Why am I not losing weight in a calorie deficit?

If the scale is not moving, you are almost certainly eating more than you think, retaining water, or both. The maths of a deficit does not break. Roughly 7,700 kcal is stored in a kilogram of body fat, so if you genuinely eat less than you burn over time, fat comes off. When it does not, the deficit is usually smaller than it looks on paper, because of things that are easy to miss: untracked oils, generous weekends, portions that have crept up, or a deficit that was too gentle to see through the noise of daily weight swings. None of this means your body is broken. It means the numbers need a closer look.

Here are the usual culprits, and what to do about each.

You are under-logging (the most common one)

This is the big one, and it catches almost everyone at some point. Studies on food diaries consistently find people underestimate what they eat, often by a wide margin, and it is rarely deliberate. It is the handful of nuts, the taste while cooking, the last few bites off a child's plate, the splash of milk in five coffees.

Log everything, including the small stuff. Bites, licks and tastes are still calories.
Weigh food on a scale rather than eyeing it. A "tablespoon" of peanut butter is usually two.
Log before you eat where you can, so you are deciding, not just recording.

Fettle (stayfettle.com) has barcode scanning and UK foods to make this quicker, which matters, because the easier logging is, the more honestly you do it.

Cooking oils and fats are the silent gap

Oil is the classic blind spot. Fat carries 9 kcal per gram, more than double protein or carbs, so it is the most calorie-dense thing in your kitchen, and it is nearly invisible once it is in the pan.

A single tablespoon of olive oil is roughly 120 kcal. Cook with two or three tablespoons a day and never log them, and you have a few hundred hidden calories that can wipe out a modest deficit on their own. Measure oil with a spoon rather than pouring straight from the bottle, and log it every time. Butter, dressings and cooking sprays count too.

The weekend undoes the week

Five tight days followed by two loose ones is one of the most common reasons the scale stalls. It is easy to hold a 500 kcal deficit Monday to Friday, then eat a couple of thousand extra across Saturday and Sunday without really tracking.

The week runs on averages, not individual days. Four days at a 500 kcal deficit is 2,000 kcal saved. One relaxed weekend day can hand most of that back. Keep logging at weekends, especially, since that is where the deficit tends to leak. You do not have to eat perfectly, you just have to see it.

You are holding water, not fat

Sometimes fat is coming off and the scale hides it, because body weight bounces around day to day for reasons that have nothing to do with fat. Water is the main one.

A salty meal, a hard workout, a poor night's sleep, more carbs than usual, or hormonal changes across the monthly cycle can all park a kilogram or two of water on the scale for a few days. This masks real fat loss underneath. The fix is not to panic at a single reading. Weigh yourself under the same conditions, first thing in the morning, and watch the weekly average trend rather than any one number. Over a few weeks the water noise averages out and the real direction shows.

Portions have quietly crept up

Even careful loggers drift. The scoop gets a little more generous, the "medium" apple becomes a large one, the eyeballed portion grows a bit each week. This is portion creep, and it is gradual enough that you do not notice until the results stall.

Every few weeks, go back to weighing your regular foods for a few days to recalibrate. It is a quick reality check, and it usually explains a surprising amount.

Your deficit might just be too small

If you are being honest with the logging and the trend is still flat, the deficit itself may be too gentle to see through normal daily weight swings. A very small deficit produces very slow loss, and that slow loss can hide underneath a kilo or two of daily water movement for weeks.

A deficit of around 300 to 500 kcal a day is a sensible, sustainable target for most people, giving roughly 0.25 to 0.5 kg of fat loss a week. If your deficit is smaller than that, or your calorie target was set too high to begin with, the answer may be a modest cut to the target rather than more willpower. Recheck your maintenance number, since it drops a little as you lose weight, so a target that worked at the start may need trimming later.

Give it time before you change anything

Before you tear up the plan, give it two to three weeks of consistent, honest logging and judge the weekly average, not the daily reading. Weight loss is rarely a clean line down. It is a jagged line with a downward trend, and the trend is the only part that matters.

One honest note

This is general guidance for healthy adults, not medical advice. If the scale genuinely will not move despite careful, consistent logging over several weeks, or you have symptoms that concern you, speak to your GP or a registered dietitian. Some medical conditions and medications affect weight, and those are worth ruling out properly rather than guessing at.