Fill Power in down: quality, quantity, and how to choose

When we talk about a down jacket or sleeping bag, two concepts that are often confused arise: down quality and fill weight. Fill power helps us understand the former; fill weight helps us understand the latter. To assess real thermal performance, we need to look at both.

What is fill power?

Fill power measures the expansion capacity or loft of the down. The higher the value, the more volume a given amount of down can reach and the more air it can trap with little weight.

That is why high fill power down allows for the design of garments and sleeping bags that are very efficient in terms of warmth-to-weight ratio. But a high fill power number does not automatically mean that a garment is warmer.

Quality and quantity: the two variables

A jacket with 800FP and little fill may be less insulating than another jacket, also 800FP, with a significantly higher amount of down. Construction, baffle distribution, fit, and fabrics complete the equation.

Real-world example: Electron Pro vs Neutrino Pro

The current Rab Electron Pro and Neutrino Pro both use 800FP European goose down. However, in size M:

• Electron Pro: 179 g of down, 477 g total weight.
• Neutrino Pro: 212 g of down, 563 g total weight.

The fill power is the same, but the Neutrino incorporates 33 g more fill and Rab positions it one level higher in insulation. It is a good example of why it is not advisable to buy solely by looking at the “800FP” number.

View Electron Pro vs Neutrino Pro comparison →

What does hydrophobic down provide?

Rab uses down with a hydrophobic treatment in numerous products to help reduce moisture absorption and accelerate drying. This improves the insulation's tolerance to changing conditions, but it does not turn a down jacket into a waterproof garment.

What should you look at when comparing two down garments?

• Fill power: down efficiency.
• Fill weight: total amount of down.
• Construction: type and distribution of baffles.
• Outer fabric: weight, durability, and protection against wind and light moisture.
• Total weight: important if the garment will be in your backpack for many hours.
• Intended use: a jacket for intense movement is not designed the same way as a garment for belaying or static cold.

Down or synthetic

Down stands out for its warmth-to-weight ratio and compressibility. Synthetic insulation, on the other hand, usually offers more stable performance in humidity and faster drying. Neither is universally better: it depends on the activity and the conditions.

Read Down vs synthetic: how to choose insulation →

Mountain Group Video: quality or quantity

Armando Escudero, a technical advisor at Mountain Group, explains in this video how to interpret down quality and why you have to look at more than just the fill power. In the second part, he also reviews the different types of Lowe Alpine backpacks according to activity.

What backpack do you need?

The logic is similar: there is no “best” backpack in the abstract. Volume, carrying system, ventilation, stability, and access must respond to the type of activity.

View backpacks and bags →

The data for Electron Pro and Neutrino Pro correspond to the current official Rab specifications for QDB-81 and QDB-85.