What Size Is a King Bed Mattress?

The Answer First

A UK king size mattress measures 150cm wide by 200cm long, or 5ft by 6ft 6in. Depth is not standardised and varies by construction, commonly falling between 18cm and 32cm.

Those three figures are the ones you need. Width and length determine whether the mattress fits your frame and which bedding to buy. Depth determines whether standard or deep fitted sheets will stay put, and whether a divan base with a tall headboard will still look right once the mattress is on.

Depth Is the Variable People Miss

Width and length for a king are fixed in the UK. Depth is not, and it changes the practical experience of the bed more than most people expect.

Entry level open coil mattresses often sit around 18cm to 22cm. Mid range pocket sprung models commonly land between 24cm and 28cm. Premium pocket sprung mattresses with multiple comfort layers and hybrid foam and spring designs can reach 30cm to 35cm.

Deeper is not automatically better. A deep mattress made of thin layers stacked up gives no advantage over a well engineered shallower one. What depth does affect is the height you sit at when getting out of bed, how the mattress looks against the headboard, and which sheets you need. If your headboard is 60cm above the base, a 32cm mattress leaves only 28cm of headboard visible, which can look truncated.

King Mattress Construction Types

Open coil mattresses use a single continuous spring unit. They are the most economical construction and lighter to handle, which is a genuine advantage at king size. The trade off is motion transfer: because the springs are linked, movement on one side is felt on the other.

Pocket sprung mattresses house each spring in its own fabric pocket so it can respond independently. At king size, with two people 150cm apart, this makes a clear difference to how much you notice a partner moving. Spring counts in king pocket sprung mattresses commonly range from around 1000 to well over 2000.

Memory foam and hybrid mattresses contour to the body and virtually eliminate motion transfer. Some sleepers find pure foam retains too much heat, which is why hybrids combining a spring core with foam comfort layers have become the popular middle ground. Our modern mattresses UK sale range covers all three constructions in king size.

What Your Bed Base Needs to Provide

A 150cm span requires more support than a double, and this is where premature sagging usually originates. A king base must have a central support rail running from head to foot, with its own legs meeting the floor. Without that, the middle of the base flexes and the mattress follows.

Slat spacing matters as much as slat count. Gaps of more than about 7cm allow the mattress to bulge downwards between slats, which over time produces a rippled surface. Sprung beech slats add a slight give that works well with pocket sprung mattresses.

Solid platform bases and divans provide even support but need ventilation. Mattresses release moisture overnight, and a completely sealed base traps it. Slatted bases handle this naturally, while solid bases should have ventilation holes.

If you are pairing a new king mattress with a new frame, our modern king size beds UK sale range lists slat and support specifications so you can check before ordering.

Firmness at King Size

Firmness is personal and depends on weight and sleeping position. Side sleepers generally suit a softer to medium feel that lets hips and shoulders sink slightly. Back and front sleepers usually prefer medium to firm to keep the spine aligned. Heavier sleepers need more support to avoid bottoming out.

The advantage of a king is that dual tension becomes viable. Because the mattress is wide enough for two distinct zones, some manufacturers offer different firmness on each side within a single mattress. If you and your partner disagree on firmness, this solves a problem that no amount of compromise on a single tension will.

Zip and link mattresses take that further, using two 75cm halves joined together. They also make delivery up tight stairs far easier, since each half is roughly single width.

Bedding to Fit a King Mattress

A king duvet measures around 225cm by 220cm, giving a good drop on both sides of a 150cm mattress. Some sleepers prefer to size up to super king bedding on a king bed for extra coverage, which is a matter of preference rather than a fitting requirement.

Fitted sheets must match depth as well as footprint. Standard depth typically handles up to 25cm, deep up to 35cm. Measure your mattress at the side, not including any topper, then add the topper depth separately if you use one.

A mattress protector is worth using from day one, particularly on a more expensive mattress. Look for one that fits the depth properly, since a tight protector pulls at the corners and eventually tears.

Looking After a King Mattress

Rotation matters more on larger mattresses because the load is spread across a wider area and uneven wear shows up as visible dips. Rotate head to foot every three months for the first year, then twice a year afterwards. Only flip if the mattress is double sided, as many modern ones have a designated sleep surface.

Weight is a genuine consideration. A pocket sprung king can be heavy enough that rotation is a two person job. If that is likely to be a problem, look for handles on the sides or consider a zip and link construction.

Air the mattress regularly by pulling the bedding back for half an hour in the morning before making the bed. It costs nothing and helps moisture disperse rather than accumulate in the fillings.

Completing the Bedroom

A king mattress raises the sleeping surface higher than many people anticipate, particularly on a divan base. Bedside cabinets should ideally sit level with or slightly below the top of the mattress so a glass or book is easy to reach. Around 55cm to 65cm tall works with most king setups, and our modern bedside cabinets UK sale range includes taller designs suited to deeper mattresses.

For a coordinated room, matching the frame finish across drawers and wardrobes keeps things calm. Our modern bedroom furniture sets UK sale pair king frames with storage in the same finish.

At Furniture in Fashion we list mattress depths as well as widths and lengths, which makes matching sheets and headboards straightforward. Free delivery is available to most UK mainland postcodes and returns are available up to 30 days.

Frequently Asked Questions

What are the dimensions of a UK king mattress?

150cm wide by 200cm long. Depth varies by construction, usually between 18cm and 32cm.

How deep should a king mattress be?

There is no correct depth. Choose based on the construction quality and how the height works with your headboard and bedside furniture, then buy sheets to match.

Do I need deep fitted sheets for a king mattress?

If the mattress is deeper than about 25cm, yes. Standard depth sheets will keep pulling off the corners on deeper models.

Can two people have different firmness on a king mattress?

Yes. Dual tension mattresses and zip and link designs both allow a different feel on each side of a king.

How often should a king mattress be rotated?

Every three months in the first year, then twice yearly. Rotate head to foot unless the mattress is designed to be flipped.