One pound per cubic inch is 27.6799047102 grams per cubic centimetre. It is the densest-sounding unit in common use — almost everything measured in it comes out as a small decimal, because a pound in a cubic inch is a very concentrated amount of matter.
About the units
Pounds per cubic inch is the American unit for the density of metals and dense engineering materials. Grams per cubic centimetre is the metric equivalent used everywhere else for the same purpose. The conversion combines the pound-to-gram factor of 453.59237 with the cubic-inch-to-cubic-centimetre factor of 16.387064, and dividing them gives the figure above. Because only a handful of materials exceed 1 lb/in³ — lead, tungsten, gold, uranium — the unit is usually written with three or four decimals.
The exact factor
The exact derivation is 453.59237 ÷ 16.387064 = 27.6799047102031 g/cm³ per lb/in³. Reference values: aluminium 0.0975 lb/in³ or 2.70 g/cm³; steel 0.284 or 7.85; copper 0.324 or 8.96; lead 0.410 or 11.34; and tungsten 0.697 or 19.3. Gold at 0.698 lb/in³ is 19.32 g/cm³, essentially identical to tungsten, which is the physical fact behind tungsten-cored counterfeit gold bars.
Where you meet this conversion
American materials engineering and machining, where stock weight is calculated from dimensions in inches and density in pounds per cubic inch. Aerospace weight budgets, mould filling calculations and metal purchasing all use it. Comparing an American alloy datasheet to a European one requires the conversion, and the numbers look so different — 0.284 against 7.85 — that no one confuses the units, which is one advantage of a factor this large.