Short answer: A calorie deficit means eating less energy than your body uses. The NHS suggests eating about 600 kcal a day less than you do now and aiming to lose 0.5–1 kg (1–2 lb) a week. Start from an estimate of your maintenance calories, judge progress by your weekly average weight rather than single readings, and expect loss to slow as you get lighter. That is normal, not failure.
Almost every weight-loss approach – low-carb, low-fat, smaller portions, fewer meals – works the same way underneath: it helps you take in less energy than you use. NICE, England’s health guidance body, adds that the method should fit your food preferences, culture and circumstances. The hard part is choosing a sensible deficit, measuring progress without being fooled by water, and knowing what to change when the scale stops moving.
If you are pregnant or breastfeeding, under 18, managing diabetes or another medical condition, taking regular medication, or have a history of eating disorders, talk to your doctor or a registered dietitian before you cut calories.
What is a calorie deficit?
Your body uses energy all day: to keep your organs working, to digest food and to move. When you eat and drink less than that, the gap – the deficit – is covered from stored energy, mainly body fat along with some lean tissue. Eat more and the surplus is stored. Eat about the same and your weight stays roughly stable; that level is your maintenance intake.
How to estimate your maintenance calories
You can’t measure maintenance directly at home, so you estimate it twice: once with a calculator, then again with your own data.
Step 1: start with a calculator
Population averages are only a rough guide: the NHS uses about 2,000 kcal a day for women and 2,500 kcal for men. A personal calculator does better. The free Body Weight Planner from the US National Institutes of Health uses your age, sex, height, weight and activity level; be honest about how active you really are.
Step 2: check it against real life
Log what you eat for a few typical weeks without dieting, and weigh yourself on several mornings each week. If your weekly average weight holds steady, your average logged intake is a good working figure for maintenance. If it drifts up or down, your true maintenance is a little lower or higher than what you logged.
Both steps carry error: a 2011 analysis in The Lancet notes that even lab methods can’t pin down daily energy needs more precisely than about 5%, and the NIH planner notes that people often underestimate how much they eat. If you are new to logging, our guide on how to count calories accurately covers the oils, drinks and sauces that are easiest to miss.
How big should your calorie deficit be?
Health services recommend a moderate gap. The NHS suggests reducing your intake by around 600 kcal a day and aiming to lose 0.5–1 kg (1–2 lb) a week. The US CDC notes that people who lose weight gradually, at about 1–2 lb a week, are more likely to keep it off than people who lose it faster.
Bigger is not better. NICE advises that low-energy diets of 800–1,200 kcal a day should only be used as part of a structured program with long-term specialist support, that diets under 800 kcal belong in specialist services too, and that neither is a long-term strategy. It also advises against restrictive, nutritionally unbalanced diets, which it says don’t work in the long term and can be harmful.
| Step | Example | Notes |
|---|---|---|
| Estimated maintenance | 2,300 kcal a day | Calculator estimate, checked against a few weeks of logs |
| Deficit | 600 kcal a day | In line with the NHS suggestion |
| Daily target | 1,700 kcal | 2,300 − 600 |
| Weekly budget (optional) | 11,900 kcal | 1,700 × 7; lets you eat a little less on weekdays and more at a weekend meal |
| Target pace | 0.5–1 kg (1–2 lb) a week | A guide, not a promise; loss slows as you get lighter |
To find the 600 kcal, the NHS suggests spreading the cut across the day instead of skipping a meal: roughly 100 kcal at breakfast, 200 at lunch, 200 at dinner and 100 from snacks. That might mean water instead of a sugary drink, a measured spoon of oil for a stir-fry or curry, one flatbread instead of two, and fruit instead of crisps. If you prefer to eat by eye, see our guide to estimating portions without a scale.
Why the “3,500 calories = 1 pound” rule is too simple
You will often read that a deficit of 3,500 kcal equals 1 lb of body weight, so cutting 500 kcal a day loses about 1 lb (0.5 kg) every week, indefinitely. The 2011 Lancet analysis showed why that rule overpromises: as you lose weight, your body needs less energy, both at rest and to move a lighter body around.
In the paper’s example, a sedentary man weighing 100 kg (220 lb) permanently eats about 480 kcal a day less. The simple rule predicts a loss of 22 kg (about 49 lb) in the first year and more after that. The researchers’ model predicts about half that in year one, with weight levelling off at around 75 kg (165 lb), about 95% of the change happening within three years.
Their better rule of thumb for an average adult with overweight: each lasting change of about 24 kcal (100 kJ) a day shifts your eventual weight by about 1 kg. Turned around, after losing 5 kg (11 lb) the same menu gives you a deficit about 120 kcal a day smaller than when you started. That is one reason progress slows, and why your target should follow your weight.
Water weight: why the scale jumps around
Your weight on any morning includes water, stored carbohydrate and food still in your gut. Glycogen (the body’s carbohydrate store) is held with water, and carbohydrate and salt affect how much fluid you retain. So a salty takeaway, a pasta-heavy weekend or a switch to low-carb eating can move the scale without any change in body fat.
The fix is a trend weight. Weigh yourself under the same conditions – after waking and using the toilet, before eating or drinking – on as many days as you are comfortable with, and compare weekly averages rather than single days.
| Day | Week 1 | Week 2 |
|---|---|---|
| Mon | 82.4 kg | 81.9 kg |
| Tue | 82.9 kg | 82.6 kg |
| Wed | 82.1 kg | 81.5 kg |
| Thu | 81.8 kg | 81.8 kg |
| Fri | 82.6 kg | 82.3 kg |
| Sat | 82.0 kg | 81.2 kg |
| Sun | 81.7 kg | 81.4 kg |
| Average | 82.2 kg (181.2 lb) | 81.8 kg (180.3 lb) |
Day by day, week 2 could feel like a stall; the averages show a drop of 0.4 kg (about 0.9 lb). If daily weighing makes you anxious, weigh less often or stop, and talk to a doctor or dietitian: tracking is a tool, not a test.
Not losing weight in a calorie deficit? A plateau checklist
First, make sure it is a real plateau: your weekly average, not your daily weight, has been flat for several weeks. In typical programs, weight often bottoms out after 6–8 months, and the Lancet authors concluded this is more likely explained by the diet gradually loosening than by metabolism halting weight loss. Drift is something you can check:
- Portions have crept up. Weigh the foods you eat most for a few days. The NHS example: a rice packet lists a 75 g (2.6 oz) portion as 260 kcal, but if you cook 150 g (5.3 oz), you are eating 520 kcal.
- Extras have gone unlogged. Alcohol, soft drinks and juice, cooking oil and spreads, cheese toppings and sauces all count.
- Weekends undo weekdays. Compare your weekend logs with Monday to Friday. A weekly budget can make room for a social meal without hiding it.
- Your target is out of date. Each kilogram lost lowers your needs a little. Recalculate maintenance at your current weight.
- You are moving less. If your step count has fallen, you are using less energy than when you set your target.
- Water is masking fat loss. A salty week or more carbohydrate than usual can hold the average up for a while.
If everything checks out and the trend is still flat, lower your target slightly to match your new maintenance, or hold at maintenance for a few weeks and then resume. Avoid jumping to a very low intake. While in a deficit, keep protein adequate and include some strength training – see how much protein you need per day.
How Portiva helps
Your calorie and macro targets in Portiva adapt to your actual weight trend and your logs, with a plain-words explanation of each change, so you don’t have to redo the math when progress slows.
- Targets update weekly with Plus and monthly on the free plan, and you can choose a weekly budget instead of a daily one.
- Weight from Apple Health feeds your trend automatically.
- Log by photo, voice, text or barcode. When oil, sauce or portion size would change the calories a lot, Portiva asks one short question instead of guessing.
Frequently asked questions
How many calories should I eat to lose weight?
There is no single number. Estimate your maintenance calories with a calculator, check it against a few weeks of logs, and subtract a moderate amount: the NHS suggests about 600 kcal a day less than you currently eat. NICE reserves intakes of 800–1,200 kcal a day for programs with specialist support, so don’t go that low on your own.
Is a 500-calorie deficit enough to lose weight?
For many adults, a lasting 500 kcal a day gap is a moderate deficit, close to the roughly 600 kcal the NHS suggests. Just don’t expect the “1 lb a week forever” of the 3,500-calorie rule: loss slows as your body gets lighter, so the same deficit produces smaller weekly losses over time. Update your target as your weight changes.
How long does a weight-loss plateau last?
A stall of a week or so is often just water and normal variation, which weekly averages smooth out. If your average has been flat for several weeks, your deficit is probably smaller than you think – from portion creep, unlogged extras or a target that hasn’t been updated since you lost weight. Check those first, then adjust your target.
Does eating too little put you in “starvation mode”?
Your energy needs do fall as you lose weight, so the same intake gives a smaller deficit over time. That slows progress gradually; it doesn’t make a real deficit stop working overnight. Very aggressive diets bring other problems, and NICE advises against restrictive, unbalanced diets because they don’t work long term and can be harmful.
Why did my weight go up when I’m in a calorie deficit?
Day-to-day weight includes water, stored carbohydrate and food in your gut, so a salty meal or a large late dinner can push the scale up for a few days without any gain in fat. Compare weekly averages instead of reacting to one reading, and give a new plan a few weeks.
Sources
- Calories and weight loss, NHS Better Health.
- Overweight and obesity in adults, NHS (2026).
- Overweight and obesity management (NG246): physical activity and diet, NICE (2025).
- Body Weight Planner, National Institute of Diabetes and Digestive and Kidney Diseases (NIH).
- Quantification of the effect of energy imbalance on bodyweight, Hall KD et al., The Lancet (2011).
- Steps for Losing Weight, Centers for Disease Control and Prevention.