Use when buying diapers in bulk, planning a Costco/Amazon stock-up run, checking how long current diaper stock will last, or deciding size transitions — size-by-size quantity math from the baby's birthday (growth-band durations + tapering changes/day), days-of-stock countdown with order verdicts, and per-size box-lifespan and cost estimates.
Bulk diaper buying fails in two opposite ways: people either buy three
boxes of size 1 right before the baby grows out of size 1 (donated,
unused), or run out at 2 AM and pay convenience-store prices. The math
that prevents both is boring but decisive — how many days a baby
typically spends in each size, how many changes per day that age band
uses, and what's actually on the shelf. This skill runs that math from
a single input every parent knows exactly: the birthday.
When to Use
Planning a bulk stock-up (warehouse club, Amazon subscribe box,
baby-shower registry) — what sizes and how many of each
Deciding whether to buy the jumbo box of the current size or
already start the next size
Checking how many days of diapers are left in the closet and when
to order
Comparing box sizes and price tiers ("does a 240-count box of
size 3 even get used before size 4?")
Building a registry: what to put on it in what quantities
Don't use for: cloth diaper wash schedules (different math entirely),
or medical advice about rash/urine output — output-count questions go
to a pediatrician.
The Model
Sizes N-7 are modeled as time bands a 50th-percentile baby passes
through, each with its own changes-per-day range that tapers inside
the band (a fresh size-2 baby uses ~9/day; one about to size up uses
~6/day):
Size
Weight (kg)
Typical days in size
Changes/day
N
0-4.5
~3 weeks
8-12
1
4-6.5
~6 weeks
7-10
2
6-8.5
~8 weeks
6-9
3
8-11.5
~10 weeks
5-8
4
10-14.5
~5 months
4-7
5
13-18.5
~6 months
4-6
6
16-23
~7 months
4-6
7
20-30+
pulls-up territory
3-5
Key truth: sizes 1-3 are short stops (a big box often outlives the
size), while size 4 is where babies park for months — that's the size
to overstock, not size 1.
Workflow
Get the birthday (exact input, no guessing) and today's stock count.
Run the planner for the horizon you're buying for:
bash
python3 scripts/diaper_stock_calculator.py plan \
--from 2026-10-20 --to 2027-01-20 --birthday 2026-08-15
Output: per-size days × avg changes/day = need, rounded up to
case multiples of 20. A plan JSON is saved to scripts/data/.
Verdicts: ORDER NOW (<7 days), order within a few days (<14),
stocked (with days until next order).
For single-size sanity checks (registry items, one box at the
store): burn --size 3 --lbs --tier premium → weight band, box
lifespans, cost.
Round up and prefer buying late over early for sizes 1-3;
babies run big or small — adjust after the next well-visit weigh-in.
Interpreting for a big or small baby
The model is calibrated to the 50th percentile. Adjust by observed
weight, not age: if the pediatrician says 90th percentile, expect size
transitions earlier than the day math says (buy one box less of
each small size); 10th percentile means longer stays. The burn mode
weight bands are the cross-check — size is chosen by weight, never by
age alone.
Common Pitfalls
Overbuying sizes 1-2. New parents get size-1 boxes from
everyone; the band is ~6 weeks. One jumbo box + one backup is
plenty; put size 4 on the registry instead.
Buying newborn size at all without reason. Many babies skip N
entirely (>3.5 kg at birth). Wait for the birth weight.
Ignoring the taper. A flat "8/day for size 2" overbuys by ~20%;
the last weeks in a size use 6/day.
Case-count tunnel vision. A 240-box that gets outgrown at
diaper 160 is not a bargain. The planner's need vs buy columns
expose exactly this gap.
Trusting brand age labels. "Size 3, 16-28 lbs" on the package
is marketing elastic; actual fit is belly-to-thigh geometry. When
blowouts or thigh marks appear, size up regardless of the table.
Measuring stock in "boxes" not "diapers." Counts per box vary
30-250 by size and brand; always count units.
Verification Checklist
Birthday confirmed against an actual document, not memory
Current stock counted in units (not boxes) for usage
Plan horizon matches the sale/shipping cadence you're buying for
Sizes 1-3 rounded down (short bands), size 4+ rounded up
Re-check after every well-visit weigh-in — rescale the plan
One-Shot Recipe: baby-shower registry quantities
bash
python3 scripts/diaper_stock_calculator.py plan \
--from 2026-11-01 --to 2027-05-01 --birthday 2026-11-01
Take the buy column as the registry ceiling per size, then halve
sizes N-1 (guests oversupply small sizes) and keep size 4 whole. Add
one wipe pack per 100 diapers — wipes don't expire or size out.
