The gap between two 90 cm mattresses almost always comes from three things: mattresses that are slightly narrower than their rated size, bed bases that drift apart, and two different thicknesses. To eliminate the gap, first secure the bed bases, then fill the hollow with a mattress wedge or a 180 × 200 mattress topper placed over both.
Two 90 cm mattresses on a 180 cm frame is practical: everyone chooses their own firmness, and it is easier to carry up stairs. The price to pay is the gap in the middle. You feel it when you move closer, and it swallows pillows. The good news: it can be measured, so it can be fixed.
Why is there a gap between two 90 cm mattresses?
The first reason, and the least known: a "90" mattress is not necessarily 90 cm wide. The CTB Literie certification framework from FCBA, the furniture technology institute, which is based on the NF EN 1334 standard, allows for a tolerance of "nominal -20 + 0 mm" for length and width. In other words, up to 2 cm less, but never more.
Do the math: two 88 cm mattresses on a 180 cm frame leave 4 cm of play. This space does not stay distributed on the sides. As you toss and turn, it ends up in the middle.
Second reason: height. The same standard allows ±10 mm for thickness. Two mattresses can therefore differ by 2 cm, even if they are the same model. Add two different firmness levels, and the step becomes deeper as soon as you lie down.
Third reason: the bed bases. Two 90 cm bed bases placed side by side, without attachments, will drift apart slightly every night. The mattresses follow.
How do you know where your gap comes from?
Before buying anything, try four quick tests with a tape measure, a pencil, and a broom handle.
| Test | What you are measuring | If the result is… | The problem is |
|---|---|---|---|
| Total width | Both mattresses pushed together, from one outer edge to the other | Less than 180 cm | The mattresses are narrower than the frame |
| Broom handle across | Placed flat on both mattresses, in the middle of the bed | It tips or leaves a gap of more than 1 cm | A difference in height |
| Pencil mark | A line on each bed base, at the foot of the bed, realigned at night | The line has shifted in a week | The bed bases are sliding |
| Hand flat in the center | Press on each side of the seam | One side sinks much more than the other | Two firmness levels that are too different |
Often, two causes combine. Treat them in order: bed bases, then width, then height.
How to connect two 90 cm mattresses: what are the solutions?
Each solution solves a specific problem. None of them solve all four.
| Solution | What it solves | What it doesn't solve | Installation |
|---|---|---|---|
| Coupling kit (brackets or connecting feet) | Bed bases drifting apart | The hollow between the mattresses | A few screws or tightening, once |
| Coupling strap around both mattresses | Mattresses moving away from each other | The difference in height | Goes under the mattresses, ten minutes |
| Mattress wedge (T-shaped foam) | The hollow in the center, along the entire length | A significant height difference | Slips into the gap |
| 180 × 200 mattress topper | The hollow and a small height difference | Bed bases that slide | Placed on top of both mattresses |
| A single 180 × 200 fitted sheet | A small gap, by holding both mattresses together | A gap of several centimeters | None |
The combination that works best for most beds: a coupling kit for the bed bases, then a mattress wedge or mattress topper depending on the result of the broom handle test.
Mattress wedge or mattress topper: which one to choose for the space between two mattresses?
The mattress wedge fills the void, but it does not raise a mattress that is lower than the other. If your broom handle remains flat, this is the simplest solution.
The mattress topper covers the entire surface. It eliminates the seam and a small height difference. On the downside, it adds thickness: make sure your fitted sheet's pocket depth is sufficient, otherwise it will pull up at the corners.
A difference of 2 cm or more, with two very different firmness levels? No accessory will make the step disappear completely. The day one of the mattresses reaches the end of its life, a single 180 cm mattress remains the only permanent solution.
And the space between the mattresses and the headboard?
With two mattresses, there is often a second hole: at the top, against the headboard or the wall. This is where the central gap opens up like a funnel and the pillow disappears. We explain where this space comes from and how to measure it in our article on the space between the mattress and the headboard.
For that hole, a mattress wedge is useless: you need a piece that covers the entire width of the bed, from one edge to the other.
At NUVIA
Our headboard wedge pillow wedges between the top of the mattresses and the headboard or wall. For two 90 cm mattresses on a 180 cm frame, choose the 180 format as a single piece: it closes the top of the gap where pillows and phones fall. High-density foam, machine-washable removable cover, two side pockets. It does not replace a mattress wedge along the entire length, and it does not stop sliding bed bases.
Frequently Asked Questions
How do you eliminate the space between two 90 cm mattresses?
First, secure the bed bases with a coupling kit, then fill the hollow with a mattress wedge. If there is also a difference in height, a 180 × 200 mattress topper placed over both mattresses is more effective.
Why do two 90 cm mattresses leave a gap on a 180 cm bed?
The FCBA's CTB Literie standard allows for up to 2 cm less than the stated width. Two 90 cm mattresses can therefore measure 176 cm in total and leave 4 cm of play on a 180 cm frame.
Is the mattress wedge effective?
Yes, for filling the hollow in the center when both mattresses are the same height. It does not correct a step between a higher mattress and a lower one.
Can you use a 180 × 200 fitted sheet on two 90 cm mattresses?
Yes, and it helps hold the two mattresses together. Choose a pocket depth suitable for the thickness of the higher mattress, including any topper.
How do you prevent two bed bases from drifting apart?
With a coupling kit: brackets or connecting feet that secure the two frames together. This is the first step; without it, the gap will return within a few nights.
Written by the NUVIA team · Updated October 7, 2026