Calorie Deficit Calculator
Calculate the daily calorie deficit needed to reach your goal weight in your chosen timeframe. Includes safety warnings and milestone tracking
⚠️ High Daily Deficit
A deficit of 1375 cal/day exceeds the safe limit of 1000 cal/day. Consider extending your timeframe.
Total Weight Loss
15.0
kg to lose
Weekly Loss Target
1.25
kg per week
Daily Deficit Needed
1375
calories/day
Daily Calorie Target
1260
to consume
Estimated Timeline
12 weeks
At 1375 cal/day deficit
How a Calorie Deficit Actually Causes Fat Loss
There are roughly 7,700 calories in a kilogram of body fat. To lose 1kg you need to be 7,700 calories below your maintenance level over time, which can be split however you like: 1,100 calories a day for a week, or 250 calories a day for a month. The calculator works out your maintenance calories from your weight and activity level, then subtracts the deficit needed to reach your goal in your chosen timeframe.
A safe deficit sits between 300 and 700 calories per day for most adults. Anything beyond 1,000 a day risks muscle loss, energy crashes, and the kind of hunger that ends in a Saturday-night binge that wipes out the week's progress. The NHS recommends losing no more than 0.5 to 1kg per week, which lines up with a daily deficit of about 500 to 1,000 calories.
Why the Scale Stalls After a Few Weeks
Almost everyone hits a plateau around week 3 or 4. There are three reasons. Your metabolism adapts: as you lose weight your body needs fewer calories to run, so the deficit that once produced loss now equals maintenance. Water retention masks fat loss: a stressful week or a salty takeaway can hide 3kg of real fat loss for days. And calorie creep: portion sizes drift up as the diet feels familiar, particularly with oils, nut butters, and cheese which are dense and easy to under-count.
If the scale has been flat for 3 weeks or more, recalculate using your current weight and shave another 100-200 calories off the daily target. Equally check tracking accuracy: weigh five everyday foods you eat regularly and you will probably find at least one is 30% larger than your eyeball estimate. Pair this calculator with the [TDEE calculator](/tdee-calculator) to see how your maintenance number changes as you lose weight.
How Slow Is Too Slow, How Fast Is Too Fast
A sustainable rate is 0.5-1% of your body weight per week. For an 80kg person that is 400-800g per week, around 200-400g for a 40kg child or teenager. Anything faster than 1% per week and you are mostly losing water and muscle, which is why crash dieters always rebound: they regain water immediately, look bloated, get demoralised, and quit.
If your deficit comes back warning red on the calculator, your goal weight is too aggressive for your timeframe. Either extend the timeframe or move the goal weight closer. A 15kg loss is realistic in 6 months, exhausting in 3 months, and almost guaranteed to fail in 6 weeks. Slower diets keep more muscle, look better at the end, and stay off.
Frequently Asked Questions
How many calories should I cut to lose 1lb a week?
About 500 calories per day. There are roughly 3,500 calories in a pound of fat, so a 500-calorie daily deficit produces about 1lb of loss per week. For 2lb a week you need 1,000 calories per day, which is uncomfortable for most people and not generally recommended unless supervised by a GP or dietitian.
Can I exercise instead of eating less?
You can, but it is much harder than it sounds. A 30-minute jog burns around 300 calories, which is undone by one chocolate bar. Most successful weight loss happens through eating less, with exercise added for general health, mood, and to preserve muscle. The NHS Eatwell Guide recommends combining a modest deficit (around 500 cal/day) with at least 150 minutes of moderate activity per week, not relying on exercise alone.
Will I lose muscle in a deficit?
Some, unless you actively work to keep it. Eat at least 1.6g of protein per kilogram of body weight per day, do resistance training 2-3 times a week, and keep your deficit moderate (under 700 calories per day). With those three things in place you will lose mostly fat. Without them, expect about 25-30% of your weight loss to come from muscle, particularly if you are over 40.
Is this calculator suitable if I have a medical condition?
These numbers are general estimates only. If you have diabetes, thyroid issues, PCOS, are pregnant or breastfeeding, are recovering from an eating disorder, or are taking medications that affect appetite or metabolism, please speak to your GP or a registered dietitian before starting any calorie deficit. The right plan for you may look very different from a generic calculator output.
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