Loft is an important international indicator for measuring the warmth of down, referring to the volume in cubic inches occupied by one ounce (30 grams) of down under certain conditions. For example, if one ounce of down occupies 600 cubic inches, its loft is said to be 600. Generally speaking, plump, mature birds have higher loft, while smaller birds have lower loft.
But what exactly is the use of loft? One explanation is that fill power represents the "quantity" of down, while down content determines the "quality," with loft being a crucial indicator. With the same fill power, a higher loft indicates better warmth. This is because higher loft means more insulating air is contained within the down, resulting in better insulation.