The '3,500 calories = 1 pound of fat' rule is an outdated shortcut
The classic rule states: to lose a pound of fat, you need to create a deficit of roughly 3,500 calories. The formula came from averaged lab calculations of fat tissue's energy density and has long been used as a quick reference. The problem is it assumes the body is a static system where a calorie deficit converts to fat loss linearly and predictably at every stage of a diet.
In practice, energy expenditure drops as you lose weight — a smaller body burns fewer calories just to keep running, and a sustained calorie deficit adaptively slows metabolism further to protect the body from depletion. Because of this, the same deficit number produces a different amount of weight loss at the start of a diet versus a few months in — the formula stops being linear.
Practical takeaway: don't be surprised if a strict calculation of 'this much deficit equals this much weight loss per month' stops matching reality over the long haul — the body isn't a calculator, it dynamically adjusts to change, and a plan is better corrected against actual results periodically than locked to one formula from day one.