Free, sourced calculators for everyday decisions.
No signup, no paywall, no invented numbers — every figure links to its source.
Pet & Vet
4 calculatorsPet questions where a wrong number has real consequences — how much chocolate is dangerous, how old your dog actually is in human terms, how much to feed. Each figure here comes from veterinary references such as the Merck Veterinary Manual and published clinical guidance, not a plausible-sounding rule of thumb (the “×7” dog-age myth being the classic example). Treat every result as a starting point for a conversation with your vet, never a substitute for one.
Looking for something else? See our Sleep & Rest calculators
Sleep & Rest
4 calculatorsRest is mostly arithmetic once you know the constants: sleep runs in roughly 90-minute cycles, caffeine clears on an about-five-hour half-life, and how much sleep you need shifts predictably with age. Each calculator applies figures from published sources — for example the National Sleep Foundation’s consensus recommendations — to your own times, rather than handing you generic advice. They’re guides to better rest, not medical advice for a diagnosed sleep disorder.
Looking for something else? See our Pet & Vet calculators
How Hearth works
Hearth is an independent project built around one rule: every number on the page has to trace back to a real, citable source — a veterinary manual, a published guideline, a peer-reviewed figure — never a plausible-sounding guess. Most "quick calculator" sites optimize for looking authoritative; Hearth optimizes for actually being right, and shows its work so you can check.
-
Find the source
We start from a named, published reference — a veterinary manual, a clinical guideline, a research figure — and fetch and read it, rather than approximating from memory.
-
Verify by hand
Before a calculator ships, a person checks its output against that source across real inputs. If the math and the source disagree, it doesn't go live.
-
Show the receipts
Each calculator names its sources on its own page, so you never have to take our number on faith — you can follow it straight back to where it came from.
Everything runs in your browser: no accounts, no logins, no third-party trackers, and nothing to buy. See the privacy policy for the specifics.
Frequently asked questions
How accurate are these calculators?
Each calculator implements a formula or table taken from a named, published source, and we hand-check its output against that source before it ships. They are built to be genuinely correct within the limits of the underlying research — but they are general-purpose estimators, so a result is a well-sourced starting point, not a guarantee for your specific situation.
Where do the numbers come from?
Every numeric output traces back to a real, citable source — a veterinary manual, a published guideline, a peer-reviewed figure — which we fetch and read rather than approximate from memory. Each calculator names its own sources on its page so you can check the math yourself.
Is my data collected?
No. Every calculator runs entirely in your browser — the numbers you type stay on your device and are never sent to a server. There are no accounts, no logins, and no third-party trackers. See the privacy policy for the full detail.
Do I need to sign up or pay?
Never. Hearth has no signup, no paywall, and nothing to buy. Every calculator is free and fully usable the moment the page loads.
Can I use these instead of professional advice?
No. The pet calculators are estimates to inform a conversation with your veterinarian, and the sleep tools are general guidance, not treatment for a diagnosed condition. When a result worries you or a real decision is riding on it, talk to a qualified professional.