Size guide
School bag size guide: will A4 fit, and what size by age
A4 paper is 29.7 cm by 21 cm, so a school bag needs internal dimensions of at least about 31 cm by 23 cm to take books flat; around 10 litres suits most children aged 3 to 10.
Litre figures are the least useful number on a school bag listing. Two 10 litre bags can be shaped completely differently, and only one of them takes a reading folder flat.
Use the measurements instead. Here is the arithmetic, and a rough guide by age.
plop in short
- 10L waterproof roll-top backpack for children aged 3 to 10.
- 32 cm high, 25 cm wide, 12 cm deep, so A4 books sit flat.
- Soft matte TPU fabric, fully waterproof, with welded seams and a reinforced base.
- Padded adjustable shoulder straps and a chest strap a three-year-old can clip alone.
- £39 with free UK shipping, 30-day returns and a 2-year guarantee.
The A4 arithmetic
A4 is 29.7 cm tall and 21 cm wide. Add a little for a folder's covers and for the bag's own seams and lining, and you want at least about 31 cm of internal height and 23 cm of internal width.
A bag around 32 cm high and 25 cm wide takes A4 flat with room to spare. Anything shorter and the folder goes in diagonally, which is how corners get bent and pages get creased.
- A4 sheet: 29.7 × 21 cm.
- Minimum practical internal size: about 31 × 23 cm.
- Comfortable internal size: about 32 × 25 cm.
Capacity by age
These are working numbers rather than rules; the contents list matters more than the birthday.
- Ages 1 to 3: about 2 to 6 litres. Snack, beaker, spare clothes.
- Ages 3 to 5: about 10 litres. Change of clothes, lunch box, bottle, nursery folder.
- Ages 5 to 10: about 10 to 14 litres. Reading folder, A4 workbooks, lunch, bottle, PE kit on kit days.
- Ages 11 and up: 15 litres and above, because textbooks and a laptop change the maths entirely.
How to check a bag properly
Look for internal dimensions rather than external ones, since padding and a stiff base can eat 2 cm. If a listing gives only litres, ask the seller for height and width.
Depth is the number most people forget. Around 12 cm is enough for a lunch box next to a folder; much less and the bag has to bulge to fit both.
Fit on the child, not just the contents
The bag should sit between the shoulders and the waist, not hang below the bottom, and it should be no wider than the child's shoulders. Padded adjustable straps and a chest strap keep it there.
A 32 cm tall bag suits a three-year-old and still looks right on a ten-year-old, which is why one well-sized bag can cover the whole of primary school.
Common questions
- What size bag fits A4 books?
- A4 is 29.7 cm by 21 cm, so you need at least about 31 cm of internal height and 23 cm of internal width. Around 32 cm by 25 cm fits comfortably.
- How many litres should a primary school bag be?
- Around 10 litres covers a reading folder, lunch box, spare clothes and a water bottle for most children aged 3 to 10.
- How big should a school bag be for a 5 year old?
- About 10 litres, roughly 32 cm high by 25 cm wide by 12 cm deep, and no wider than their shoulders.