Mattress Online Rock Hard Ultra Firm Small Double Mattress
The Rock Hard Ultra Firm from Mattress Online is an all-foam orthopaedic mattress rated extra firm, which means very little sink and a sleeping surface that keeps you on top of the bed rather than in it.
- Very firm, no sinking feel
- Moulds to ease pressure points
- 100-night trial
£369
Get the lowest price we've found
Save with the previous model
Some retailers still sell the earlier version of this mattress for less than the current model's best price.
Mattress Online Rock Hard Ultra Firm Mattress
More available sizes
Mattress details
Mattress
Mattress Online Rock Hard Ultra Firm Mattress
Size
Firmness
Firm
Construction
Foam
Orthopaedic
Depth (cm)
25
Layers
3
Layer Details
Reflex Foam
A flexible reflex foam layer sits at the top, which means responsive support that helps keep your spine aligned. It has a little give, but this is not a plush comfort layer.
Ortho PU Foam
The ortho PU foam core provides the extra-firm support this mattress is built around. It moulds to your shape to reduce pressure points and encourage better posture.
Reflex Foam
A second reflex foam layer at the base promotes spinal alignment from underneath. It moulds slightly to your body for a more personalised feel.
Layer details from mattressonline.co.uk product page, September 2026.
Key benefits
- Room to spread out, alone: At 120cm wide, it gives a solo sleeper far more space than a UK single to shift position through the night.
- Fits where a double will not: It is 15cm narrower than a UK double, which can be the difference between a door that clears the bed and one that doesn't.
- Firmness that stays firm at this width: The ortho PU foam core keeps you on top of the mattress rather than in it, so the extra width is usable, level sleeping space.
- Easy bedding and frames: 4ft is a standard UK size, so fitted sheets and bed frames are simple to find on the high street.
- Works as a firm guest bed: Two adults can share for the occasional night, and the unyielding surface means neither guest rolls into the other.
This size suits you if...
You sleep alone, want a very firm bed for your back, and a UK single feels cramped but your room can't take a full double. Or you're furnishing a box room, loft conversion or small guest bedroom where every centimetre of floor space matters and visiting couples only stay the odd night.
Consider another size if...
You share the bed every night, because 120cm is a squeeze for two adults long term. We'd recommend the UK double instead, which gives 67.5cm each for two sharing; allow at least 2.6m x 2.8m of room.
Similar Mattresses in This Price Range
Did you know? On Mattress Unboxed we have over 1,000 different mattresses to suit all types of sleepers. View all mattresses.
FAQ's
Is a small double Rock Hard Ultra Firm big enough for a couple?
For occasional nights, yes. The very firm surface helps here, since neither of you sinks into a dip in the middle. For every night, though, 120cm across is tight for two adults, and the UK double gives 67.5cm each for two sharing, which is a more comfortable long-term arrangement.
What room size do I need for a UK small double mattress?
We suggest allowing at least 2.4m x 2.8m. That leaves enough space to walk round the bed and open drawers or a wardrobe. It's our own guidance rather than the manufacturer's, so treat it as a comfortable minimum, and measure your room before ordering.
How much bigger is the small double than a UK single?
The UK small double is 30cm wider than the UK single, and the two are the same length at 190cm. That extra width matters on a mattress this firm: because you sleep on top of the surface rather than sinking in, you feel every centimetre of the wider footprint.
Will standard double bedding fit this mattress?
Use small double (4ft) fitted sheets, which are widely stocked as 4ft is a standard UK size. A standard double duvet works well on top, giving a generous overhang on both sides for one sleeper, or reasonable coverage on the odd night two people share.
Where can I buy the Rock Hard Ultra Firm in a small double?
Compare the retailers listed on this page. The prices shown here are live and updated as retailers change them, so it's worth checking back before you buy rather than relying on a figure you saw last week.