Mattress Online Rock Hard Super Firm Foam Super King 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
£454
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 Super Firm Foam Mattress
More available sizes
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
- 90cm each for two sharing: Each of you gets 90cm of width, which means a restless partner is far enough away that distance does more than any comfort layer could.
- Room for night-time visitors: When a child pads in at 3am, or a large dog treats the bed as routine, the 6ft width means everyone still keeps a fair share.
- A stable surface across the full width: The 23cm reflex foam build stays solid edge to edge, which means no roll-together even on a mattress this wide.
- 15cm more each than a UK king: The UK king gives 75cm per sleeper; the UK super king gives 15cm more each, a difference you notice every night rather than a spec-sheet detail.
- Suits larger master bedrooms: In a big master or a period property a UK king can look slightly undersized, and the UK super king fills the space properly.
This size suits you if...
You share the bed every night and want the most space a standard UK size offers, you're regularly joined by children or pets and need everyone to keep their own patch, or one of you is a genuinely restless sleeper and the extra 15cm each over a UK king matters. You also know you want a very firm feel, since this size doesn't soften the mattress's extra-firm character.
Consider another size if...
Your bedroom is smaller than our suggested 3m x 2.9m minimum, or your stairs and landing turns make a 180cm-wide foam mattress a struggle. We'd recommend the UK king instead, which is the same length, and allow at least 2.7m x 2.9m for it.
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
What room size do I need for a super king Rock Hard mattress?
We suggest a minimum of 3m x 2.9m, with around 3.5m x 3.2m feeling comfortable once you add wardrobes and walking space. At 180 x 200cm, or 6ft x 6ft 6in, this is the largest size UK retailers stock as standard, so measure before you order rather than after.
Is the Rock Hard super king too heavy to move upstairs?
It's a substantial 23cm foam mattress at the largest standard UK width, so treat moving it as a two-person job and check door swings and landing turns first. Unlike zip and link beds, it arrives as one piece, so tight staircases in older houses deserve a proper measure beforehand.
Is a super king firm mattress good for a restless partner?
Yes, and this size arguably helps more than any construction detail. With 90cm each, you're simply further from a fidgeting sleeper, and the solid reflex foam surface transfers little movement anyway. The combination suits couples where one person tosses and turns and the other wants to sleep through it.
Can two adults and a child share the Rock Hard super king?
Comfortably, for occasional night-time visits. The 180cm width matches two UK singles side by side, so a small child slotting in still leaves both adults a reasonable share. The very firm surface also stays stable rather than dipping when a third body climbs aboard.
How much wider is the super king than the double?
The UK super king is 45cm wider and 10cm longer than the UK double. For two people sharing, the UK double gives 67.5cm each, so the UK super king gives 22.5cm more each. If you're upgrading from a double, the difference feels dramatic on the first night.