What problem this solves
The standard pillow chart says “insert should be 5 cm larger than the cover.” This works for one specific scenario: a 50 cm square cover with a 55 cm down insert, achieving a choppy look. It doesn’t work for a 60 cm lumbar pillow (which needs less upsize because the shape is rectangular) or a poly-fill insert (which compresses less than down). The result is pillows that look under-stuffed or over-stuffed relative to the designer’s intent. The calculator applies the upsize by look and fill.
The 5 cm rule is roughly correct for down-filled, square, choppy pillows. For poly-filled inserts, the upsize drops to 2 cm. For overstuffed looks, the upsize rises to 7–10 cm. For lumbar (rectangular) covers, the upsize is smaller because the long axis doesn’t need as much push to fill out.
Who it’s for
- Interior designers specifying pillow inserts before ordering covers
- Homeowners buying covers online and wanting the right insert
- Bedding designers mixing pillow sizes for a styled bed
- Sofa stylists mixing choppy and plump pillows on the same piece
Why look matters more than fill
A pillow’s visual character is mostly about how the corners behave. Choppy corners (look = choppy) read as casual, lived-in, approachable. Plump corners read as finished, hotel-like, formal. Overstuffed corners (rounded, no corner visible) read as decorative, sculptural. The insert upsize is what controls corner behaviour: more upsize = more push on the corners. Fill matters less because most modern inserts come in a range of fills at the same upsize.
Limitations
- Upsize assumes a standard cover fabric (cotton, linen, velvet). Heavy fabrics (leather, heavy wool) need slightly less upsize.
- Stretched covers over-stuff — the calculator returns the same upsize regardless of fabric stretch.
- Lumbar covers (rectangular) work better with a smaller upsize (3 cm) regardless of look — the long axis fills itself.
- Bolster and round pillows follow different rules — the calculator only handles squares and rectangles.
- Inserts don’t last forever; down inserts lose loft over 3–5 years and need fluffing or replacement.
Practical rules of thumb
Standard 50 × 50 cm cover with down insert: 55 × 55 cm. Standard 60 × 60 cm cover with down: 65 × 65 cm. Lumbar 50 × 30 cm cover: 50 × 35 cm insert. Mix sizes on a sofa (40, 50, 60) for a layered look. Always use a cover with a hidden zipper if the pillow will be removed frequently — the insert can be swapped without changing the cover.