What a calorie deficit is and where the 3,500 figure comes from
A calorie deficit is simply energy out minus energy in. Sustain one and your body makes up the difference from stored energy, most of which is adipose tissue. The arithmetic that turns a deficit into a weight-loss rate needs one more number: how much energy a unit of body-weight change represents.
That number is the 3,500 kcal per pound rule, published by Max Wishnofsky in 1958. He reasoned that a pound of adipose tissue is roughly 87% fat by weight, that fat yields about 9.5 kcal per gram in the body, and that 454 × 0.87 × 9.5 lands near 3,500. Divide by seven and you get the familiar target: a 500 kcal daily deficit for a pound a week.
The rule works because it is roughly right in the short term and easy to remember. Its weakness is that it treats energy expenditure as a constant when it is not. As you get lighter you burn less — a smaller body costs less to maintain and less to move — so the same absolute intake produces a shrinking deficit. Modelling work by Kevin Hall and colleagues shows this compounds: a fixed intake reduction produces rapid early loss that flattens towards a new plateau rather than continuing in a straight line. Over the first month or two the linear rule is close. Over a year it substantially overstates the total.
The practical response is not to abandon the arithmetic but to re-run it. Recalculate your maintenance and your deficit every few kilograms, and the linear approximation stays inside its useful range.
How the calculation works, term by term
The chain has three steps and no hidden assumptions beyond the energy density.
Turn the weekly rate into weekly energy. Multiply the target loss by 3,500 kcal per pound. One pound is 3,500 kcal; two pounds is 7,000; half a kilogram is 3,858 kcal, because a kilogram is 7,716 kcal at this conversion.
Divide by seven. Deficits are managed daily, so the weekly energy figure becomes a daily one. Nothing requires the deficit to be identical every day — a weekly total achieved with larger deficits on some days and smaller on others produces the same result, which is the whole basis of intermittent approaches.
Subtract from maintenance. The intake target is TDEE minus the daily deficit. This is where the accuracy of your TDEE estimate matters: if your maintenance is really 2,200 and you assumed 2,400, then a nominal 500 kcal deficit is actually 300, and you will lose at 60% of the planned rate. That is the single most common reason a diet appears not to work.
Then check two ratios. The deficit as a share of maintenance tells you how aggressive the plan is in relative terms — a 500 kcal deficit is modest at 3,000 kcal maintenance and severe at 1,600. The loss rate as a share of body weight tells you whether the rate is appropriate for your size: 1 lb a week is 0.5% for a 200 lb person and 1.0% for a 100 lb person, and those are very different propositions.
Worked example: 2,400 kcal maintenance, targeting 1 lb a week at 200 lb
- Weekly energy. 1 lb × 3,500 kcal/lb = 3,500 kcal a week.
- Daily deficit. 3,500 ÷ 7 = 500 kcal a day.
- Intake target. 2,400 − 500 = 1,900 kcal a day.
- Deficit share. 500 ÷ 2,400 × 100 = 20.8% of maintenance.
- Rate against body weight. 1 ÷ 200 × 100 = 0.50% of body weight a week — inside the 0.5–1% band that sports nutrition guidance suggests.
- Floor check. 1,900 kcal is comfortably above the 1,500 kcal minimum used for men, so no flag.
Now push the target to 2 lb a week. The deficit doubles to 1,000 kcal a day and intake falls to 1,400 kcal, which is below the floor, and the rate becomes 1.0% of body weight — at the very top of the recommended band. The calculator reports that eating exactly at 1,500 kcal would instead give (2,400 − 1,500) × 7 ÷ 7,716 = 0.82 kg, or 1.80 lb, a week. That is the honest maximum for this person by diet alone; going faster means raising expenditure rather than cutting intake further.
Raising expenditure is a real option, and the calories burned by activity calculator will tell you what an hour of any given activity is worth. Forty minutes of brisk walking at 4.3 METs for a 91 kg person is about 270 kcal, which buys back more than half of the extra deficit without touching the plate.
Choosing a rate you can actually hold
Judge the rate as a percentage of body weight, not in pounds. The ISSN position stand on diets and body composition suggests roughly 0.5 to 1% of body weight per week for people who want to keep muscle while losing fat. Above that, the share of weight lost as lean tissue climbs, training performance drops and adherence falls off.
The deficit share is the second lens. Somewhere around 20 to 25% of maintenance, most people find hunger manageable and training unaffected. Beyond that the trade-offs bite quickly, and the total time to goal often gets longer rather than shorter because of the weeks lost to breaking the diet.
Three things protect lean mass while you are in a deficit, and none of them is the size of the deficit itself. Eat enough protein — see the protein intake calculator for a number based on your weight and goal. Keep resistance training in the programme, with load rather than volume as the priority. And keep the deficit moderate enough that you can do both.
Finally, expect the scale to lie in the short term. Glycogen and its associated water can move body weight two kilograms in either direction inside a week, which will swamp a 500 kcal daily deficit. Judge progress on a seven-day rolling average and on at least three weeks of data.
Daily deficit for each weekly loss rate
| Weekly loss | Metric | Daily deficit | Weekly deficit | Intake at 2,400 TDEE |
|---|---|---|---|---|
| 0.25 lb | 0.11 kg | 125 kcal | 875 kcal | 2,275 kcal |
| 0.5 lb | 0.23 kg | 250 kcal | 1,750 kcal | 2,150 kcal |
| 0.75 lb | 0.34 kg | 375 kcal | 2,625 kcal | 2,025 kcal |
| 1 lb | 0.45 kg | 500 kcal | 3,500 kcal | 1,900 kcal |
| 1.5 lb | 0.68 kg | 750 kcal | 5,250 kcal | 1,650 kcal |
| 2 lb | 0.91 kg | 1,000 kcal | 7,000 kcal | 1,400 kcal |
In metric terms, 0.5 kg a week needs 551 kcal a day and 1 kg a week needs 1,102 kcal a day.
Why the deficit you planned is not the deficit you got
- The TDEE estimate was wrong. Prediction equations return the average for people your size. If your true maintenance is 200 kcal lower than estimated, a 500 kcal planned deficit is really 300, and the scale moves at 60% of the expected rate.
- Intake is under-reported. Unmeasured cooking oil, drinks and tastes while cooking routinely add a few hundred calories a day. Weighing food for two weeks is the cheapest diagnostic there is.
- Expenditure falls as you lose weight. A lighter body burns less at rest and less in motion. Re-run the calculation every few kilograms rather than holding the original target for months.
- Non-exercise activity drops. People move less spontaneously in a deficit — fewer steps, less fidgeting — and the effect can be worth a few hundred calories a day without any conscious change.
- Water masks fat loss. Glycogen binds water, so a low-carbohydrate week shows a large drop and a high-carbohydrate week shows a rebound. Neither is fat.
- The 3,500 rule is linear and your body is not. It fits the first weeks well and overstates cumulative loss over months.
Minimum intake is not a suggestion
This calculator flags intakes below 1,200 kcal a day for women and 1,500 for men, the figures widely used for unsupervised weight loss. Below those levels it becomes difficult to meet protein and micronutrient requirements from food, and very-low-calorie diets are conducted under medical supervision for that reason. If your target rate pushes you under the floor, either lengthen the timeline or raise expenditure — do not simply eat less.
Where this fits with the rest of the arithmetic
This calculator answers one question: how large a gap does a given rate require. Three neighbouring questions need different tools.
What is my maintenance in the first place? That is total daily energy expenditure, and it is the input with the most leverage over everything downstream. Work it out with the TDEE calculator, which multiplies your basal metabolic rate by an activity factor.
When will I get there? A goal date needs the deficit applied repeatedly against a falling body weight, which is what the weight loss timeline calculator does. It will give you a later date than dividing total loss by weekly rate, and that later date is the more realistic one.
What do I eat inside that budget? Once you have an intake number, the split between protein, carbohydrate and fat determines how much of the loss is fat rather than muscle. The macro split calculator turns the calorie target into grams.
One last point on framing. A deficit is a rate, not a debt. Missing the target by 200 kcal on one day does not have to be repaid on the next; what matters is the average over weeks. People who track weekly totals rather than daily ones tend to hold a deficit longer, and duration beats depth for total fat lost.