Fill power, also written as Fill Power or FP, refers to the loft measured in cubic inches (cuin) when a fixed load is applied to 1 ounce (approximately 28 g) of down. Even with the same amount of down, materials that loft more easily achieve higher values and superior insulation. The higher the number, the more air layers can be retained, resulting in greater warmth with less down.

General guidelines by fill power value are as follows. Visualize the intended use and temperature to choose the range that best fits your needs.

While higher FP generally means greater insulation, it can feel too warm depending on transportation and environment. Choose the appropriate range to suit your lifestyle.

Warmth is basically determined by “fill power × down fill amount.” Inner down or lightweight down jackets typically contain around 50–100 g, while winter main outerwear is often around 100–150 g or more. However, some products do not disclose fill amounts, so checking fit and warmth through in-store try-ons and actual wear is important.

• Down consists of soft clusters without quills, trapping a large amount of air and offering excellent insulation.
• Feathers have quills and provide resilience, but their insulation performance is slightly lower.
• The ratio is displayed as “Down XX% / Feather XX%”.

Down 90% / Feather 10%

Known as the “golden ratio,” this high-quality balance offers exceptional lightness and warmth. Commonly used in mountaineering and top-tier models.

Down 70–80% / Feather 20–30%

A standard ratio with a good balance of lightness and resilience, making it ideal for everyday urban wear.

Down 50% / Feather 50%

Often used in cost-conscious, light cold-weather models. Insulation is limited, making it suitable for spring/autumn or inner down layers.

Products made with 100% down do not exist; even the highest-end items contain a small percentage of feathers. Feathers help prevent uneven distribution and maintain the garment’s shape.

• Nylon: Soft and highly abrasion-resistant; easy to apply water-repellent finishes and resistant to rain and snow.
• Polyester: Wrinkle-resistant and maintains shape well; works effectively with water-repellent treatments.
• Waterproof/Water-repellent performance: Down is vulnerable to moisture, so these treatments greatly affect usability. Wind resistance also directly impacts perceived warmth.

• Stitched (quilted) construction: The outer and inner fabrics are sewn directly together, resulting in a lightweight, stylish design, but cold air can enter through seams—best for city wear.
• Box (baffle) construction: Internal partitions create three-dimensional chambers that prevent down shifting and cold air intrusion, delivering high insulation for harsh, cold environments.

For everyday wear or commuting in the city, ease of movement and styling versatility are essential. Matte-finish nylon materials and minimalist, hoodless designs work well for both casual and formal settings.

Thin and lightweight inner down jackets are highly useful during transitional seasons and for layering. They work well on their own and provide extra warmth when worn under a coat in midwinter. Slim styles that fit easily under suits are also popular.

For leisure activities and travel to cold regions, outdoor-oriented models that combine wind resistance, water repellency, and insulation are reassuring. Pay attention to functional details such as ample pockets and detachable hoods that support active use.

In extreme cold environments, not only high FP but also sufficient down fill, strong sealing, and water resistance are required. Look for voluminous silhouettes with well-packed down, as well as designs that block cold air, such as drawcords at the cuffs and hood.