How big should your calorie deficit be?

Why a percentage works better than a flat 500 kcal

A fixed figure like “500 kcal less” hits people very differently. For a small, not very active woman with a total expenditure of 1,500 kcal, that's a third of her energy. For a tall, active man at 3,000 kcal, it's a sixth. A percentage deficit grows with your expenditure, so it feels similar for both.

The Diet workbook uses the same starting point: around 20% below your total expenditure, never below your basal metabolic rate.

Step 1: Basal metabolic rate using Mifflin-St Jeor

Your basal metabolic rate is the energy your body uses at complete rest. The calorie calculator estimates it with the Mifflin-St Jeor equation:

• Men: 10 × weight in kg + 6.25 × height in cm − 5 × age + 5
• Women: 10 × weight in kg + 6.25 × height in cm − 5 × age − 161

The equation was developed on 498 healthy adults of normal weight and with overweight (Mifflin et al., 1990). In a systematic review, it came within 10% of the measured value more often than other common equations (Frankenfield et al., 2005). But that also means it can be noticeably off for individual people. Every number built on it is a starting value, not a measurement.

Steps 2 and 3: Daily life and training counted separately

The calculator multiplies your basal metabolic rate by a factor for your daily life without training. Training is then added as a separate amount. That way a gym session doesn't count twice, once in the factor and once on top.

For training, the calculator allows 3.5 kcal per kilo of body weight for each session of around 60 minutes and spreads the total over seven days. If you're between two activity levels, take the lower one and check after three to four weeks, based on your progress, whether it fits.

Step 4: 20% less, but never below your basal metabolic rate

Basal metabolic rate times the activity factor, plus the training amount, gives your total expenditure: roughly what you burn on an average day. For a diet, the calculator subtracts 20% from that and rounds to 10 kcal.

There is a floor, though. If the 20% would put you below your basal metabolic rate, the calculator sets your target calories to your basal metabolic rate. This mainly affects smaller, lighter and less active people. An example: a 55-year-old woman, 158 cm, 58 kg (128 lb), sedentary, no training, has a basal metabolic rate of 1,132 kcal and a total expenditure of 1,358 kcal. 20% less would be 1,086 kcal. So the calculator stays at 1,130 kcal, and the deficit here is only 17%. In a case like this, more movement in daily life is a better lever than eating even less.

Worked example: a woman at 72 kg

Let's assume: a woman, 32, 168 cm, 72 kg (159 lb), goal weight 66 kg (146 lb), lightly active daily life, three training sessions a week. This is how the calculation goes, number for number as in the calculator:

The deficit is around 414 kcal a day. On paper that's 0.38 kg per week, a little over 0.5% of her body weight. After twelve weeks she would be at about 67.5 kg, and she would reach her goal weight of 66 kg after around 16 weeks. The calculator also shows protein (2.0 g per kg of goal weight, here 132 g), fat (0.9 g per kg of goal weight, here 59 g) and carbs (the rest, here 148 g). There's more on protein intake in the article How much protein do you really need? For muscle gain and on a diet.

The 7,700 kcal per kilo is a rule of thumb. For leaner people, a smaller deficit per kilo is often enough, because a larger share of what they lose is fat-free mass (Hall, 2008). The progress the calculator shows is therefore a guide, not a timetable.

How fast is too fast?

For keeping your muscle, a pace of about 0.5 to 1% of body weight per week has proven itself. This range comes from a review on contest preparation in natural bodybuilding, so it was written for people who want to get very lean without losing muscle (Helms et al., 2014).

How much the pace matters is shown by a study of 24 elite athletes who did strength training four times a week (Garthe et al., 2011). One group ate around 19% less and lost 0.7% per week. The other ate around 30% less; the plan was 1.4% per week, and in practice it came to about 1%. In the end, both groups had lost a similar amount of weight. The slower group took longer, but gained 2.1% fat-free mass along the way. In the faster group, it stayed unchanged. The 20% in the calculator is close to the slower group.

That's why I see 1% per week as an upper limit rather than a target. The leaner you already are, the more you should stay at the lower end.

Check and adjust after three to four weeks

The first one to two weeks tell you little, because water and glycogen move the scale a lot. Why it can even go up in a deficit is explained in the article Weight going up in a calorie deficit: water or fat? What counts is the weekly average over three to four weeks:

• Within the planned range: Change nothing.
• Clearly slower: First check whether you're really eating those calories and measuring correctly. Only then adjust in a small step, for example by 100 to 150 kcal.
• Clearly faster: Don't cut further. Increase slightly instead.

Only ever change one variable at a time, so you can see what made the difference. And once you've lost a few kilos, recalculate with your new weight: for the woman in the example, total expenditure drops to 1,974 kcal at 66 kg.

Signs that your deficit is too big

A deficit is allowed to be noticeable. It shouldn't hold you back. Four warning signs:

• Hunger: constant and hard to control.
• Performance: a clear drop in training over several sessions.
• Recovery: sleep, mood and energy get worse.
• Daily life: your concentration slips, and food takes up a disproportionate amount of space in your thoughts.

If several of these apply for one to two weeks, check the structure first: enough protein, filling foods with plenty of volume, regular meals. If that doesn't help, raise your calories slightly instead of cutting further. When a diet should come to an end altogether is covered in the article Ending a diet without the yo-yo effect: what comes next.

When to check with a doctor

The calculator and this article are aimed at healthy adults. Talk to your doctor before starting a diet if you are pregnant or breastfeeding, take medication for diabetes or high blood pressure, have a kidney or metabolic condition, or have had an eating disorder in the past. The same applies if your weight drops quickly for no obvious reason. This article does not replace medical advice.