Calorie Deficit Calculator
Estimate how long it takes to reach a goal weight at a given daily calorie deficit, with the weekly rate of loss and the total energy that has to come from fat.
How to use this calculator
- 1Enter your current weight and your goal weight in the same units.
- 2Enter the daily calorie deficit you plan to maintain.
- 3Read the estimated time to reach your goal and the weekly rate of loss.
- 4Check that the rate stays within a sustainable 0.5–1% of body weight per week.
How it works
Calorie deficit and weight loss
energy to lose = weight to lose × 3,500 kcal/lb (7,700 kcal/kg) days to goal = total energy ÷ daily deficit weekly loss = daily deficit × 7 ÷ 3,500 (or ÷ 7,700 for kg) a 500 kcal/day deficit ≈ 1 lb per week
Weight loss, at its simplest, follows energy balance: to lose stored fat you must consume fewer calories than you burn, creating a deficit that the body makes up by drawing on its reserves. A pound of body fat holds roughly 3,500 kilocalories of energy, so accumulating a 3,500-kcal deficit corresponds, in this idealised model, to losing about a pound. Dividing the total energy in the weight you want to lose by your daily deficit gives the number of days it should take. A 500-kcal daily deficit works out to about a pound a week, which is the classic guideline. The model is a helpful planning tool, but it treats the body as a simple calorie bank, which it is not — metabolic adaptation means the real rate slows as you go.
Worked example
To go from 180 lb to 160 lb is 20 lb, or about 20 × 3,500 = 70,000 kcal that must come from fat. At a 500-kcal daily deficit that is 140 days, or roughly 20 weeks, losing about a pound a week — a rate of just over half a percent of body weight weekly, which is squarely in the sustainable range.
Calorie Deficit Calculator: the complete guide
Energy balance, the foundation of weight change
Weight change is governed by energy balance: take in more energy than you expend and the surplus is stored, mostly as fat; take in less and the body makes up the shortfall by burning stored energy. A calorie deficit is simply the amount by which your intake falls below your expenditure each day. Sustained over time, that daily gap adds up to the total energy the body must pull from its reserves, which shows up on the scale as weight lost. This is the fundamental principle behind every diet, however it is dressed up.
The 3,500-calories-per-pound figure comes from the energy density of body fat and gives the model its arithmetic. It lets you convert a daily deficit into a rate of loss and a timeline, which is genuinely useful for setting expectations. Someone maintaining a 500-calorie daily deficit can reasonably expect to lose in the region of a pound a week, and planning around that keeps goals realistic. The trap is treating the number as a precise law rather than a first approximation.
Why real weight loss is slower than the maths
The simple calorie model consistently overpredicts weight loss over the long term, and the main reason is metabolic adaptation. As you lose weight, your body becomes smaller and needs fewer calories to run, so the same food intake produces a shrinking deficit. Beyond that, the body actively defends its weight: prolonged dieting can lower the metabolic rate more than size alone would predict, and it ramps up hunger hormones. The deficit you started with quietly erodes, and the straight-line prediction pulls ahead of reality.
This is not a reason to distrust the approach, but to expect the curve to flatten. Plateaus are normal and usually mean the deficit has narrowed as your maintenance needs fell — the fix is to recalculate your calorie needs at your new, lower weight rather than to despair. The honest takeaway is that the timeline from this calculator is a best case that assumes a constant deficit; real journeys take somewhat longer and are rarely linear. Building that expectation in prevents the discouragement that derails many attempts.
Choosing a safe and sustainable rate
It is tempting to chase the fastest possible loss with a huge deficit, but aggressive dieting backfires. Losing weight too quickly sheds more muscle alongside fat, lowers metabolism further, and is very hard to sustain, which sets up the rebound that follows most crash diets. Health authorities generally recommend losing about 0.5 to 1 kilogram (roughly 1 to 2 pounds) per week, corresponding to a daily deficit somewhere in the 500 to 1,000 calorie range, adjusted for body size.
A useful way to keep the rate sensible is to think in percentage terms: aiming for around 0.5 to 1 percent of body weight per week scales the target to the person, so a heavier individual can safely lose more in absolute terms than a lighter one. Preserving muscle also matters, and that means pairing the deficit with adequate protein and resistance exercise, which the pure energy-balance model ignores entirely. A moderate, sustained deficit that you can actually live with beats an extreme one you abandon — consistency over weeks and months is what produces lasting results.
Frequently asked questions
How big a calorie deficit do I need to lose weight?
A deficit of about 500 calories a day yields roughly a pound of loss per week, since a pound of fat stores about 3,500 calories. A 1,000-calorie deficit doubles that but is an aggressive upper limit. Sustainable loss usually comes from a 500–1,000 calorie daily deficit.
How long will it take to lose the weight?
Divide the total energy in the weight you want to lose (about 3,500 kcal per pound) by your daily deficit. Losing 20 lb at a 500-calorie deficit is roughly 140 days, or 20 weeks. Real loss is usually a bit slower because metabolism falls as you shrink.
Why am I losing weight slower than calculated?
Because of metabolic adaptation. As you lose weight your body needs fewer calories, so a fixed food intake creates a smaller deficit over time, and dieting can lower metabolism further. The simple formula assumes a constant deficit, so real loss lags the prediction and plateaus are normal.
What is a safe rate of weight loss?
About 0.5 to 1% of body weight per week, or roughly 1 to 2 pounds, from a 500–1,000 calorie daily deficit. Faster loss risks shedding muscle, slowing metabolism, and rebounding. Pairing the deficit with enough protein and resistance training helps preserve muscle.