Your maintenance calories are tied to your body, so when your body changes, the number changes with it. A target that produced steady fat loss in January can be doing nothing at all by April, and the usual explanation is not a broken metabolism. It is that you are now a smaller person eating a number calculated for a larger one.
Why it drifts
A lighter body costs less to run. There is less tissue to maintain and less mass to move, so both your resting burn and the cost of every walk go down.
There is a second effect on top of that. During a sustained deficit, people move less without noticing. Fewer trips up the stairs, less pacing, more sitting still. That unconscious activity is a large and variable slice of daily expenditure, and it quietly shrinks when energy is scarce.
Put together, that is why a deficit narrows over time even when you keep eating exactly the same amount.
When to run the numbers again
Every 5 to 10 kg of weight change, in either direction. That is the simple rule and it covers most people.
Recalculate sooner if any of these apply:
Your training has genuinely changed. Adding four sessions a week, or dropping from four to none, moves your activity factor, and that has a larger effect than a couple of kilograms does.
Progress has stalled for three weeks or more, with honest tracking behind it. Not one flat week. Three.
Your job or daily routine changed. Going from a job on your feet to a desk is worth several hundred calories a day.
You finished a diet and moved to maintenance. The maintenance figure for your new weight is not the one you started with.
How to actually do it
Put your current numbers into the calculator. Current weight, not the weight you started at, and be honest about the activity level. If you are unsure between two factors, take the lower one.
Then treat the output as a hypothesis rather than an answer. Eat at it for two or three weeks, weigh yourself several mornings a week, and compare weekly averages rather than individual days.
If the average holds steady, you have found maintenance. If it moves when it should not, adjust by 100 to 200 calories and give it another two weeks. Your own data beats any equation, because no formula knows how much you fidget.
Reading the scale properly
Weight is noisy. Water shifts of a kilogram or more happen routinely with salt, carbohydrate, hormones, travel and a hard session that leaves your muscles inflamed.
So never make a decision from a single weigh-in. Weigh most mornings under the same conditions, after the toilet and before breakfast, and look at the weekly average. Two averages three weeks apart tell you far more than any two individual readings.
The mistake almost everyone makes
Cutting calories again as soon as things slow down.
It feels like the obvious move and it is usually the wrong one. Each time you drop lower, you get a bit hungrier, you move a bit less, and you buy a smaller and smaller deficit. Do that repeatedly and you end up eating very little and losing nothing, which is where a lot of people get stuck.
Before you cut, check the other side of the ledger. Have you stopped logging carefully? Has your daily movement dropped? Could you add a couple of thousand steps instead of removing 200 calories?
And consider stopping. A few weeks at maintenance is not lost time. It gives your appetite and your training a chance to recover before the next push, and our piece on why a maintenance phase matters explains why that beats grinding on.
Recalculating while gaining
The same logic runs backwards. A bigger body costs more, so a surplus shrinks as you gain, and eventually your bulk stops working.
Recalculate every 5 kg or so and add food to keep the surplus where you want it. Do not add much more than that, because faster weight gain is mostly fat rather than muscle. Our guide to cutting versus bulking covers sensible rates.
A reasonable rhythm
Recalculate every 5 to 10 kg. Review your weekly average every fortnight. Adjust in steps of 100 to 200 calories rather than large jumps. And take the whole thing less seriously than the arithmetic implies, because it is an estimate you are steering with the scale, not a measurement of your body.