers calculate a dimensional weight by dividing the package volume in cubic inches by a fixed divisor, commonly 139, to obtain a chargeable weight in pounds. A light bulky parcel is therefore priced by its cubic inches rather than by what it weighs, which changes how goods are packed.

The conversion to metric is exact but unhelpful. Because the inch is defined as exactly 2.54 centimetres, a cubic inch is exactly 16.387064 cubic centimetres, and no rounding is involved. That precision does not make the number easier to use, and it is the main reason engineering drawings outside the United States moved to millimetres.

One cubic inch equals 16.387 cubic centimetres, about 0.0164 litres, about 0.554 US fluid ounces, or 1/1728 of a cubic foot.