How a calorie deficit works
Weight loss, stripped to its arithmetic, is an energy balance: eat fewer calories than you burn, consistently, and your body covers the gap from stored energy, mostly fat. The size of that daily gap is your calorie deficit. This calculator builds yours in three steps: it estimates your BMR with the Mifflin-St Jeor equation, multiplies it by your activity level to get your maintenance calories — the intake that holds your current weight — and then subtracts the deficit you choose to give a daily target. Everything else on the page, including the timeline to your goal weight, follows from those three numbers, so the honest work is choosing a deficit you can actually live with for months, not weeks.
The formula behind the plan
The timeline uses the traditional 3,500 kcal per pound rule: a pound of body fat stores roughly 3,500 kcal, so a 500 kcal daily deficit sums to 3,500 kcal — about a pound — per week. The deficit options here run from 250 kcal (gentle, roughly half a pound a week) through 500 kcal (the standard recommendation) to 750 and 1,000 kcal, which suit larger bodies with high maintenance calories but are punishing, and often counterproductive, for smaller ones.
The honest caveat: 3,500 is a rough estimate, and loss slows
The 3,500 kcal-per-pound framing is a rough estimate, not a law, and this page would mislead you if it pretended otherwise. It assumes every calorie of deficit comes from fat at a fixed energy density and that your maintenance calories never change. Neither holds for long. As you lose weight you are a smaller body that needs fewer calories, so the same target intake becomes a shrinking deficit; some of the weight lost is lean tissue and water, which carry different energy values; and the body adapts subtly — you fidget less, move a little more economically, and burn slightly less than the equations predict. The practical result is that real loss is usually slower than the straight line, especially after the first weeks, when early water-weight changes flatter the rate. Use the weeks figure as an optimistic guide, judge progress on a four-week weight trend, and when loss stalls for several weeks, recalculate: your maintenance has moved, so your target must move with it.
Picking a deficit you can keep
The best deficit is the largest one that does not feel like a punishment, because adherence over months beats intensity over days. A 250 to 500 kcal deficit preserves energy, training performance and muscle far better than an aggressive cut, and hunger stays manageable. Protect protein and do some resistance training if you can, since the scale cannot tell fat loss from muscle loss but your strength and mirror can. Avoid very low targets: intakes under roughly 1,200 kcal a day for women or 1,500 for men are difficult to make nutritionally complete and belong under professional supervision. This calculator gives general information only and is not medical advice — if you are pregnant, breastfeeding, under 18, or have a medical condition or a history of disordered eating, talk to a qualified professional before dieting.
Worked example
A 30-year-old man, 90 kg, 175 cm, moderately active: BMR = 10 × 90 + 6.25 × 175 − 5 × 30 + 5 = 1,848.75 kcal. Maintenance = 1,848.75 × 1.55 ≈ 2,865.56 kcal. Minus a 500 kcal deficit gives a target of 2,365.56 kcal a day. To lose 10 kg (22.05 lb) at about 1 lb a week takes roughly 22.05 weeks on the simple rule.
How to use this calculator
- Select your sex and enter your age, current weight in kilograms and height in centimetres.
- Choose the activity level that honestly matches a typical week.
- Enter your goal weight in kilograms.
- Pick a daily deficit — 500 kcal is the standard starting point; 250 is gentler.
- Read your daily calorie target in the headline panel and the estimated weeks to your goal in the stats, remembering the timeline is optimistic.
What to do when the loss stalls
A stall of one or two weeks is usually water, not failure: glycogen, salt, a hard training week and hormonal cycles all move water weight by more than a week's fat loss. Judge by the trend over three to four weeks of consistent eating, weighing under similar conditions. If the trend is genuinely flat for a month, something real has changed — most often your maintenance calories, because a lighter body burns less. Rerun this calculator at your new weight; typically the target drops by 100 to 200 kcal, and loss resumes. Also audit the quiet leaks: cooking oils, drinks, weekends and portion drift are the usual suspects, and tightening tracking for two weeks answers the question honestly.
Finally, plan the end at the start. When you reach your goal weight, your new maintenance — BMR at the new weight times your activity factor — is your long-term intake, and it will be lower than the maintenance you started with. Easing the deficit away gradually, rather than returning to old habits the day the scale cooperates, is what separates a finished diet from a kept result.
Important note
For information only — not medical advice. This calculator uses standard published formulas to estimate values for general education and planning. It does not diagnose, treat, or replace the advice of a doctor or other qualified health professional. If you have a health concern, are pregnant, or are making decisions about treatment, medication, or a major diet or exercise change, please talk to a healthcare professional first.