Most of us buy a school backpack the same way: we find one our child likes, check it isn't falling apart, and move on. Size rarely comes into it and weight almost never does.
But a backpack is the heaviest thing your child carries every single day. Get the size wrong and it sits too low, drags on one shoulder, or swallows so much that it invites overpacking. Get the weight wrong and you start hearing about sore shoulders halfway through the term.
Here's how to choose properly, in about five minutes.
The weight rule that matters most
Before size, weight. The American Academy of Pediatrics advises that a loaded backpack should not exceed 10–15% of a child's body weight, and many paediatricians recommend staying nearer 10% or lower still for smaller children and anyone with a long walk to the school gate.
The easiest way to check isn't a guess. It's your bathroom scale.
Weigh your child.
Weigh the packed backpack on its own.
If the bag is more than a tenth of your child's weight, take something out.
To save you the maths:
| Your child weighs | Aim for no more than | Absolute ceiling |
|---|---|---|
| 15 kg | 1.5 kg | 2.2 kg |
| 20 kg | 2.0 kg | 3.0 kg |
| 25 kg | 2.5 kg | 3.7 kg |
| 30 kg | 3.0 kg | 4.5 kg |
| 35 kg | 3.5 kg | 5.2 kg |
| 40 kg | 4.0 kg | 6.0 kg |
| 50 kg | 5.0 kg | 7.5 kg |
The number that surprises most parents is how fast a bag fills up. A large insulated water bottle is close to a kilo once it's full. Add a lunch bag with an ice pack, a PE kit, a reading folder and a tablet, and a Year 2 child is already at their limit before a single book goes in.
Backpack size by age and stage
Size should follow your child's back length, not their year group but age is a reasonable starting point.
Nursery and toddlers (2–4 years)
Look for: roughly 25–30 cm tall, 4–7 litres.
At this stage the bag carries a change of clothes, a water bottle, a snack box and a comfort toy. That's it. A bag that reaches past their bottom will swing when they run and catch on chairs. Small is the point a mini backpack should look almost too small on them.
Chest straps matter more here than anywhere else, because little ones let straps slide off their shoulders constantly.
FS1 and FS2 (4–6 years)
Look for: roughly 30–36 cm tall, 8–12 litres.
Now you need room for a lunch bag alongside the bottle, plus a folder that shouldn't have to be bent to fit. Check that an A4 book sits flat inside if it has to go in diagonally, the bag is too small and everything inside it will get creased for the next two years.
Independence becomes a factor: your child should be able to open and close every zip alone, and get the bag on and off without help.
Primary, Years 1–4 (6–9 years)
Look for: roughly 36–42 cm tall, 14–18 litres.
The classic school backpack size. Look for a padded back panel and genuinely padded straps, because the loads start getting real textbooks, library books, homework folders, sometimes a laptop or tablet.
This is also the age when kids start caring intensely about how their bag looks, which is worth taking seriously. A child who likes their backpack will actually wear it properly instead of dangling it off one shoulder.
Years 5 and up (9–12 years)
Look for: roughly 42–48 cm tall, 18–25 litres, with a padded laptop or tablet sleeve.
Bigger loads and more subjects. A separate padded compartment stops a device from sliding around against everything else. Check the shoulder straps are wide enough not to dig in narrow straps concentrate weight onto a small patch of shoulder, which is where the complaints come from.
Teens
Look for: 45 cm and up, 20–30 litres.
By now the bag is a laptop bag as much as a school bag. Sternum straps stop shifting during a walk between buildings, and a bottom compartment or side pockets keep a gym kit away from books.
How to tell it fits
Once the bag is on, run through this quickly:
The top sits at or just below shoulder level not above the shoulders, not up against the back of the head.
The bottom rests in the curve of the lower back, no more than about 10 cm below the waistline. If it's sagging towards the bottom, tighten the straps.
Both straps on, always. One-shoulder carrying loads everything onto one side of a growing spine. It's the single most common habit worth correcting.
Straps snug, not tight. You should slide a flat hand between the strap and the shoulder no more.
No leaning. If your child tips forward to balance the bag, it's too heavy, regardless of what the scale said.
Warning signs worth acting on
Red marks or grooves on the shoulders after school
Tingling or numbness in the arms or hands
Struggling to lift the bag on or off without swinging it
Complaints of upper or mid-back pain, particularly on school days
Persistent back pain in a child isn't something to wait out. If it continues, or shows up on weekends too, it's worth mentioning to your paediatrician.
Five habits that cut the load
Sort the bag by the timetable, not by default. Most bags carry a permanent layer of things that never come out. Empty it completely once a week.
Weigh it at the start of term. Once you've seen what 3 kg actually feels like on your child's back, you'll spot an overloaded bag by hand from then on.
Heaviest items closest to the spine. Books flat against the back panel, lighter items towards the front. The same load feels noticeably lighter when it's packed this way.
Use the outer pockets. A water bottle held in a side pocket rather than rolling loose inside keeps the weight low and balanced and stops it landing on top of a sandwich.
Right-size the bottle. A 750 ml bottle is excellent for a teenager on a sports day. For a five-year-old who has water fountains at school, a smaller bottle saves them carrying half a kilo they don't need.
Choosing a Citron backpack
Our backpacks come in sizes built around these stages a mini for nursery and early years, and a larger size for primary and up with padded straps, a supportive back panel and side pockets sized for our own water bottles, so nothing rides loose.
Every bag comes with free personalisation, which does more than look lovely: a named bag is a bag that comes home. And our Smart QR tag means that even if it does go astray, whoever finds it can reach you.
FAQs
How heavy should a school bag be? No more than 10–15% of your child's body weight, according to the American Academy of Pediatrics so around 2 kg for a 20 kg child. Weigh the packed bag on a bathroom scale rather than estimating.
What size backpack does a 5-year-old need? Around 30–36 cm tall, roughly 8–12 litres. The key test is that an A4 folder lies flat inside without bending.
Should a backpack sit above or below the waist? The bottom should rest in the curve of the lower back and shouldn't hang more than about 10 cm below the waistline. If it's lower, tighten the shoulder straps.
Is a trolley bag better than a backpack? It can help if your child's load is genuinely unavoidable and their school allows wheels. But trolley bags twist the body when pulled and are awkward on stairs, so a correctly sized, correctly packed backpack is usually the better everyday option.
Can my child use the same backpack for a few years? Usually two to three years, as long as it still fits their back length the top near shoulder level, the bottom at the waist. Once they've outgrown it, the bag starts sitting too low, and that's when posture problems creep in.
