How big a deficit should you pick?
The bigger your deficit, the faster you lose weight, and the harder it is to keep going. The right size is the biggest one you can keep up week after week without feeling run down. For most people that's somewhere between 300 and 750 kcal a day below maintenance.
What each size gets you
With roughly 7,700 kcal in a kilogram of body fat, here's what a steady daily deficit adds up to:
| Daily deficit | Per week | Per month |
|---|---|---|
| 250 kcal | 1,750 kcal | about 1 kg |
| 500 kcal | 3,500 kcal | about 2 kg |
| 750 kcal | 5,250 kcal | about 3 kg |
| 1,000 kcal | 7,000 kcal | about 4 kg |
Expect the scale to drop faster than this in the first week or two, as you lose water as well as fat, and a little slower as you get lighter. Over a few months, these figures are a fair guide.
A sensible place to start
The NHS suggests aiming to lose 0.5 to 1 kg a week. A good way to scale that to your size is to aim for 0.5 to 1% of your body weight a week: around 0.5 to 1 kg if you weigh 100 kg, but 0.3 to 0.6 kg if you weigh 60 kg. The less you have to lose, the smaller your deficit should be.
If you're unsure, start at 500 kcal a day. It's big enough to see on the scale within a few weeks and small enough that most people can live with it. After a month, look back: if it felt easy, you can go a little bigger; if you were hungry and tired, ease off.
Bigger isn't always faster
Hunger and energy
A bigger deficit means more hunger, less energy for exercise and a shorter fuse. A deficit that leaves you exhausted tends to get made up for at the weekend, which can wipe out the week.
Keeping your muscle
When you eat a lot less than you need, more of the weight you lose comes from muscle rather than fat. Eating plenty of protein and doing some strength training helps you hold on to it, and a moderate deficit makes both easier.
Sticking with it
The deficit that works is the one you keep. Three weeks at 1,000 kcal a day loses about 2.7 kg before most people give up. Twelve weeks at 400 kcal a day loses about 4.4 kg, and leaves you with habits you can carry on with.
Don't go too low
However keen you are, don't cut your calories in too far. Very low calorie diets, of 800 kcal a day or less, should only be followed with medical supervision. As a rough rule, most women shouldn't eat less than about 1,200 kcal a day and most men less than about 1,500 without advice from a GP or dietitian. If you have a lot of weight to lose, your GP can help you plan a bigger deficit safely.
Maintaining or gaining
Not everyone wants to lose weight. If you're maintaining, your aim is to stay close to break-even, and a band of about 100 kcal either side is plenty. If you're gaining, for example to build muscle, a surplus of 250 to 500 kcal a day is enough; much more mostly adds fat.
Setting it in the tracker
Choose your goal and daily target on the week page. Days that meet it get a green tick, and the week total shows whether the week as a whole did. You don't need to hit it every day: a week at 500 a day is 3,500 kcal however you spread it. The weekly calorie budget helps you plan a week with a bigger weekend, and the calorie deficit calculator turns a goal weight and date into a daily figure.
Questions
Is a 1,000 calorie deficit too much?
For many people it is more than they can keep up, and it risks losing muscle. It can suit someone with a lot of weight to lose, ideally with support from a GP. For most, 300 to 750 kcal a day is easier to stick to.
How much weight will I lose with a 500 calorie deficit?
About 3,500 kcal a week, which is roughly 0.45 kg (1 lb) of fat. Expect a faster drop in the first week or two from water, then something close to that.
Can I have a bigger deficit some days and a smaller one on others?
Yes. What matters is the total over the week, so you can leave room for a weekend or a meal out, as long as the week still adds up.
What is the minimum I should eat a day?
As a rough rule, about 1,200 kcal for most women and 1,500 for most men. Very low calorie diets of 800 kcal or less should only be followed with medical supervision.
Sources
- NHS. Lose weight.
- NHS. Very low calorie diets.
- Helms ER, Aragon AA, Fitschen PJ. Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. Journal of the International Society of Sports Nutrition, 2014; 11: 20.
- Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. The Lancet, 2011; 378(9793): 826–837.
This guide is general information, not medical advice. If you have a health condition, are pregnant, or have a history of disordered eating, talk to your GP or a registered dietitian before changing how you eat.