What is fill power?
Fill power, also known as loft and abbreviated to FP, is a measure of the quality of down. In simple terms, it measures the loftiness or fluffiness of the down clusters within the jacket. The higher the fill power, the more the down lofts, the more air is trapped, and the better the insulation.
The technical bit
Technically, fill power indicates how much space one ounce (1oz) of down will fill, measured in cubic inches, or CUIN.
This measurement is done in a lab under specific conditions for consistency. Everything is controlled: the temperature, humidity, process for preparing the down, diameter of the measuring cylinder, weight used to compress the down and so on.
TODO
Confusingly, there's a US and an EU method to measuring fill power. The EU method is sometimes also known as Lorch
The standard Lorch test gives a fillpower figure about 4% lower than the US Federal method for the same down
https://idfl.com/info/conditioning-methods-evaluation/
Fill power in practice
But what does that mean in practice? The higher the fill power, the higher the loft, the more air is trapped, the warmer it is, and the more compressible it is.
High fill power sounds great doesn't it? Well, it is, but you pay for it. High fill power, eg in the 850-1000, is usually only reserved for the most specialist gear
Jackets usually sit between about 550 FP (like the North Ridge Men's Tech Down Jacket) up to 1000 (like the Rab Zero G Jacket), with 850-1000 usually reserved for only the most specialist gear.
As a rough guide:
500-650
650-800
850-1000 is usually reserved for the most specialist gear, where you need the highest warmth to weight ratio and minimum pack size.