Calorie Deficit Calculator

What a calorie deficit actually is

A calorie deficit is the whole of weight loss in one sentence: you take in less energy than you spend, so your body makes up the difference from what it has stored. Everything else, every diet name and every meal plan, is a method for arriving at that gap. There is no way round it and no trick that replaces it.

So you entered your details and the tool gave you four numbers: what to eat to hold steady, and three targets for losing at different speeds. This page explains where they come from, and then spends most of its time on the question that actually brings people back to a page like this a month later, which is why the scale stops agreeing with the arithmetic.

The formula this calculator uses

First your resting burn, from the Mifflin-St Jeor equation:

Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161

Then an activity multiplier to get your maintenance level, and the targets are simple subtractions from there:

The tool labels those as roughly 0.25, 0.5 and 1 kilogram a week. Treat the labels as approximate rather than exact, because they lean on the rule of thumb that a kilogram of body fat is worth a fixed number of calories, and as the next-but-one section explains, it is not fixed at all.

Height goes in as feet and inches, weight in kilograms, and the tool will not run for anyone under 18.

Careful with the first activity option

One thing in this tool is worth flagging plainly, because it can catch you out.

The top of the activity dropdown says "Basal Metabolic Rate". Choose it and no activity multiplier is applied at all, so the tool reports your bare resting burn on the line labelled "Maintain Weight". That label is not right, and we would rather tell you than let you act on it. Your BMR is what you would burn lying still in a dark room all day. It is not the amount that maintains your weight while you live an actual life, and eating at it would put most people into a fairly steep deficit before they had subtracted a single calorie.

So unless you specifically want to see your resting figure, skip that first option and pick the line that honestly describes your week. If all you wanted was the resting number itself, our BMR Calculator is the right tool and will explain what it means.

A worked example

Take a 35 year old man, 5 feet 10 inches tall, 85 kg, doing light exercise 1 to 3 times a week. First his resting burn, with his height converted to 177.8 cm:

BMR = (10 × 85) + (6.25 × 177.8) − (5 × 35) + 5
BMR ≈ 1791 Calories/day

Multiplied by the light activity factor of 1.375, his maintenance comes to about 2,463 calories a day. From there:

Notice the last row. The arithmetic wanted to send him to about 1,463 calories, and the tool refused, holding at the floor of 1,500 for men. It does the same at 1,200 for women. That floor is there because below it a target cannot reliably carry the protein, vitamins and minerals you need, and our Calorie Intake Calculator goes into why in more detail.

Why the first few weeks lie to you

Here is the pattern almost everyone who uses a deficit calculator eventually runs into. You start the diet. The first fortnight is brilliant, the scale drops faster than the calculator promised, and you think you have found something that works. Then it slows. Then it seems to stop, even though you have not changed a thing. You conclude your metabolism is broken, or the calculator was rubbish, or you are secretly cheating.

None of those is the answer. The answer is that the exchange rate between calories and body weight is not a constant, and this is the single most useful thing to understand about dieting.

The whole rule-of-thumb apparatus, the one behind the labels on this tool, assumes that every kilogram you lose is worth the same fixed number of calories, somewhere around 7,700. Researchers have measured this directly, and it does not hold. In a study that tracked dieters over 24 weeks, the energy content of the weight they were losing came out at roughly 4,900 calories per kilogram at week four, climbed to about 6,000 by week six, and only settled around there afterwards. Shorter studies catching the very first days found figures lower still.

The reason is what you are actually losing. At the start it is not mostly fat. It is the glycogen stored in your muscles and liver, and the water that glycogen holds onto. That weight is cheap: it costs very few calories to shed and it goes fast. The same study put a number on it. In the fourth week of dieting, around 60 percent of what the men lost was not fat at all, and around half for the women. By week 24 those figures had fallen to roughly 35 and 24 percent, meaning that by then two-thirds or more of what they were losing was genuinely fat.

So read the pattern again with that in mind. Your fast fortnight was largely water. Your disappointing month three is fat, which is expensive, which is why the scale moves more slowly. The diet did not stop working. It started working. The researchers who ran that study said plainly that this slowdown, arriving despite honest adherence, is one of the main reasons people get disheartened and give up, and that people should be told about it in advance so they expect it.

Consider yourself told. Two things follow. Do not judge your plan on the first two weeks, because they are the least representative fortnight of the whole thing. And do not panic when the rate drops, because a slower scale at week six is a diet doing its job properly, not one falling apart.

Where the estimate can be off

Beyond the exchange rate, a few honest limits:

• The activity multiplier is the weakest link. This tool's multipliers top out at 1.55, so if you train hard most days your real maintenance is likely above what it shows. Our TDEE Calculator uses a wider range and may suit you better.
• It cannot see your body composition. Same age, height, weight and sex gives the same figure regardless of how much muscle you carry.
• Your maintenance drifts down as you lose. Less body to run, and a genuine metabolic slowdown on top, which our Maintenance Calorie Calculator explains. Recalculate every few kilograms.
• Weigh the trend, not the day. Water, salt, hormones and what is currently in your gut swing daily weight by a kilogram or more, which has nothing to do with fat. Look at weekly averages.

If you are managing a health condition, are pregnant or breastfeeding, or have a history of disordered eating, treat this page as background reading and take the decision to a doctor or a registered dietitian instead.

Questions people ask

What size deficit should I actually pick?

For most people, the mild or normal option. They are easier to live with, they cost you less muscle, and staying with a plan is what decides the outcome far more than the size of the gap.

I am eating the target and the scale has stalled. Why?

Most often it is the exchange rate explained above, plus a maintenance that has drifted down since you started. Recalculate at your current weight, check your portions honestly, and give any change a fortnight before judging it.

Why did I lose so much in week one?

Glycogen and the water stored with it. That early drop is real weight but it is not fat, and it does not repeat. Judge the diet from week three onward.

Should I create the deficit by eating less or moving more?

Both work, and doing some of each tends to be easiest, because a gap made entirely by eating less can get uncomfortable and a gap made entirely by exercise takes a lot of hours. Exercise also helps you keep muscle while the fat comes off.

References

Where the figures come from. The resting equation this tool runs is Mifflin-St Jeor, published in the American Journal of Clinical Nutrition. The finding that the energy content of weight loss is not a fixed 7,700 calories per kilogram, but starts far lower and rises over weeks, comes from Heymsfield and colleagues. The low-calorie diet figures behind the floor come from MedlinePlus, the patient information service of the US National Library of Medicine.

1. Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition. 1990;51(2):241-247. https://doi.org/10.1093/ajcn/51.2.241
2. Heymsfield SB, Thomas D, Martin CK, et al. Energy content of weight loss: kinetic features during voluntary caloric restriction. Metabolism. 2012;61(7):1058-1061. https://pmc.ncbi.nlm.nih.gov/articles/PMC3810417/
3. Thomas DM, Martin CK, Redman LM, et al. Time to correctly predict the amount of weight loss with dieting. Journal of the Academy of Nutrition and Dietetics. 2014;114(6):857-861. https://pmc.ncbi.nlm.nih.gov/articles/PMC4035446/
4. MedlinePlus. Diet for rapid weight loss. US National Library of Medicine. https://medlineplus.gov/ency/patientinstructions/000885.htm