The short answer
Buy the mattress size and thickness specified for your exact cot model. Do not assume that an Australian cot, a compact cot, a round cot or a travel cot takes one universal mattress size. The ACCC’s household cot guide says to use only the mattress designed for the cot. Red Nose says a safe mattress is firm, flat, clean, in good condition and the right size.
This guide is a fit checklist, not an endorsement of a particular mattress. Guidance was checked on 29 September 2026.
Find the cot’s specified dimensions first
Look for the mattress length, width and maximum or required thickness on the cot’s permanent label, in its manual or on the manufacturer’s current product page. Record the dimensions in millimetres if possible. A retail label such as “standard cot mattress” or “compact mattress” is too vague to settle the fit.
If you are buying a cot and mattress together, check what is in the box. Some cots include a mattress; others specify a separate one. Some convertible cots change their sleeping configuration, but that does not mean every mode uses the same mattress. The current instructions for the configuration you actually plan to use are the source to follow.
You can compare the Australian cot field and the cot mattress field on this site, but treat the two product pages as a matched purchase. A mattress that scores well on its own is a poor choice if it is not the one specified for your cot.
Check length, width and thickness
The three numbers have different jobs:
| Dimension | Why it matters | What to check |
|---|---|---|
| Length | A short mattress leaves an end gap. | Compare the mattress label with the cot’s required length. |
| Width | A narrow mattress leaves a side gap. | Check both sides once the mattress is placed flat and centred. |
| Thickness | A thick mattress reduces the effective height of the cot sides. | Follow the cot’s stated thickness limit as well as its length and width. |
The ACCC’s supplier guidance describes gap requirements in the mandatory standard. For a household cot, the limit is assessed differently with the mattress centred and pushed to one side. For a parent, the practical rule is simpler: use the size specified for the cot and check the fit once it is assembled. Red Nose’s consumer advice says there should be no more than a 20 mm gap between the mattress and cot sides.
Do not make an undersized mattress “fit” by filling a gap with rolled towels, cushions or extra bedding. The ACCC warns that the wrong mattress and extra padding create entrapment and suffocation risks.
Firm and flat still matter
Size is necessary, but it is not the whole check. Red Nose advises a firm, flat mattress. A soft or elevated surface is not made suitable by having the right outer dimensions. A cover or protector must fit tightly and be used as instructed; avoid products that add loose layers or change the sleep surface.
For a portable folding cot, Red Nose specifically advises using the firm, clean, well-fitting mattress supplied with it and not adding another mattress or padding. The household cot and travel cot rules cannot be collapsed into one shopping list. See our porta cot reviews for published product differences, then follow the chosen model’s manual.
A five-minute buying check
- Identify the exact cot model and mode you will use.
- Read its required mattress length, width and thickness from its label or current instructions.
- Match the mattress’s printed dimensions, rather than its marketing name.
- Place it flat in the assembled cot and check the fit on every side. If it does not match the specified size, do not improvise a fix.
- Check firmness, condition, cover fit and current product recalls.
Our safe sleeping guide covers the rest of the sleep setup, and our cot standards guide explains the current product rules. Neither a “breathable” name nor a review score replaces the cot maker’s dimensions.
Checked 29 September 2026 against ACCC Product Safety and Red Nose guidance.
Evidence record
Sources checked
This guide draws on the following primary sources. Rules and guidance can change, so check the linked authority before relying on a detail.