About these tools
A health calculator that prints a single number and nothing else is doing something dishonest with a true figure. BMI was designed in the 1830s to describe populations, not people, and it cannot distinguish muscle from fat — so a fit rugby player and a sedentary person of the same height and weight get the same number and it means completely different things. Calorie estimates from height, weight and age carry an error of roughly ±10% before you account for how wrong anybody's self-reported activity level is.
None of that makes the figures useless. It makes them estimates, which is a different thing from a verdict, and the difference belongs on the page rather than in a disclaimer nobody reads. So every calculator here returns its caveats with its result, in the readout, where the number is.
The calorie calculator will also refuse to give you an unsafe target. A large enough deficit applied to a small sedentary person produces a figure under 1,000 kcal, and the arithmetic is perfectly correct — which is exactly why the tool clamps it and says so instead.
Everything runs in your browser. Health data is the most sensitive kind there is, and none of what you type here goes anywhere: the site makes no external requests at all.