Quantity formulas

All eight rules, what uses them, and exactly when a number recomputes.

Checked against the app on 2026-10-12 (1.1).

Rule	Math	Floor	2 nights	5 nights	14 nights
Always 1	1	—	1	1	1
Always 2	2	—	2	2	2
Always Custom	your number	—	yours	yours	yours
Days + 2	nights + 2	—	4	7	16
Days ÷ 2	⌈nights ÷ 2⌉	2	2	3	7
Days ÷ 3	⌈nights ÷ 3⌉	—	1	2	5
Days ÷ 4	⌈nights ÷ 4⌉	—	1	2	4
Days ÷ 5	⌈nights ÷ 5⌉	—	1	1	3

"Days" means nights away — the gap between departure and return.

What the library actually uses

The formulas do nearly all their work inside Clothes, which is where counting is annoying and where over-packing happens.

Rule	Applied to	Reasoning
Days + 2	Underwear, socks, onesies, dress socks, crewneck undershirts	The things you want spares of. Two extra covers a spill and a delay
Days ÷ 2	T-shirts, tank tops, camis, undershirts, polos, dress shirts	Worn against the skin, so roughly every other day; floor of 2
Days ÷ 3	Blouses, button-front shirts, long-sleeve shirts, dresses, trousers	Re-wearable, but not indefinitely
Days ÷ 4	Jeans, pants, shorts, skirts, leggings, bras, ties, wool socks	Nobody needs seven pairs of jeans for a week
Days ÷ 5	Sweaters, sweatshirts	The outer layer you re-wear most
Always 2	a handful of specifics	—
Always 1	everything else — the large majority of them	A hairbrush is a hairbrush

209 CSV rows carry a formula. The rest are Always 1 by default.

Where a formula is set

Formulas live in the library. Trips hold numbers.

Surface	What you set
Settings → Item Library → item → Default Quantity	the rule
Sections & Kits Library → Browse Kits → kit → item	a per-kit override of the rule
A row on a trip	a number, for that trip only

There is deliberately no formula picker inside a trip. By the time the trip exists the math has happened; what you want then is "make it four."

A per-kit override lives on the kit and doesn't leak — the same item can be ×1 in one kit and ×2 in another. Overridden entries display as "Customized for [kit name]".

Recomputation — the part that changed

At trip creation: every formula-driven quantity is computed against the trip's dates and stored on the trip.

When you change the trip's dates: formula-driven quantities are recomputed, after confirmation. This shipped 2026-09-18.

Three categories are never touched:

Hand-set quantities. Anything you typed, or anything on an Always Custom rule.
Rows you added yourself.
Rows the same change is removing — a date edit also changes which items belong.

The confirmation flow: the alert counts, the review sheet names. A real date edit produced twenty-two quantity changes in testing, and an alert that tried to list them spent three lines naming four. So the alert says how many; Review Updates shows which.

Names read at the quantity they're moving to — a row going 1 → 3 reads "T-shirts", not "T-shirt".

Why this changed

Before it, the two halves of a date edit disagreed. Stretching a weekend into two weeks added the longer trip's items and left every quantity at the weekend's figure — silently. That's an under-pack, which is the specific failure the app exists to prevent.

Templates and length

A template carries a nominal length in nights. When you create a trip from it, quantities compute against your trip's dates, not the template's — so one template gives three shirts for a weekend and seven for two weeks.

The template's own number matters only when nothing else is available to count against.

⚠️ The nominal length does a second, less obvious thing. At authoring time it becomes a trip duration category, which filters which library items seed into the template at all. A 3-night template and a 10-night template come out with genuinely different contents. The default is 3 nights, chosen silently.

Edge cases

Situation	Behavior
One-night trip, round-up rule	2 — the floor, not 1
One-night trip, round-down rule	1 — the floor, not 0
A To-Do, Errand or Note	no quantity shown, ever, even if a number is stored
Setting a number on a trip	replaces the calculated one for that trip only; the library rule is untouched
Plural display	the library stores both singular and plural, so a calculated quantity reads properly

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