Calculate a daily calorie deficit, a calorie target that matches your goal, and a realistic weight-loss timeline range — built from your TDEE with an adaptive, week-by-week projection instead of one fixed date.
Adaptive timeline Planning range, not one date Safety-floor caution
Not sure about your maintenance calories? Start with the TDEE Calculator, or use “Calculate my maintenance calories” below.
Step 1 — Your maintenance calories
cm
years
Step 2 — Your goal
kg
kg
Goal method
%
5%–30%. Deficits over 20% show a caution.
Calorie-target strategy
The same calorie target every day while your estimated TDEE gradually changes.
Result
Select an equation reference to calculate the standard Mifflin-St Jeor estimate.
What is a calorie deficit?
A calorie deficit occurs when average intake is lower than average expenditure (TDEE). The difference between the two — the deficit — is what this calculator estimates and turns into a percentage, a pace, or a target date.
A percentage deficit (e.g. 15% below maintenance) and a fixed-number deficit (e.g. 500 calories/day) describe the same idea two ways. A percentage scales with your own maintenance calories; a fixed number doesn’t — why this calculator defaults to percentage-based presets.
Why the timeline is shown as a range
A naive calculation — kilograms to lose × 7,700 kcal, divided by the daily deficit — produces one exact-looking date. That’s false precision: energy needs change as weight changes, and real progress isn’t linear — water weight, adherence, sleep, and stress all move the actual date.
Instead, this calculator simulates progress week by week, recalculating BMR and TDEE from your projected weight at each step, then runs the simulation three times — varying the effective deficit by roughly ±10% — to produce a faster and slower bound around the central estimate. The range is wider than the simple arithmetic estimate on purpose: it’s honest about uncertainty rather than falsely precise.
Fixed vs. adaptive calorie targets
Fixed calorie intake keeps the same daily number throughout the plan — simpler to follow, and the default here. But because TDEE falls as weight drops, a fixed intake becomes a gradually shrinking deficit, which is why weight loss commonly slows down even without changing anything.
Adaptive calorie target recalculates periodically to hold roughly the same percentage deficit as your estimated TDEE changes — closer to a constant relative effort, but needs reassessing more often.
Worked example
Take someone with a starting TDEE of 2,760 calories/day (from the TDEE Calculator’s worked example) who selects a 15% deficit:
Daily target = 2,760 × 0.85 ≈ 2,350 cal/day — a deficit of about 410 calories/day, or roughly 2,900 calories/week.
A simple arithmetic estimate for losing 5kg would be 5 × 7,700 ÷ 410 ≈ 94 days, or about 13.4 weeks. But because BMR and TDEE fall as weight drops, this calculator’s simulation projects a slightly longer central estimate — and shows a range around it (roughly 13–16 weeks) instead of that single figure, since the naive division assumes a constant deficit a fixed intake can’t actually sustain.
Safety floor
NIDDK diabetes-prevention guidance says eating fewer than 1200 calories/day is not advised. If your target falls below this threshold, this calculator still shows the number for transparency — but marks it “Below the general calculator safety threshold” rather than a normal result, and does not project weeks below that threshold. This is a general lower-bound warning, not a personalized prescription: requirements vary by individual, and some people need substantially more. If your target falls this low, consider a gentler deficit and speak with a qualified clinician or registered dietitian.
Protecting nutrition while losing weight
A calorie deficit is about energy balance, not nutrient quality — the two aren’t the same. Adequate protein and resistance exercise may help maintain lean tissue (muscle) while in a deficit, rather than losing muscle and fat equally. Larger deficits leave less room for the vitamins, minerals, and protein the body needs — one reason very large or fast deficits are harder to sustain.
This calculator must not be used to start, stop, or adjust any medication. If you take a prescription weight-management medication, follow your clinician’s instructions for nutrition and calorie intake — this tool does not model how such medications affect appetite, energy expenditure, or a safe calorie floor.
Limitations
This is a simplified planning model — it cannot reproduce individual metabolic adaptation, water-weight fluctuations, body-composition changes (fat vs. muscle), or the NIH Body Weight Planner’s more detailed physiological model.
The 7,700 kcal/kg (3,500 kcal/lb) energy-equivalent is a widely used planning approximation, not an exact physical constant for every person.
“Method B” (already knowing maintenance calories) holds that number constant throughout, since it doesn’t collect the age/height/equation inputs needed to recalculate as weight changes — use “Calculate my maintenance calories” for the full adaptive projection.
This tool may not be suitable without professional guidance for people under 18, during pregnancy or breastfeeding, with an eating-disorder history, for competitive athletes, for diabetes or metabolic disease, for kidney, liver, or heart conditions, when recovering from surgery or illness, for anyone on prescription weight-management medication, or for anyone on a medically supervised diet.
Sources and methodology
Maintenance calories: the Mifflin-St Jeor BMR/TDEE engine shared with the TDEE Calculator (see its Sources section for the citation). Energy-equivalent approximation: 7,700 kcal/kg (3,500 kcal/lb), a widely used planning approximation, not an exact constant. Pace and safety guidance: informed by CDC and NIDDK guidance, cited below.
This calculator’s simplified adaptive model is an independent implementation and is not the NIH Body Weight Planner and not affiliated with or endorsed by NIDDK, CDC, or NIH/NHLBI.
A calorie deficit occurs when intake is lower than expenditure (TDEE), so the body draws on stored energy — associated with weight loss over time. A surplus is the reverse, associated with weight gain.
How large should a calorie deficit be?
This calculator offers gentle (10%), moderate (15%), and larger (20%) presets, plus a custom 5–30% range. CDC guidance describes roughly 1–2 lb (0.5–1 kg) per week as more sustainable for many adults — general guidance, not a prescription; a deficit above 20% shows a caution here.
How do I calculate a 10%, 15% or 20% deficit?
Multiply maintenance calories by (1 − the percentage): at 2,760 calories, a 15% deficit is 2,760 × 0.85 ≈ 2,350 calories/day. This calculator does the arithmetic and lets you pick a preset or custom percentage.
How long will it take to reach my target weight?
This calculator runs a week-by-week simulation — not a single division — recalculating BMR and TDEE as weight changes, then shows a faster/slower planning range rather than one exact date, because real timelines vary with adherence, water weight, and metabolism.
Why is the timeline shown as a range?
A single-number timeline implies more precision than the model can support. This calculator varies the effective deficit by about ±10% to produce a faster/slower bound around the central estimate — a planning range, not a medical confidence interval.
Is a 1,000-calorie deficit too large?
It can be, depending on your maintenance calories and body size — 1,000 calories is a much larger percentage cut at lower maintenance calories than at higher ones. Check your daily target against the safety-floor caution below, and consider a gentler deficit or professional guidance for very large cuts.
Why did weight loss slow down?
This is expected, not a sign anything is wrong. As weight drops, BMR and TDEE fall too, so a fixed calorie intake represents a shrinking deficit over time — one reason many plans reassess and adjust every few weeks.
Should calorie targets change as weight decreases?
It depends on the strategy. Fixed intake keeps the same daily number throughout, so the effective deficit narrows as TDEE falls; adaptive recalculates periodically to hold roughly the same percentage deficit. Neither is inherently "right" — fixed is simpler, adaptive stays closer to a constant relative deficit.
Can I use this calculator with a weight-loss medication?
This calculator must not be used to start, stop, or adjust any medication. If you use a prescription weight-management medication, follow your clinician's instructions for nutrition and calorie intake — this tool does not account for how such medications affect appetite, energy expenditure, or safe calorie floors.
What happens when the calculated intake is below 1,200 calories?
This calculator still shows the mathematical result for transparency, but marks it "Below the general calculator safety threshold" rather than a normal target. NIDDK guidance says eating fewer than 1200 calories/day is not advised — a general lower-bound warning, not a personalized prescription. Speak with a qualified clinician or registered dietitian before following an intake this low.
Is this calculator suitable for gaining weight?
Yes — enter a target weight above your current weight and choose the target-date or custom-calorie-target method. The projection and planning-range logic work the same way for a gaining goal, just with a surplus instead of a deficit.
Health & medical disclaimer
This calculator provides an educational estimate, not medical advice.
Actual energy needs and weight changes vary by age, sex, body composition, medical conditions, activity level, and professional guidance.
It is not intended for people under 18, and may not be suitable without professional guidance during pregnancy or breastfeeding,
for a history of an eating disorder, for competitive athletes, for diabetes or metabolic disease, for kidney, liver,
or heart conditions, for recovery from surgery or illness, for anyone using prescription weight-management medication, or for anyone advised to follow a medically supervised diet.
This calculator must not be used to start, stop, or adjust any medication.