Losing weight comes down to arithmetic: your maintenance calories minus the calories you actually eat. This calorie deficit calculator works out your basal metabolic rate, multiplies it by an activity factor for your daily maintenance calories, then subtracts the deficit you want to create so you can see exactly how many calories to eat per day to hit a target rate of loss.
What Is the Calorie Deficit Calculator?
This calculator separates the two numbers that get confused. Your basal metabolic rate is the energy your body uses just to stay alive, and your maintenance calories add the energy you spend moving and working on top. Your actual intake should sit below that second figure if you want to lose weight.
Once maintenance is known, the deficit you choose determines the rate of loss and nothing else does. The calculator shows the arithmetic so you can judge whether the target you have set is realistic.
How the Calculations Work
BMR (Mifflin-St Jeor) = 10 × weight in kg + 6.25 × height in cm − 5 × age + 5 for men or − 161 for women
Maintenance calories are the BMR multiplied by an activity factor, and your target intake is maintenance minus the deficit. The weekly loss figure comes from the rough rule that a pound of body fat represents about 3,500 calories of stored energy.
The last row is the one worth watching. If your target intake falls below a sensible floor, the deficit is too aggressive for you, and a smaller deficit that you can hold will produce a better result than a larger one you abandon.
Choosing a Realistic Deficit
A deficit of roughly 500 calories a day is the most common sustainable choice, and it corresponds to about half a kilo of loss per week. Deficits of 1,000 calories or more work in the short term but usually lead to hunger, lower energy and a rebound.
Slow loss is also the kind that stays off. Research consistently favours gradual changes that you can hold for years over aggressive ones that produce a large initial drop and a full regain.
What This Calculator Cannot Do
The Mifflin-St Jeor equation is a population average and it can be off by several hundred calories for an individual, particularly for people who are very muscular, very tall, pregnant or training hard. Treat the maintenance figure as a starting point and adjust it using what actually happens over two to three weeks.
It also says nothing about the quality of the food you eat. A deficit of 500 calories made up of whole foods will feel very different from the same deficit made up of processed food, and it will do different things to your body.
Frequently Asked Questions
How many calories should I eat to lose weight?
Your maintenance calories minus a deficit of around 500 a day is the usual starting point. The calculator works out your maintenance from your age, height, weight and activity, then shows the exact target.
How fast can I safely lose weight?
Around half a kilo, or one pound, per week is the figure most guidelines support. Faster loss usually means losing muscle along with fat, and it is much harder to maintain.
Is 500 calories a day too little to eat?
It can be, especially for women, small men and anyone with a high activity level. If the target intake falls below roughly 1,200 calories for a woman or 1,500 for a man, it is worth choosing a smaller deficit or seeking advice from a clinician or registered dietitian.
Why does my weight loss stall after a few weeks?
Your body adapts by reducing energy expenditure slightly and by often increasing the efficiency of how you burn fat. A stall of two to three weeks is normal, and the usual answer is to check your actual intake for a fortnight rather than to cut harder.
Does exercise change the numbers?
Yes, through the activity factor. Choose the level that describes your normal week rather than your most active week, because a factor that is too high will overestimate your maintenance and leave you eating more than you need.