Mattress Online Rock Hard Super Firm Foam King Size Mattress
The Rock Hard Super Firm Foam Mattress from Mattress Online does exactly what the name suggests.
- Very firm, full-height foam build
- Helps keep your spine aligned
- 100-night trial & 10-year guarantee
£374
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Mattress Online Rock Hard Super Firm Foam Mattress
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Mattress details
Mattress
Mattress Online Rock Hard Super Firm Foam Mattress
Size
Firmness
Firm
Construction
Foam
Orthopaedic
Depth (cm)
23
Layers
Layer Details
Reflex Foam
A supportive foam that moulds to your body, which means it promotes spinal alignment and a slightly personalised feel despite the very firm surface. It's durable and resilient, holding its shape for an extended period of time.
Layer details from mattressonline.co.uk product page, September 2026.
Key benefits
- 75cm each for two sharing: You each get 75cm of a very firm, stable surface, which means you can sprawl or change position without meeting in the middle.
- Less disturbance across the bed: The solid reflex foam has very little bounce, so a partner leaving hours before you is less likely to jolt you awake at the far side.
- Extra length for taller sleepers: At 200cm long, the UK king is 10cm longer than the UK double, welcome relief for anyone approaching six foot on a surface with no give at the feet.
- 7.5cm more each than a double: The UK king is 15cm wider than the UK double, giving each sleeper 7.5cm more, a difference you feel every night on a firm bed.
- Firm support at full width: The 23cm reflex foam build stays consistent right across the 150cm width, so the edge sleeper gets the same sturdy support as the middle.
This size suits you if...
You share every night and have outgrown a UK double, especially if one of you keeps different hours and you want the least possible disturbance from the other side of the bed. It also fits if one of you is approaching six foot and needs the 200cm length, or if you both want genuine extra-firm support without compromising on personal space.
Consider another size if...
Your bedroom falls short of the 2.7m x 2.9m we suggest for a UK king, or you sleep alone. We'd recommend the UK double instead, which needs at least 2.6m x 2.8m and still gives a couple 67.5cm each.
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FAQ's
Is the UK king Rock Hard Super Firm big enough for a couple?
Yes, comfortably. At 150cm wide it gives two people 75cm each, and that space counts for more here than on a softer bed. Because the reflex foam barely gives, you can't burrow into your own hollow to make room; you rely on genuine width instead, and the UK king provides it.
Should we get the Rock Hard mattress in a UK double or a UK king?
If the room allows it, the UK king. It's 15cm wider and 10cm longer than the UK double, giving each of you 7.5cm more width. On a mattress this firm, where neither of you sinks in, that extra elbow room reduces contact and disturbance. Short on space? The UK double needs a smaller room, at least 2.6m x 2.8m.
Is the UK king long enough for someone over six foot?
At 200cm, the UK king gives 10cm more length than the UK single, small double and double, which all run 190cm. On a very firm foam surface there's no softness at the foot end to absorb overhanging feet, so that extra length is a practical upgrade rather than a nice-to-have for taller sleepers.
What room size do I need for a king size Rock Hard mattress?
We suggest a room of at least 2.7m x 2.9m for a UK king, with 3.2m x 3.2m feeling comfortable. That's our guidance rather than the manufacturer's, and it allows walking space and bedside tables on both sides. New build master bedrooms can be tighter than they first appear once wardrobes go in, so measure carefully.
Will a partner getting up early wake me on the king Rock Hard?
Less than on most beds. The 23cm reflex foam is very firm with little bounce, so movement doesn't ripple across the surface the way it can on springs, and the UK king's 75cm per sleeper adds physical distance too. Compare the retailers listed on this page for the current price in this size.