Calorie deficit

What a calorie deficit is

A calorie deficit means taking in less energy than you spend. When that happens, your body makes up the shortfall from storage — mostly fat — and you get lighter.

That is the entire mechanism. Low-carb, intermittent fasting, keto, Mediterranean, counting macros, cutting out bread: every one of these works if, and only if, it leaves you eating less than you burn. They differ in how easy they make that, not in whether they bypass it. Nothing bypasses it.

Which is good news, because it means the question is never "which diet", it is "which way of eating less do I find tolerable".

The arithmetic

Body fat stores roughly 3,500 kcal per pound, or7,700 kcal per kilogram. That figure converts a vague goal into a fixed quantity:

It is an approximation rather than a constant —where the number comes from covers why, and why it gets more optimistic the longer you diet.

Step one: find your maintenance

A deficit is defined relative to maintenance, so maintenance is the number you need first. Estimate it from the Mifflin–St Jeor equation plus an activity factor, then correct it against reality:

• Eat roughly consistently for two to three weeks, no deliberate deficit.
• Weigh daily, same conditions, and use the weekly average.
• Steady weight means you have found it. Drifting down means you were already under; drifting up means over.

The maintenance calculator does the estimate, and the longer explanation covers the measurement method.

Step two: pick a size

Subtract 15–25% from maintenance. For most people that lands between 300 and 700 kcal a day.

Bigger is not better. Aggressive deficits cost more lean tissue, make training miserable, and — the part that decides outcomes — are much harder to stay on. A 500 kcal deficit held for five months beats a 1,000 kcal deficit abandoned in three weeks. How big yours should be goes into choosing, including the floor you should not go under.

Step three: expect the scale to misbehave

This is where most people quit, and it is entirely predictable.

Weeks one to two flatter you. Glycogen leaves early, and each gram of it takes about three grams of water along. The scale drops faster than fat loss justifies.

Week three appears to stall. The water is gone, so what remains is actual fat loss — roughly half a kilogram a week, which is small against daily fluctuations of a kilogram or more from food volume, salt, hormones and sleep. Nothing has gone wrong. The signal is simply smaller than the noise.

The fix is to judge a four-week trend of weekly averages and ignore single mornings entirely. If it still has not moved,why the scale stops moving works through the actual causes.

Protecting muscle on the way down

A deficit does not care where the energy comes from, so some of what you lose can be lean tissue. Two things shift the ratio: eating enough protein, and giving your muscles a reason to stay. Resistance training two or three times a week does the second job even when you are not getting stronger.

Whether you can actually build muscle in a deficit has a more interesting answer than most people expect.

Tracking it without going mad

The practical difficulty with a deficit is that it is invisible. You cannot feel 500 kcal, the scale is noisy for weeks, and a target calculated in January is describing someone else by March — your maintenance falls as you get lighter.

That is the gap Caldown fills. It converts the goal into a single total, subtracts energy as Apple Health reports it, adds back what you log eating, and recomputes the finish date from your real rolling average rather than the number you typed at setup.