The NHS puts average daily needs at around 2,500 kcal for men and 2,000 kcal for women, and suggests aiming to lose about 0.5kg to 1kg a week by eating roughly 600 calories a day fewer than your body uses. For many people that works out at around 1,900 kcal a day for men and 1,400 kcal a day for women, which is the basis of the NHS weight loss plan.
Those are starting estimates, not personal prescriptions, and there is a floor beneath them that matters more than the target does. Eating too little is not a shortcut. It is the most reliable way to feel awful, lose muscle alongside fat, and end up back where you started. This guide covers both halves: how a deficit works, and where the line is that you should not cross.
Read this part first
What a calorie deficit actually is
Your body uses energy all day: keeping you alive, digesting food, moving you around, and whatever training you do. A calorie deficit means taking in less energy than you use over time, so your body makes up the difference from its own stores. That is the whole mechanism. Everything else is detail about how to do it without being miserable.
Two pieces of that detail matter. First, the deficit is measured over weeks, not days. A single large meal does not undo a week, and a single very light day does not achieve anything either. Second, your daily needs are an estimate. Height, weight, age, muscle mass and how much you move all change the number, and any calculator, including ours, is giving you a starting point to adjust from.
The NHS figures, plainly
- Typical daily needs: the NHS gives around 2,500 kcal for an average man and 2,000 kcal for an average woman, noting these vary with age, size and activity.
- A sensible rate of loss: the NHS suggests aiming for about 0.5kg to 1kg (1lb to 2lb) a week, achieved by eating roughly 600 calories a day fewer than you need to maintain your weight.
- The NHS weight loss plan works out at no more than about 1,900 kcal a day for most men and about 1,400 kcal a day for most women.
Notice how modest that is. A 600 calorie daily deficit is a meaningful change but not a dramatic one, and the NHS is explicit that a slower approach is more likely to work in the long run than a strict regime that produces fast early results and cannot be sustained.
The minimums, which are not negotiable
Trael will not build a plan below 1,200 kcal a day for anyone, and will not go below 1,500 kcal a day for men. These are hard floors in the software, not defaults you can talk your way past, and they exist because below them it becomes difficult to get enough protein, iron, calcium and the rest of what a body needs while also training.
This is not an arbitrary product choice. The NHS classifies eating around 800 calories a day or fewer as a very low calorie diet, says it should only be followed under medical supervision and for no more than 12 weeks, and is clear that you should not follow one unless a GP has suggested it. Intakes in that territory are a clinical intervention for specific situations, prescribed and monitored. They are not a dieting tactic.
Why crash diets tend to backfire
Severe restriction does produce fast movement on the scales in the first fortnight, much of which is water and the contents of your gut rather than fat. That early number is exactly what makes crash diets feel like they are working, and exactly why the following month feels like a failure when the rate slows to something normal.
The practical problems compound. Very low intakes are hard to hold for more than a few weeks, so most people stop, and stopping abruptly after weeks of restriction tends to be messy. Training quality drops, so you lose more muscle than you would have on a moderate deficit, which leaves you needing slightly fewer calories at the same weight afterwards. And the whole experience teaches you that eating normally is what caused the problem, which is both untrue and unhelpful.
A smaller deficit you can hold for three months does more than an enormous one you hold for three weeks. That is the least exciting sentence in this article and the most useful one.
Working out your own starting number
- Start from the NHS estimate for your sex, or from a maintenance figure you already trust from experience.
- Subtract around 500 to 600 calories a day. Do not subtract more because you are in a hurry.
- Check the result against the floors. If it lands below 1,200 kcal, or below 1,500 kcal if you are a man, raise it back up to the floor and accept a slower rate.
- Hold it for two to three weeks before judging it. Weight moves around day to day for reasons that have nothing to do with fat.
- Adjust once, by a small amount, based on what actually happened rather than what you expected.
Two things make the same deficit far easier to live with. Get enough protein, which helps with fullness and with holding on to muscle while you lose weight, covered in our guide to how much protein you need. And keep training, particularly strength work, which is the difference between losing weight and losing fat.
When calories are the wrong tool
Counting is a tool, not a virtue, and it does not suit everyone. If tracking makes you anxious, if you find yourself skipping meals to protect a number, or if the number starts mattering more than how you feel, put it down. Plenty of people do better by changing what is on the plate rather than measuring it: more protein and vegetables, fewer drinks with calories in them, meals cooked rather than ordered.
And if you are pregnant or recently gave birth, if you are underweight, if you have a condition affecting your metabolism, thyroid, or digestion, or if you take medication that affects appetite or weight, this is a conversation for your GP or a registered dietitian rather than an article or an app.
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How Trael handles this
Trael sets a daily calorie and protein target from your questionnaire answers and adjusts it from your weekly check-in. The floors described above are enforced in the software rather than suggested, week-to-week changes are deliberately capped so a target cannot drift downwards, and the questionnaire screens for eating-disorder risk before any number is produced. It is £24.99 a month with a 3-day free trial. The FAQ sets out what the nutrition guidance covers and what it does not, and the support and safety page lists the services above.
Trael provides general fitness guidance only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a GP or qualified professional before starting any new exercise or diet programme, and stop and seek help if you feel unwell.