Fill Power (Down)
Published 2026-08-03. Last updated 2026-08-03. Desk research by Kaiser Khan — we have not product-tested fill power. This is not a shop review and not medical advice.
In short
Fill power is a loft-efficiency rating for down. Higher numbers mean more loft volume from the same mass of down. It is not the same as TOG, which measures a finished duvet’s thermal resistance. Synthetic hollowfibre fills do not use fill power. They rely on fibre geometry instead. Compare down quality with fill power, then check TOG for warmth.
About 1 minutes to read
Definition. Fill power measures how many cubic inches one ounce of down lofts under a standard test. Higher fill power means more loft per mass of down.
Fill power describes loft efficiency of down clusters. Pair it with duvet fill and TOG when reading product specs. Warmth band still comes from the finished TOG test.