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A guide, not medical advice. Use a room thermometer, never add loose blankets over a sleeping bag, and follow safe-sleep guidance (baby on their back, no hat indoors, feet to the foot of the cot). Check your bag's own label for its tog and minimum weight. Babies vary, feel baby's chest or back of the neck to check they're comfortable. For trusted advice see The Lullaby Trust.
Sleeping bag tog & temperature chart
Match the tog to the room, then dress baby in the layers below. Most families own a 2.5 tog bag plus a lighter one for summer.
Tog
Room temp
What to dress baby in
0.5 tog
24°C +
Nappy, or a short-sleeved bodysuit
1.0 tog
20-23°C
Short-sleeved bodysuit, or a light sleepsuit
2.5 tog
16-19°C
Sleepsuit (add a bodysuit if cooler)
3.5 tog
Under 16°C
Sleepsuit + bodysuit
Sleeping bag tog questions
The things parents ask most before buying.
What tog sleeping bag should my baby use?
A 2.5 tog bag is the most common all-rounder for a 16-20°C room. Use 1.0 tog for warmer rooms (20-24°C), 0.5 tog above 24°C, and 3.5 tog for cold rooms under 16°C. Always check the room with a thermometer.
Can a baby wear a sleeping bag in summer?
Yes, choose a low tog (0.5 or 1.0) and dress baby in just a short-sleeved bodysuit, or a nappy only above about 27°C. Never add blankets on top of a sleeping bag.
What should a newborn wear in a sleeping bag?
In a 2.5 tog bag at 16-20°C, a short-sleeved bodysuit plus a sleepsuit is typical. Check baby isn't too hot by feeling the chest or back of the neck. Many bags have a minimum weight (often around 4 kg / 8.8 lb), check the label before use.
How do I know if my baby is too hot?
Feel the chest, tummy or back of the neck, it should feel warm, not sweaty or clammy. Hands and feet are normally cooler and aren't a reliable guide. If baby is hot, remove a layer.
Do you need more than one tog?
Most families end up with two bags, a 2.5 tog for spring, autumn and winter, and a 1.0 or 0.5 tog for summer. With central heating, a nursery rarely drops below 16°C, so a single 2.5 tog bag actually covers most of the UK year, you adjust the warmth with what baby wears underneath, not by swapping the bag.
It's only worth a 3.5 tog if your nursery genuinely runs cold, like an unheated room in an older house. And when you're between choices, a lower tog plus an extra layer of clothing is safer than a thick bag in a warm room.
What tog sleeping bag for a newborn?
For a newborn in a typical 16-20°C room, a 2.5 tog bag with a short-sleeved bodysuit and a sleepsuit is the usual starting point. Check the bag's minimum weight first, many don't start until around 4 kg (8.8 lb), and a bag that's too big is a slip-down risk.
Newborns can't regulate their temperature well, so a room thermometer and a quick chest-or-neck check matter more here than at any other age. If in doubt, a lower tog plus a light layer is safer than a thick bag.
Sleeping bag or blankets?
A well-fitted sleeping bag is generally the easier option for safe sleep. It can't ride up over the face the way a loose blanket can, and there's nothing to kick off and get cold. The key words are "well-fitted", the right size for baby's weight, with a neck that doesn't gape.
If you do use blankets, official safe-sleep advice is to keep baby's feet at the foot of the cot and tuck the blankets no higher than the shoulders. When you're unsure, your health visitor is the best person to ask.
Baby sleeping bag size guide
Tog keeps baby at the right warmth; size keeps them safe. A bag that's too big lets baby slip down inside it, so buy for their current weight and height, not to "grow into".
Size
Age (guide)
Baby length
Bag length
0-6 months
Newborn-6m
~50-68 cm
~60-70 cm
6-18 months
6-18m
~68-86 cm
~80-90 cm
18-36 months
18m-3yr
~86-98 cm
~100-110 cm
Ages are a rough guide, always check the bag's own weight range on the label. Many newborn bags start at 4 kg (8.8 lb), and some have poppers to shorten them for the smallest babies.
Getting the tog right through the year
The bag is only half the equation, what baby wears under it does the fine-tuning. One 2.5 tog bag can carry you through most of the UK year if you layer sensibly underneath.
Warm summer nights (24°C+): switch to a 0.5 or 1.0 tog, or just a short-sleeved bodysuit in the bag. A room thermometer matters most here, heatwaves are when babies overheat.
Spring and autumn (18-21°C): a 2.5 tog with a sleepsuit is the sweet spot for most nurseries.
Cold winter rooms (under 16°C): a 3.5 tog, or a 2.5 tog with an extra bodysuit layer. Never add a loose blanket over the bag, and skip hats indoors, babies lose heat through the head and can't cool down with one on.
More baby sleeping bag questions
When can my baby start using a sleeping bag?
From birth, as long as the bag fits their weight, most start at around 4 kg (8.8 lb). Below that a bag can be too big and baby may slip down inside it, so check the label's minimum weight before you use it.
Can a baby wear a sleepsuit under a sleeping bag?
Yes, that's the normal setup. In a 2.5 tog bag at 16-20°C, a sleepsuit (with or without a bodysuit under it) is typical. Add or drop that layer to fine-tune the warmth, rather than changing the bag itself.
What if my baby is between sizes or togs?
For size, go with the smaller bag, snug is safer than loose. For tog, pick the lower one and add a clothing layer if needed. It's easier and safer to add warmth than to cool an overheated baby.
Affiliate disclosure. Some links lead to retailers, and we may earn a small commission if you buy, it never changes the price you pay. This tool offers general guidance and is not a substitute for medical advice or the manufacturer's label.