Find the right diaper size by weight and how many you'll need
Diaper size is set by weight, not age. Most reliable input.
Used to estimate daily diaper count & weight-gain pace. Leave blank to infer from weight.
Leave blank to use the typical count for your baby's stage.
Enter a per-diaper price to estimate spend. Typical $0.15–$0.35.
Recommended diaper size
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Estimate only — every brand fits a little differently. Weight ranges and daily counts are typical averages; your baby's actual fit and output may differ. Move up a size if you see red marks, frequent leaks, or a snug waistband, even if the weight still says the current size. This is general parenting guidance, not medical advice.
Diaper Size & Quantity Calculator — guide & how to use
What it does
This diaper size and quantity calculator tells you which diaper size fits your baby
based on weight, estimates how many diapers per day and per month you'll go through, and projects
how many of the current size you'll need before the next size up — so you can buy the right amount
without over-stocking a size your baby will quickly outgrow. Enter a cost per diaper and it also estimates your spend.
How to use it
Enter your baby's current weight (the strongest signal — switch lb/kg as needed).
Optionally add age in months for a more accurate daily count and weight-gain pace.
Override diapers per day if your baby differs, and add a cost per diaper for spend estimates.
Read your recommended size, daily/monthly counts, and the estimate of how many to buy before sizing up.
Diaper size by weight (US)
Newborn (N): up to 10 lb
Size 1: 8–14 lb
Size 2: 12–18 lb
Size 3: 16–28 lb
Size 4: 22–37 lb
Size 5: 27 lb and up
Size 6: 35 lb and up
Ranges overlap on purpose — when your baby's weight sits in an overlap, fit beats the number. Go up a size at the first sign of leaks or red marks.
How the math works
Size: the smallest size whose weight range still contains your baby's weight; at the very top of a range we suggest sizing up soon.
Daily count: typical by stage — ~10/day for newborns, ~8 for 1–4 months, ~7 for 5–8 months, ~6 for 9–12 months, ~5 for toddlers — unless you override it.
Weeks to next size: (top-of-range weight − current weight) ÷ typical weekly weight gain (about 5 oz/week early, slowing to ~2 oz/week for older babies).
Diapers to next size: weeks to next size × 7 × diapers per day, rounded up to whole packs of ~30 isn't assumed — we give the raw count so you can match it to any pack size.
Monthly & spend: diapers per day × 30; multiplied by your cost per diaper when provided.
Worked example
A 3-month-old at 14 lb:
14 lb falls in Size 2 (12–18 lb) — and still inside Size 1's overlap, but Size 2 gives more room, so the tool recommends Size 2.
At 3 months the typical daily count is ~8 diapers/day → about 240 a month.
Top of Size 2 is 18 lb; at ~4.5 oz/week gain, 4 lb (64 oz) takes ≈ 14 weeks → ≈ 14 × 7 × 8 = 784 diapers of Size 2 before Size 3.
At $0.22/diaper, that's about $53/month.
Stocking tips
Don't over-buy Newborn or Size 1 — babies blow through them in weeks, and a big baby may skip Newborn entirely.
Sizes 3 and 4 last the longest — those are the safe ones to buy in bulk or subscribe to.
Keep one open pack of the next size up on hand so a growth spurt never leaves you short overnight.
Many retailers let you exchange unopened diaper packs for a larger size — check the return policy before stocking up.
Frequently asked questions
What diaper size does my baby need?
Go by weight, not age: Newborn up to 10 lb, Size 1 (8–14), Size 2 (12–18), Size 3 (16–28), Size 4 (22–37), Size 5 (27+), Size 6 (35+). In an overlap, size up if you see leaks or red marks.
How many diapers a day does a baby use?
About 8–12/day as a newborn, tapering to ~5–7/day for older babies and toddlers. The tool uses the stage midpoint and lets you override it.
How many will I need before the next size?
The tool estimates weeks until your baby hits the top of the current weight range using typical weekly gain, then multiplies by your daily count.
Should I stock up on newborn diapers?
No — buy a small amount. Newborn and Size 1 are outgrown fastest. Stock up on Sizes 3 and 4 instead.
How many diapers in the first year?
Roughly 2,500–3,000, heaviest in the newborn months. This tool projects a personalized monthly figure from your inputs.
Is it really free?
Yes. 100% free, and everything runs privately in your browser.