text: *1 kg/m = 1 g/L (exactly)
  *1 kg/m = 0.001 g/cm (exactly)
  *1 kg/m ≈ 0.06243 lb/ft (approximately)
  *1 kg/m ≈ 0.1335 oz/US gal (approximately)
  *1 kg/m ≈ 0.1604 oz/imp gal (approximately)
  *1 g/cm = 1000 kg/m (exactly)
  *1 lb/ft ≈ 16.02 kg/m (approximately)
  *1 oz/(US gal) ≈ 7.489 kg/m (approximately)
  *1 oz/(imp gal) ≈ 6.236 kg/m (approximately)