Why firmness changes the height you need
Your pillow fills the space between your head and the mattress. When you sleep on your side, your shoulder mostly determines the size of that space. On a softer mattress, your shoulder sinks deeper. The gap gets smaller, so you need less pillow height to keep your head level with your spine. On a firmer surface, your shoulder sinks less and the gap stays larger.
Firmness is one factor among several. Sleep position has a bigger effect. In our fittings, the difference between stomach sleepers (median 3.7 inches) and side sleepers (median 4.9 inches) is about 1.2 inches. The soft-versus-medium difference is about 0.4 inches. Shoulder width also matters: among side sleepers, the median height rises from 4.1 inches below 15.0 inches shoulder width to 5.3 inches above 18.1 inches.
What our fittings show
Based on 300+ fittings, this is the median pillow height for side sleepers by mattress type:
| Mattress | Fittings (n) | Median pillow height |
|---|---|---|
| Medium | 184 | 4.9 in |
| Soft | 21 | 4.5 in |
The soft-mattress group is small, so read it as a direction, not a rule. We don't have enough fittings on firm mattresses to give you a number.
A quick check on your own mattress
Lie on your side on your usual pillow and have someone take a photo from behind at head level. Your head should sit in line with your spine, not tilted up or down. If your head tips toward the mattress, the pillow is likely too low for how your shoulder sinks. If it tips upward, the pillow is likely too high. Check again after you change your mattress, because your old pillow was matched to the old surface.
What this means for your pillow
Think of mattress firmness as a small adjustment on top of your sleep position and shoulder width. A softer mattress usually means a slightly lower pillow, and a firmer one usually means you won't need to go lower. YOUP pillows range from 3.1 to 5.7 inches, and every fitting lands within that range. For more on how height works, read our guide to pillow height, the sleep environment checklist and our page on pillow fit. The YOUP pillow is made to your measurements: body, sleep position and mattress.