How we build calculators — methodology

Five rules every calculator follows

Sourced data. Every number in every formula has a citation. If a value is sampled (food prices, insurance quotes), it is sampled from a documented set of providers on a named date.
Documented formula. The exact math is shown next to the calculator in a methodology sidecar. No black boxes.
Listed variables. Every multiplier, every constant, every band has a name, a value, and a one-line description of what it represents.
Stated assumptions and limitations. Models break at the edges. The sidecar names where this one does.
Unit-tested math. The functions that drive the calculator are pure, importable from a sourced data module, and covered by tests that run on every change.

How the data modules are structured

Each calculator has a matching src/data/calculators/<name>.ts file that exports:

A LAST_VERIFIED ISO date — the day the data was last sampled or audited.
A SOURCES array — every primary reference, with URL, organization, year published, and the date Sam accessed it.
The data constants (multipliers, ranges, lookup tables) with inline comments explaining where each value comes from.
Pure functions for the math — no Preact, no DOM. They take an input object and return a result object so tests can pin behavior at the boundary.

What gets re-verified

Three kinds of data drift, on three different cadences:

Pricing data (food cost per cup, insurance premium bands) — re-sampled every 6 months or when a major provider re-prices.
Cost-of-care data (hip dysplasia surgery, EPI treatment, bloat) — re-checked annually against Merck Veterinary Manual updates and Pawlicy / Trupanion claims data.
Formula and breed-standard data (RER coefficients, AKC weight ranges, OFA prevalence) — re-checked when the issuing body publishes a revision. The NRC RER formula has been stable since 2006.

What these tools are not

They are estimators. They give you a sensible starting point grounded in published research and current US pricing. They are not a feeding prescription, a veterinary diagnosis, a financial guarantee, or a replacement for talking to your vet or your insurance provider.

The full disclaimer is on the disclaimer page. If you find an error in any calculator’s math, sources, or assumptions, the contact page is the fastest way to flag it.

Where to go next

Feeding calculator — daily calories, cups per meal, monthly food cost.
Cost calculator — first-year, annual, and lifetime cost projection.
Puppy growth calculator — current vs breed-standard, predicted adult weight.
Insurance estimator — premium estimates and the savings-vs-out-of-pocket math.