Short answer: medium to medium firm, 5 to 6 on the 1 to 10 scale, for most side sleepers. The exact number depends on how much wider your shoulders and hips are than your waist, and on your weight, which is why a partner your size can be comfortable on the same mattress and someone shaped differently cannot.
Side sleeping is the only position where the body has a shape the mattress has to work around rather than a plane it has to support.
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What the number depends on
Lie on your side and your outline has two peaks and a valley. The mattress has to let the peaks in far enough that the valley still touches. Three things decide how much give that takes.
- How much wider your shoulders are than your waist. Broad shoulders on a narrow waist need a deeper comfort layer, because the shoulder has further to travel before the waist is supported.
- Your weight. Force per square inch is what compresses foam. The same mattress reads two steps firmer to someone 80 pounds lighter.
- Where your weight sits. Weight carried at the hips loads the lower half of the mattress harder, so the hip sinks first and the shoulder feels unsupported even though the surface is fine.
The number by build
| Build | Firmness | Comfort layer |
|---|---|---|
| Under 130 lb, narrow frame | 3 to 4, plush | 2 to 3 in. There is not enough weight to compress more |
| Under 130 lb, broad shoulders | 4, plush | 3 in or more |
| 130 to 230 lb, average frame | 5 to 6, medium | 3 in |
| 130 to 230 lb, broad shoulders or wide hips | 5, medium | 3 to 4 in |
| Over 230 lb | 6 to 7, medium firm | 4 in, over a firm core |
| Over 230 lb, broad frame | 6, medium firm | 4 in, hybrid preferred |
Notice that heavier side sleepers need a firmer rating and a thicker comfort layer at the same time. Those are not contradictory: the firmness comes from the core and the cushioning from the top, which is the construction described in firm mattress vs plush.
The two minute test on your current bed
- Lie on your side in your normal position with your usual pillow, on the side of the bed you sleep on every night.
- Have someone slide a hand under your waist, in the gap between your ribs and your hip.
- Read the result. The hand should slide in with a little resistance. If it goes through with room to spare, the mattress is too firm and your shoulder and hip are carrying you. If it will not go in at all, your waist is resting on the mattress because the hips have sunk too far.
- Check the other end. Look at the line of the spine from behind. It should run straight from the neck to the tailbone rather than curving up or down at the waist.
This test is worth doing before shopping. It turns “the bed feels wrong” into a direction, and the direction is what you cannot recover once you are looking at product pages.
What the wrong firmness feels like
| Symptom | Meaning |
|---|---|
| Sore shoulder on the side you sleep on | Too firm. The shoulder is bearing weight instead of sinking in |
| Aching hip, same side | Too firm, usually with too thin a comfort layer |
| Arm going numb overnight | Too firm. Circulation is being restricted at the shoulder |
| Lower back sore on waking | Too soft. The hips have dropped and the spine has curved |
| You keep rolling onto your back | Too firm at the pressure points. The body is escaping the position |
| Sleeping hot | Too soft, or too much foam. Side sleeping buries more of you than any other position |
Where those symptoms overlap with back pain, the fuller diagnosis is in mattresses for side sleepers with back pain.
Firmness is not the only lever
- The pillow does half the job. Side sleeping puts the widest gap between your ear and the mattress of any position, so the pillow has to be thicker than a back sleeper’s. Get this wrong and no mattress firmness fixes the neck. Options are in pillows for side sleepers.
- Material changes the feel at the same number. Memory foam at 6 feels softer than a hybrid at 6, because it keeps giving under sustained pressure while coils push back. The comparison is in memory foam vs hybrid.
- A topper moves you one step softer, and only one. It cannot rescue a mattress two steps too firm, and it cannot help at all with one that sags.
What to buy at each level
- Medium, average build: the Novilla Bliss 12 inch gel memory foam gives at the shoulder without being a sink hole.
- Medium firm, heavier side sleepers: the Vesgantti 12 inch medium firm hybrid, or the BedStory 14 inch firm hybrid above 230 pounds, for the reasons in support for heavier sleepers.
- Your mattress is one step too firm: the 2 inch gel memory foam topper is the cheapest correct fix.
- Shoulder pressure is the main complaint: a thicker 4 inch memory foam topper gives the shoulder somewhere to go, though it also raises the bed by 4 inches, which matters if you are near the top of your comfortable height.
Common questions
How firm should a mattress be for a side sleeper? Medium to medium firm, 5 to 6 for an average build. Lighter sleepers go softer, heavier sleepers go firmer, and the comfort layer should be at least 3 inches whichever way you go.
Is a firm mattress bad for side sleepers? Usually. A firm surface holds the shoulder and hip up and leaves the waist unsupported, which curves the spine sideways. Heavier side sleepers are the exception, because their weight compresses a firm surface into a medium one.
Do broad shoulders change the answer? Yes. The wider your shoulders are relative to your waist, the deeper the comfort layer has to be for your spine to stay level. Firmness rating alone does not capture that.
Can a topper make a mattress soft enough? By about one step on the scale. A 2 inch topper on a firm mattress gets you to medium firm. It will not turn firm into plush.
Why do I wake up on my back? Often because the surface is uncomfortable at the shoulder or hip and the body moves off the pressure. It is one of the more reliable signs that a mattress is too firm for side sleeping.
The bottom line
Run the hand test before you shop. If a hand slides under your waist easily, the mattress is too firm for you. If it will not go in, it is too soft. Then use the table: 5 to 6 for average builds, softer if you are light, firmer with a thicker comfort layer if you are heavy, and sort out the pillow at the same time.
Check the Novilla Bliss gel memory foam mattress on Amazon →
