BMR is the baseline; activity level is the multiplier

Take the calculator's own default — a 30-year-old male, 70 kg, 170 cm. The Mifflin-St Jeor formula gives a basal metabolic rate (BMR) of 1,617.5 calories — roughly what the body burns just existing, with zero activity, over 24 hours. Everything above that comes from multiplying by an activity factor:

The same body, at the same BMR, has a maintenance range spanning over 1,100 calories a day depending purely on activity level — which is why honestly picking the closer activity level matters more to the accuracy of this estimate than almost anything else entered above it.

Where the ±500 calorie deficit/surplus numbers come from

The calculator shows maintenance ±500 calories as its "mild deficit" and "mild surplus" figures — on the default numbers, that's 2,007 and 3,007 calories a day. The 500-calorie figure isn't arbitrary: roughly 3,500 calories is the commonly cited approximate energy equivalent of a pound of body fat, and 500 calories a day, sustained for 7 days, is 3,500 for the week — the basis for the widely repeated "about a pound a week" rule of thumb for a mild, sustainable deficit. It's an approximation, not a precise metabolic law — actual results vary by individual — but it's why 500 is the number most guidance reaches for rather than a rounder figure like 400 or 600.