Nappy and diaper sizes: which size and when to move up

Newborn · Care · Reviewed 20 June 2026 · All articles

Buying nappies for a newborn can feel overwhelming. The supermarket aisle offers half a dozen brands, each with their own weight guide and sizing numbering, and a pack that fits perfectly this week can start leaking by next week as your baby grows. Understanding how nappy sizing works, what the weight ranges mean in practice, and how to spot a poor fit early will save you from blowouts, red marks, and unnecessary spending on the wrong pack.

Why nappy size matters

A nappy that is too small puts pressure on your baby's legs and waist, leaves red marks on the skin, and is far more likely to leak up the back or around the thighs. A nappy that is too large has gaps at the leg cuffs where liquid can escape, and the absorbent core may not sit in the right position to catch everything efficiently.

Getting the size right is primarily about weight, not age. Babies born at very different weights can be wearing the same size nappy at the same chronological age. A baby born at 4.5 kg will likely skip the Newborn size entirely, while a baby born at 2.5 kg may stay in Newborn nappies for six to eight weeks. Always go by the weight printed on the pack, not by what other parents tell you their baby wore at a certain age.

Fit also depends on the shape of your baby's body. Some babies are rounder in the thighs, some have a narrower waist. It is completely normal to try two or three brands before settling on one that works well for your baby's particular build.

Nappy size and weight chart

The table below shows typical weight ranges used by most mainstream brands. Ranges overlap intentionally: the overlap zone is where you use fit and leakage as the deciding factor rather than weight alone. Brand-specific ranges are printed on every pack and should always be your primary reference.

Sources for these ranges include guidance published by the NHS and major nappy manufacturers. The NHS advises parents to check weight against pack guidance and to prioritise a snug but comfortable fit around the legs and waist.

Fit guide: signs it fits and when to move up

Use the table below alongside the weight chart. Fit clues are often a more reliable signal than weight alone, since babies of the same weight can have very different body shapes.

Signs your baby has outgrown a size

The most reliable sign is red marks. If you see redness around the thigh creases or across the top of the waist after removing a nappy, the leg cuffs or waistband have been pressing into the skin. This is uncomfortable for your baby and a clear prompt to try the next size up.

Leaks are the other primary signal. If liquid is escaping around the leg cuffs even though the nappy is not fully saturated, the cuffs are not forming a seal against the skin. This typically means the nappy is too small. (Leaks from a very saturated nappy that has been on a long time suggest the nappy needs to be changed more frequently rather than sized up.)

Check where the tabs are landing. Most nappy tabs are designed to fasten in the centre of the waistband landing zone, printed with a target zone on the front panel. If you are stretching the tabs to the edges of that zone, or the tabs are not reaching the landing zone at all, the nappy is too small. Conversely, if the tabs wrap all the way past the centre and the waistband feels very loose, the nappy may be too large.

Finally, watch for nappies that look uncomfortably stretched or that seem to compress the baby's belly or thighs when the baby is lying still. Babies cannot tell you the nappy feels tight, but they may be unsettled or uncomfortable for no other apparent reason.

How brands differ

The weight ranges between brands can vary by one to two kilograms at any given size number. A Size 3 from one brand may start at 5 kg, while another's starts at 6 kg. This means that if your baby is 5.5 kg and you switch brands, you may need to reassess the size number even if the fit had been fine before.

Beyond weight ranges, brands differ in the stretchiness of the side panels, the depth of the rear waistband, the width of the leg cuffs, the scent of the materials, and the texture of the inner lining. Some brands are cut wider in the seat to accommodate cloth nappy use underneath, though this is less common in standard lines.

Budget own-label nappies often follow the same weight conventions as premium brands but may have narrower leg cuffs and less elasticity in the waistband. For daytime use with frequent changes they can work perfectly well; for overnight use many parents find a premium brand more reliable for extended absorbency and a better seal.

If you find a brand works well for your baby, it is usually more cost-effective to subscribe to a regular delivery or buy larger packs once you are confident in the size rather than rotating through several brands each week.

Cloth nappies: sizing and absorbency

Reusable cloth nappies take a different approach to sizing. Most modern systems use one of two models: sized nappies (small, medium, large) that follow similar weight ranges to disposables, or "birth to potty" one-size nappies with adjustable poppers that can be resized as the baby grows, typically covering approximately 4 kg to 16 kg.

The absorbency of a cloth nappy depends on its inserts. Common insert materials include microfibre (fast absorbing but can compress and leak under pressure), bamboo (slower to absorb but high overall capacity, good for overnight), and hemp (highest capacity by weight, slow absorbing but excellent for heavy wetters). Many parents use a combination, such as a microfibre booster on top of a hemp insert, to get fast initial absorption plus high total capacity.

Cloth nappies generally need to be changed every two to three hours during the day, slightly more frequently than a premium disposable. For overnight, a well-stuffed bamboo or hemp insert can last the full night for most babies from around three months. Fit is just as important as in disposables: leg elastics should lie flat against the skin with no gaps, and the nappy should sit high enough at the back to catch all output.

The upfront cost of a cloth nappy system is higher than a single pack of disposables, but over the full nappy years a family typically saves several hundred pounds or dollars. Cloth nappies also generate significantly less household waste and many can be used for a second or third child.

Cost comparison: disposable vs cloth

A rough cost guide for comparison: a baby typically uses around 4,000 to 6,000 nappy changes in the first two years. At an average of 15-25p per disposable nappy (budget to mid-range), that totals roughly 600 to 1,500 pounds or the equivalent in other currencies. A complete cloth nappy stash of 20-25 nappies with inserts typically costs 200 to 400 pounds new, less second-hand, with additional costs for washing (water, energy, detergent) of around 50 to 100 pounds over two years.

The financial case for cloth is strongest when nappies are used full-time from birth and for more than one child. The environmental case is more nuanced: manufacturing cloth nappies has a carbon footprint, and much depends on how they are washed and dried. The UK Environment Agency has published lifecycle analysis showing that cloth and disposable nappies have broadly similar carbon footprints when cloth nappies are washed at 60 degrees and tumble-dried, but cloth nappies have a meaningfully lower footprint when washed at 40 degrees and line-dried.

When to switch to pull-ups

Pull-up style nappies (also called training pants or pant nappies) are designed for the potty training phase, not as a direct replacement for nappies. They are usually introduced when a child is showing signs of readiness for potty training: awareness of being wet or soiled, ability to pull trousers up and down, some ability to communicate the need to go, and general developmental readiness. This typically happens somewhere between 18 months and 3 years, with significant variation between children.

Pull-ups look like underwear and can be pulled up and down without lying the child flat, which makes daytime potty training more practical. They are generally not more absorbent than a regular nappy of the same size, so they are not a better overnight option than a well-fitted nappy or dedicated overnight nappy.

There is no developmental benefit to switching to pull-ups before a child is ready to engage with potty training. If your child is not yet showing interest, continuing with regular nappies is perfectly appropriate and usually more cost-effective.

Frequently asked questions

How do I know when to move up a nappy size?

Key signs your baby has outgrown a nappy size include red marks around the legs or waist, frequent leaks (especially at the legs), the tabs barely reaching the centre of the waistband, and the nappy looking visibly stretched or bulging before it is saturated. If two or more of these signs appear together, it is time to go up a size.

Do different nappy brands use the same size ranges?

No. While brands broadly follow a Newborn through Size 6 numbering, the exact weight ranges differ between brands. For example, one brand's Size 3 may start at 5 kg while another starts at 6 kg. Always check the weight guide printed on the pack, and remember that fit matters more than the number on the label.

Are cloth nappies as absorbent as disposables?

Modern cloth nappies with microfibre, bamboo, or hemp inserts can be highly absorbent and handle overnight use when correctly stuffed. They generally require more frequent changes than premium disposables during the day, but with proper inserts they are a practical full-time option for most families.

When should I switch from nappies to pull-ups?

Pull-ups are typically introduced at the start of potty training, usually somewhere between 18 months and 3 years of age. They are not more absorbent than regular nappies and are designed to help toddlers pull them up and down independently. There is no need to switch before potty training begins.

Related articles

• Newborn sleep guide
• Skin-to-skin benefits
• Safe swaddling

Sources

• NHS: Nappies - caring for a newborn
• Which?: Nappy buying guide and brand comparisons
• UK Environment Agency: Updated lifecycle assessment for disposable and reusable nappies