Bunk Bed Mattress Depth Guide | Sizes & How Many You Need

Buying a mattress for a bunk bed is not the same as buying one for a normal bed. For bunk bed mattress depth, the key rule is that the top bunk usually needs a mattress no deeper than 15 or 16cm so the guard rail still works properly, while the bottom bunk often has more flexibility and trundle mattresses are usually around 10cm deep to slide underneath.

If you're a UK parent or family choosing a safe, practical bunk bed for children or teenagers, these details matter: a mattress that's too deep can reduce the effective guard rail height and make the top bunk less safe. Here's everything in one place, including depth limits, mattress sizes, how many mattresses different bunk setups need, which mattress types suit bunks, what changes with trundles, and the common buying mistakes to avoid.

1. Depth is a safety figure, not a preference

The top bunk of a bunk bed takes a maximum mattress depth, usually 15 or 16cm.

Guard rails are designed to sit a set height above the mattress surface. That gap is what stops a child rolling over the rail. Most high-street single mattresses are 20cm or deeper, so putting one on a bunk removes 5cm of effective rail height.

The rail is still there. It just isn't doing what it was designed to do.

Check your bed's stated maximum before ordering anything. It's on the product page, and it varies between models because the bed manufacturer sets it based on their rail height.

The bottom bunk is different

There's no rail above the bottom bunk, so no depth restriction in most cases. You can go deeper for comfort if you want to.

In practice most families buy two identical mattresses, which is simpler and means the children aren't sleeping on noticeably different beds.

Trundles are different again

A trundle mattress has to clear the underside of the bottom bunk when it slides away. That usually means around 10cm, and a standard mattress physically won't fit.

2. How many do you need?

Bed	Mattresses
Standard bunk bed	2, both within the top bunk's depth limit
Bunk with a trundle	3 two at 15cm, one at 10cm
Triple bunk bed	3
Family bunk, double bottom	2 a small double and a single
Family bunk with trundle	3
Quadruple bunk	4

This is the most commonly forgotten part of a bunk bed order. A bed arrives, gets built, and there's nothing to sleep on.

3. Which size?

Bunk bed mattress dimensions matter because most bunk beds use a standard UK single at 90 x 190cm, though some take different mattress sizes. Three exceptions worth knowing:

• Shorty bunks take 75 x 175cm. A standard single is 15cm too long and won't fit.

• Continental bunks take 90 x 200cm. A 190cm mattress leaves a gap at the end, which is uncomfortable and an entrapment risk on a raised bed.

• Family bunks usually take a small double at 120 x 190cm on the bottom and a single on the upper bunk, with that single the same width as a standard single.

Different layouts, such as l shaped models, can change how the bed fits the room even when the mattress size stays standard.

The bed's product page states the mattress size it needs. Work from that rather than measuring the bunk bed frame, since the frame is always larger than the mattress, and checking mattress sizes carefully helps it fit securely.

4. Our 15cm range

All made in the UK, these are the right mattress options for bunk beds, high sleepers, and loft beds, with each bunk mattress built to 15cm to suit the maximum mattress thickness used on many models for a safe fit, and all vacuum packed and rolled so they come up a ladder easily.

ComfortSleep 15 from 360

Reflex foam. Firm and supportive, and the most affordable of the three. Good for younger children, lighter body weights, and the bottom bunk of a bed that gets occasional use, with a simpler foam build that also suits a childs big bed after moving on from a cot or smaller first bed.

Eight sizes, from shorty to continental double.

MemoryDream 15 memory foam from 220

Memory foam mattresses can be a suitable bunk-bed option over a reflex base when kept to the correct bunk bed mattress depth. Contours to the body for overall comfort and a better sleeping experience, with the comfort layer adding contouring without needing extra depth, and it's quiet, which matters more on a bunk than people expect: a sprung mattress on the top bunk can be heard from the bottom one, so it can also suit a restless sleeper.

See our related blog post for more guidance on choosing the right bunk mattress.

Eight sizes, from shorty to continental double.

PocketRest 15 from 230

Pocket springs. More airflow, so it sleeps cooler. Worth considering for the top bunk specifically, since heat rises and the top bunk runs two to three degrees warmer than the bottom.

Single, continental single, small double, double.

TrundleEase 10 from 130

Built at 10cm for pull-out and trundle beds. This is the one that clears the frame when the trundle slides back.

Eight sizes, from shorty to continental double.

See the full bunk bed mattress range 17

5. Should the two mattresses be the same?

Usually yes, and buying matching mattresses for both bunks is usually the simplest way to avoid complaints when children swap, though the right choice can also depend on the bed's design. If one mattress is noticeably better than the other, you'll hear about it.

Where it's worth differing:

• Top bunk sleeps hot. A pocket sprung mattress on top and foam underneath is a reasonable split, and if the room allows, extra headroom gives more space above the sleeper so the top bunk feels less cramped.

• Big age gap. A firmer mattress suits a younger, lighter child; a teenager on the bottom bunk may want something different, and sleeping position or pressure points can matter more as children get older.

• Budget. If the bottom bunk is mostly for occasional sleepovers, especially those that only happen now and then, a ComfortSleep underneath and a MemoryDream on top is a sensible way to split the spend.

6. Things that catch people out

Buying one mattress. The bed listing price almost never includes them, and it's two, or three with a trundle.

Assuming a standard single fits a shorty. It's 15cm too long.

Using a supermarket mattress on the top bunk. Almost all are 20cm or more. A thicker mattress also reduces headroom between the sleeper and the ceiling on the top bunk, which can create safety issues, especially in a high sleeper or a room with limited ceiling height. Make sure there is enough space above the mattress for safe sitting up and climbing down.

A standard mattress on a trundle. It won't slide away, and you'll only find out once it's unrolled.

Forgetting the protector. Cleaning a mattress on a top bunk is genuinely difficult, and it's the one place where preventing a spill matters most as part of routine bunk bed safety checks. Take the mattress down to the floor rather than trying to clean it in place.

7. Getting it up there

A rolled mattress is far easier to carry upstairs and lift onto a top bunk than trying to manoeuvre a heavy mattress, and it also helps you stay within the weight limits set for some bunk bed designs. Unroll it on the bunk rather than on the floor.

Allow 24 to 48 hours for it to fully expand before making the bed. It'll look thinner than expected for the first few hours.

Before you order

1. Check the maximum depth on your bed's product page and follow the manufacturer's guidelines, making sure the bed still leaves enough room to use the bedroom safely.

2. Check the mattress size the bed requires, and measure your floor space so the bunk fits comfortably.

3. Count them. Two, or three with a trundle.

4. Add protectors. Cheaper than replacing a mattress, and much easier than cleaning one at height.

5. Order with the bed so everything arrives together, especially if you want options that save space around or underneath the frame.

6. Carry out regular safety checks on the bed frame and built-in safety features to help prevent accidents.

Common questions

What depth mattress for a bunk bed? Usually a 15 or 16cm maximum on the top bunk, even though the average depth of a standard mattress is often greater. Check your bed's product page for the exact figure.

Why does bunk bed mattress depth matter? Guard rails sit a set height above the mattress surface. A deeper mattress reduces that clearance, making the safety barrier less effective for bunk bed safety.

Can I use a thicker mattress on the bottom bunk? Usually yes, since there's no guard rail above it. Most people buy two identical ones anyway, though a thinner mattress is still often preferred on the top bunk for safety. The room and ceiling height still matter, because more space above the mattress makes the bed feel less cramped.

What size mattress does a bunk bed take? Most take a standard UK single at 90 x 190cm. Shorty bunks take 75 x 175cm and continental bunks take 90 x 200cm. In some models, the area below is used as a play area, which is one reason bunk beds work well in a child's room.

How many mattresses do I need for a bunk bed? Two. Three if it has a trundle, and the trundle one is shallower at around 10cm. A triple sleeper usually means a double on the bottom and a single above, while other triple bunk layouts stack three singles.

What mattress fits a trundle bed? Around 10cm deep, so it clears the frame above when it slides away. A standard mattress won't fit.

Foam or sprung for a bunk bed? Foam is quieter, which matters when someone is sleeping underneath. Sprung is cooler, which matters more on the top bunk. Either works, and low-profile foam can offer more support for a child without needing extra depth.

Are 15cm mattresses comfortable enough? Yes, for a child. Depth and comfort aren't the same thing, and a well-made 15cm mattress supports a child's weight perfectly well. The extra depth in an adult mattress is there for adult body weight.

Buying the bed too? Read our bunk bed buying guide, or browse all bunk beds.