How to Measure a Cot for the Right Mattress Size?
A cot can look like it has standard proportions, yet a few centimetres can make all the difference when choosing a mattress. Knowing how to measure cot dimensions gives you a clearer path to a snug, stable fit - and the peace of mind that comes from creating a safer, healthier sleep space for your baby.
The measurement that matters most is not the cot’s outside size. It is the usable space inside the cot where the mattress sits. This is especially relevant when purchasing a mattress for a pre-loved cot, a compact nursery cot or a cot bought from a different retailer.
How to Measure Cot Dimensions Accurately?
Measure the cot after it has been fully assembled, with its mattress base set at the height you plan to use. Remove the existing mattress, fitted sheet, mattress protector and anything else from the base. You need to measure the bare internal sleeping area, not the old mattress.
Use a rigid tape measure and record every measurement in centimetres. A metal tape measure is generally more reliable than a soft sewing tape, which can bend or stretch slightly around corners.
Measure the internal length
Place one end of the tape measure against the inside edge of the cot at the head end. Run it in a straight line along the mattress base to the inside edge at the foot end. This is the cot’s internal length.
Measure close to the centre of the base, then repeat the measurement along both sides. Most well-made cots will be consistent, but older timber cots can shift slightly over time. If the measurements differ noticeably from one point to another, check that the cot has been assembled correctly and that its frame is square and undamaged before selecting a mattress.
Measure the internal width
Next, measure from the inside of one side rail to the inside of the other. Again, take this measurement at the centre, then near the head and foot of the cot. Pay attention to curved rails, moulded ends or any internal trim that narrows the mattress space.
A mattress needs to sit flat within the cot, with no bunching, bowing or force required to press it into place. Measuring at more than one point helps uncover a cot that is not perfectly square.
Measure the mattress base and corner shape
The length and width tell you the main mattress size, but the details matter too. Look at the corners of the mattress base. Are they neatly square, gently rounded or distinctly shaped? A rectangular mattress with square corners may not sit correctly in a cot with unusually rounded internal corners.
Also check that the base itself is level and secure. A new organic mattress cannot compensate for a sagging, damaged or poorly fitted cot sheet base. If anything feels loose, uneven or worn, have the cot checked before using it for sleep.
Measure depth when choosing sheets
Cot depth is less relevant to the mattress footprint, but it is useful when buying fitted cot sheets. Measure from the mattress base to the top edge of the cot rail, then measure the depth of the mattress you choose. This helps you understand how much of the rail remains above the sleep surface as your baby grows and the base is lowered.
For sheets, measure the mattress itself: its length, width and depth. A fitted sheet should sit smoothly over the mattress, with elastic firmly held beneath it. Loose fabric can come untucked and should never be left gathered around your baby’s sleep space.
How Do I Know What Size Cot Mattress I Need?
First check the cot manufacturer’s instructions or mattress-size label, which should specify the mattress dimensions intended for that cot. Then measure the internal mattress space yourself to confirm the dimensions before ordering.
Do not choose a mattress simply because it is described as “standard cot size”. Cot dimensions vary between manufacturers, and a difference of only a few centimetres can affect how securely the mattress fits.
If you are choosing between 130 × 69 cm and 132 × 70 cm cot mattresses, use the dimensions specified by your cot manufacturer rather than choosing the closest size.
Cot Mattress Sizes in Australia: Why Labels Are Not Enough
There is no single universal cot mattress size used across every Australian cot. Two commonly encountered sizes are approximately 130 × 69 cm and 132 × 70 cm, while compact and specialty cots may require different dimensions.
That difference may sound small, but when it comes to cot mattress fit, even a few centimetres matter. This is why parents should always check the cot manufacturer’s recommended baby cot mattress dimensions and measure the internal mattress space before ordering.
Manufacturers may use terms such as standard, compact, mini or nursery cot differently. Even cots with similar advertised dimensions can have subtle differences in their internal measurements, corner construction or rail thickness. If you are replacing an existing mattress, do not assume that the size printed on its label is still the right size for the cot.
This is one of the reasons a mattress should be selected for the cot, rather than selected by appearance alone. A beautiful nursery is lovely, but a properly fitted sleep surface is the foundation of the room.
Check the Mattress Fit Before Your Baby Sleeps
Once your new mattress arrives, place it in the cot without sheets or protectors. It should lie flat on the base, with no raised edges and no gaps caused by a mattress that is too small, too narrow or the wrong shape.
Australian cot safety guidance commonly uses the 20 mm rule: when the mattress is centred or pushed to one side, the gap between the mattress edge and cot side should not exceed 20 mm. Check around all four sides, not just where you can see the widest space. Use a ruler if needed rather than estimating by eye.
A mattress that is too small can create a space where a baby’s limbs may become trapped. One that is too large may curl at the edges, sit unevenly or be difficult to remove safely. Neither is a compromise worth making.
The fit should remain snug once a fitted sheet and appropriate mattress protector are added. If an accessory makes the mattress shift, lift or no longer sit flat, remove it. Never use rolled towels, foam strips, blankets or other fillers to close a gap between a cot and mattress.
Do Not Measure Only Once
A cot may be moved between rooms, dismantled for a house move or adjusted as your child grows. It is sensible to recheck the mattress fit after reassembly, particularly with a second-hand cot or one that has been stored for some time.
Revisit the fit if the mattress has become visibly worn, if the cot base changes shape, or if you notice any new gap along the sides. Natural materials are chosen for their comfort, breathability and lower-toxicity credentials, but every infant mattress still needs to remain firm, flat and correctly fitted inside the cot.
It is also wise to check the cot manufacturer’s instructions for the recommended mattress dimensions and minimum or maximum thickness. Follow those instructions alongside current Australian safety requirements. If the cot manual is missing or the cot’s condition is uncertain, seek advice from the manufacturer or choose a cot with clear, current safety information.
A Few Common Measuring Mistakes
The most common mistake is measuring the outside of the cot. External dimensions are useful when planning where the cot will sit in the nursery, but they do not identify the mattress size. Measure internally, at mattress level, every time.
Another mistake is measuring over an old mattress. A compressed or oversized mattress can hide the actual dimensions of the cot base. Remove it first. Finally, avoid rounding measurements up because a slightly larger mattress seems more secure. A mattress must fit neatly and lie flat, not be squeezed into place.
For families choosing an organic cot mattress, accurate measurements also help protect the integrity of the materials you have chosen. A firm, breathable mattress made from considered natural materials can only perform as intended when it is supported evenly and fitted correctly within the cot.
At Milari Organics, we believe the most comfortable mattress is peace of mind. Taking five careful minutes with a tape measure is a small nursery task, but it supports the calm, clear decisions that help make bedtime feel safer from the very beginning.
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