What is a calorie deficit?
A calorie deficit means consuming less energy than your body uses over time. Your body makes up the difference from stored energy. The size of the deficit, your starting point, activity, appetite and ability to sustain the changes all influence what happens next.
This calculator estimates maintenance calories, then subtracts a choice of 300, 400 or 500 kcal per day. That range reflects the moderate deficit discussed in Richie Kirwan’s Heracles nutrition course. The default is 400 kcal; choosing a larger number does not guarantee a better result.
Build a plan you can repeat
A useful starting intake should leave room for meals you enjoy, adequate protein, dietary fat, fibre, fruit and vegetables. Resistance training can help support muscle while you lose weight. A deficit that leaves you persistently exhausted, excessively hungry or unable to recover is a reason to review the plan.
The resting energy calculation uses the rounded revised Harris–Benedict equations, followed by an activity factor. Read the TDEE explanation for the equations and how to choose your activity level. Your activity factor includes exercise; adding exercise calories again can overestimate your needs.
Why there is no exact finish date
Energy expenditure changes with body weight and activity. Fluid changes can also hide or exaggerate short-term weight changes. A fixed calorie deficit cannot reliably predict the day you will reach a goal, so this tool does not promise a weekly rate or completion date.
Track under consistent conditions and compare weekly averages over several weeks. Review your energy, hunger and strength as well as the scale. If you want a breakdown of meals and nutrients, use the macro calculator or check your protein range.
When to get an individual plan
This tool will not produce a deficit target for a BMI below 18.5 or a calculated intake below 1,500 kcal for the male equation or 1,200 kcal for the female equation. These are conservative boundaries for this self-guided tool, not proof that every intake above them is suitable.
Medical conditions, medication, pregnancy, breastfeeding or a history of disordered eating can change the right approach. If you are looking for ongoing support, explore Heracles weight-loss care.
Questions about this calculator
How large should my deficit be?
This tool offers 300–500 kcal per day as a moderate starting range from the course. The right approach depends on your needs, and a smaller sustainable change can be more useful than an aggressive target.
Will a 500-calorie deficit guarantee a fixed weekly loss?
No. The estimate is not a prediction. Energy needs adapt, and short-term weight includes fluid and digestive contents.
Why did the tool refuse to show a calorie target?
It blocks fat-loss planning below a BMI of 18.5 and does not show targets below its self-guided intake boundaries. You may need an individual nutrition plan.
Should I recalculate after losing weight?
Your energy needs may change as your body weight and activity change. Revisit the estimate and your longer-term trend rather than changing your plan in response to one day’s reading.
Method and sources
Nutrition principles adapted from Dr Richie Kirwan’s Heracles course, Optimizing Your Nutrition, Modules 1 and 3. Calculator implementation and explanations by Heracles.