What this calculator does
A change in body weight requires a sustained gap between energy in and energy out. Around 7,700 kilocalories corresponds to roughly a kilogram of body tissue, so dividing the total gap needed by the daily gap gives an approximate timeline.
The arithmetic is simple; the biology is not. Real weight changes are not linear: water shifts mask the first few weeks, energy requirements fall as body size falls, and adherence varies. Treat the result as a rough horizon rather than a schedule, and expect the later stages to take longer than the early ones.
The formula
The total energy gap is the weight change multiplied by roughly 7,700 kcal per kilogram. Dividing by the daily gap gives the number of days at that rate, assuming the gap is maintained consistently and energy requirements stay constant: the second assumption is the one that weakens over time.
| Term | Meaning |
|---|---|
| Deficit | The daily gap between energy used and energy consumed. |
| 7,700 kcal | The approximate energy equivalent of one kilogram of body tissue. |
| Weekly rate | Daily gap × 7 ÷ 7,700, in kilograms per week. |
The inputs explained
| Field | What to enter |
|---|---|
| Current weight (kg) | Your current weight in kilograms. |
| Goal weight (kg) | The weight you are working toward. |
| Daily calorie deficit (kcal) | The daily calorie gap you intend to sustain. Gradual, sustainable gaps of 250 to 500 kcal are generally more effective over time than larger ones. |
When to use it
Setting a realistic horizon
Most people substantially underestimate how long a change takes. Seeing the timeline in weeks rather than days tends to make the plan more sustainable, because it sets expectations that reality can actually meet.
Comparing a moderate and a large gap
A larger deficit shortens the timeline on paper, but adherence usually falls as the gap grows. A moderate deficit sustained for six months typically beats an aggressive one abandoned after five weeks.
Understanding why progress slows
As body weight falls, so does the energy needed to maintain it. A fixed intake therefore produces a shrinking deficit. Recalculating maintenance every few kilograms keeps the plan grounded.
Planning a gradual gain
The same arithmetic works in reverse. A modest surplus supports gradual gain; larger surpluses mostly add fat rather than lean tissue.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How long a 10 kg change takes at different daily gaps
A 10 kilogram change at a range of sustained daily calorie gaps.
| Daily gap | Time to reach goal | Change per week | Total energy gap |
|---|---|---|---|
| 200 kcal | 385 days (55.0 weeks) | 0.18 kg | 77,000 kcal |
| 300 kcal | 257 days (36.7 weeks) | 0.27 kg | 77,000 kcal |
| 400 kcal | 193 days (27.5 weeks) | 0.36 kg | 77,000 kcal |
| 500 kcal | 154 days (22.0 weeks) | 0.45 kg | 77,000 kcal |
| 600 kcal | 128 days (18.3 weeks) | 0.55 kg | 77,000 kcal |
| 750 kcal | 103 days (14.7 weeks) | 0.68 kg | 77,000 kcal |
Different amounts of change at a 500 kcal gap
A steady 500 kcal daily gap, applied to different target weights.
| Goal weight | Time to reach goal | Change per week | Total energy gap |
|---|---|---|---|
| 82 kg | 46 days (6.6 weeks) | 0.45 kg | 23,100 kcal |
| 80 kg | 77 days (11.0 weeks) | 0.45 kg | 38,500 kcal |
| 77 kg | 123 days (17.6 weeks) | 0.45 kg | 61,600 kcal |
| 75 kg | 154 days (22.0 weeks) | 0.45 kg | 77,000 kcal |
| 70 kg | 231 days (33.0 weeks) | 0.45 kg | 115,500 kcal |
| 65 kg | 308 days (44.0 weeks) | 0.45 kg | 154,000 kcal |
Questions
Is the 7,700 kcal per kilogram figure reliable?
It is a widely used approximation based on the energy density of body tissue. Real changes involve water, glycogen and lean tissue as well as fat, so early weeks often show faster movement than the arithmetic predicts and later weeks slower.
What rate of change is sensible?
Gradual change of roughly a quarter to half a kilogram a week is commonly recommended and is generally easier to sustain while preserving muscle. What suits you depends on your circumstances, and a doctor or accredited dietitian can advise properly.
Why is my actual progress slower than this?
Two common reasons. Energy requirements fall as body weight falls, so a fixed intake creates a shrinking deficit. And sustained adherence to an exact daily gap is genuinely difficult: well-documented, and not a personal failing.
Does exercise change the timeline?
Exercise adds to the daily gap, so it can shorten it. But appetite often rises to partly compensate, and the energy cost of exercise is usually smaller than people expect. Its main benefits, fitness, muscle retention, mood, health markers, sit outside this calculation.
Should I be doing this at all?
That depends entirely on your circumstances, and it is a question for a doctor rather than a calculator. If your relationship with food or your body feels difficult, that is worth raising with a health professional: support is available and effective.
To find the intake that creates the gap, use the TDEE calculator. For a body composition measure, see body fat percentage.