Nappy Sizes Overlap, and the Smaller One Usually Wins

Why the bands overlap in the first place

Weight is a proxy for a shape, and it is not a very good one. Two babies of the same weight can differ substantially in the measurements that decide whether a nappy seals — thigh circumference, waist, the length from waist to crotch — and a manufacturer covering both has to publish a band wide enough to include them. Overlapping the bands is the cheapest way to make sure no weight falls between two sizes, and it moves the choice from the box to the parent.

The smaller of two candidate sizes is the better default because leaks come from gaps rather than from capacity. A nappy that is a size too large seals badly at the legs and the back, which is where a leak actually starts; a nappy that is slightly small fills sooner but holds what it has. The exception is the night, where a size up is often the right answer precisely because capacity is the binding constraint over twelve hours rather than fit over three.

How long a size lasts, and why it stretches

The tool answers the question parents actually ask, which is not what size to buy but how many boxes of it. At 4.0 kg and about a month old, the size 1 band ends at 6.4 kg and average gain is 0.9 kg a month, so the size lasts about 2.7 months. At 7.5 kg and five months, size 2 runs to 12.7 kg at 0.6 kg a month — 8.7 months. At 11 kg and ten months, size 3 runs to 17 kg at 0.5 kg a month, which is a full year.

The sizes stretch because growth slows, not because the bands widen much. A baby gains about 0.9 kg a month for the first three months, 0.6 from three to six, and 0.5 after that — so the same 5 kg band that takes five months to cross at four months old takes ten to cross at a year. That is why the early sizes are the ones you buy in small packs and the later ones the ones worth buying in bulk, and it is the opposite of the instinct to stock up on newborn.

What the model cannot see

The weight bands are near-universal across brands, and the fit is not. Two size 4 nappies from different makers can differ by a centimetre at the waist and by more at the thigh, and a baby that leaks in one brand routinely does not in another at the same stated size. The calculator gives you a size to start from; whether that size fits is a question only the leg cuffs answer, and a child who is consistently outside the band its weight suggests is telling you about its shape rather than about the arithmetic.

The consumption figures are averages over a population and over a day, and the variance around them is wide. A newborn at ten a day is a common figure and twelve is not unusual; a fifteen-month-old at five may be at three if it is out of nappies in the daytime. The number is worth having as a budgeting input and not as a target — nobody should be changing to a schedule, and the honest use of the total is to know roughly what a year costs rather than to plan a week.

Frequently asked questions

How many nappies will we get through in the first year?

About 2,679 on the model's averages, and roughly 5,200 from birth to two and a half. Those are the figures to budget against; the month-by-month shape matters more than the total, because the first two months alone account for around 550 of them and the twelve months from eighteen to thirty for about 1,650.

When should we size up?

When the weight passes the top of the current band, or before that if the tabs no longer meet comfortably, if the leg elastics leave deep marks, or if you are getting repeated leaks at the back — the last of those is the classic sign of a nappy that is too small at night rather than too small in general. Red marks that fade within a few minutes are normal pressure; marks that persist mean the size or the fastening is wrong.

Is it worth buying a size up for the night?

Often, and it is one of the few cases where the smaller-is-better rule reverses. Twelve hours is roughly four times a daytime interval, so the constraint moves from fit to capacity and a larger nappy with more absorbent core is the direct answer. Many brands also sell a dedicated night version at the same stated size, which solves the same problem without changing the fit — worth trying first, because a size up that leaks at the legs has traded one failure for another.