How to Calculate Your Calorie Deficit (Step-by-Step)

Sophie Laurent · June 16, 2026

To calculate your calorie deficit, you need two numbers: your Total Daily Energy Expenditure (TDEE) and your chosen daily calorie target. Your deficit is the gap between them. This guide walks through each step of the calculation, with worked examples using the Mifflin-St Jeor equation — the formula validated in the peer-reviewed literature as the most accurate for estimating resting metabolic rate in healthy adults [1][2].

Step 1 — Calculate your BMR (Basal Metabolic Rate)

Basal Metabolic Rate (BMR) is the energy your body needs to keep you alive at complete rest — breathing, circulating blood, maintaining body temperature, and running basic cellular processes. Technically, BMR is measured in a fasted state, lying down, in a thermoneutral room, immediately after waking. In practice, almost no one measures it under those exact conditions. What every consumer calculator — including this one — actually estimates is Resting Metabolic Rate (RMR), a closely related figure measured under less strict conditions. The terms are used interchangeably in most fitness contexts, and the distinction rarely changes the practical number you work with.

The formula used throughout this guide is the Mifflin-St Jeor equation, published in 1990 and still the most widely validated predictive equation for RMR in healthy adults [1]:

Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5

Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161

Worked example: a 30-year-old woman, 68 kg, 165 cm tall.

BMR = (10 × 68) + (6.25 × 165) − (5 × 30) − 161 BMR = 680 + 1,031.25 − 150 − 161 BMR = 1,400.25 kcal, rounded to 1,400 kcal/day

A 2005 systematic review comparing predictive equations against measured RMR found that Mifflin-St Jeor produced the smallest average error across both non-obese and obese populations, outperforming the older Harris-Benedict equation (developed in 1919 and revised in 1984) in most subgroups [2]. That’s why it’s the default formula in this guide and in the NoGymLab Calorie Deficit Calculator — it isn’t the only equation available, but it’s the one with the strongest evidence behind it for the general population.

Step 2 — Apply your activity multiplier to get TDEE

BMR only accounts for the energy you’d burn lying still all day. To estimate your real Total Daily Energy Expenditure, multiply your BMR by an activity factor that reflects how much you move — both structured exercise and everyday activity (walking, standing, fidgeting, chores).

TDEE = BMR × activity multiplier

Activity level	Definition	Multiplier
Sedentary	Desk job, little or no exercise	× 1.2
Lightly active	Light exercise 1–3 days/week	× 1.375
Moderately active	Moderate exercise 3–5 days/week	× 1.55
Very active	Hard exercise 6–7 days/week	× 1.725
Extremely active	Physical job plus daily hard training	× 1.9

Worked example continued: our 30-year-old woman trains lightly, 1–3 sessions per week.

TDEE = 1,400 × 1.375 = 1,925 kcal/day

This is the step where most manual calculations go wrong. A 2012 measurement-error analysis of self-reported physical activity questionnaires found that people systematically overestimate their own activity level relative to objectively measured movement — sometimes substantially [5]. If you’re unsure whether you’re “lightly” or “moderately” active, choose the lower multiplier. An inflated TDEE leads to an inflated calorie target, which means a smaller real-world deficit than you think you’re running. The fix is simple: pick the conservative option, track your actual weight trend for 2–3 weeks, and adjust the multiplier up only if the data says you were too conservative.

Step 3 — Choose your deficit size

Once you have TDEE, you need to decide how much of a gap to create below it. Our companion guide on what a calorie deficit is and how big yours should be covers the evidence in depth — the short version is that 300–500 kcal/day is the most extensively studied sustainable range for most people.

Deficit size	Expected weekly loss	Muscle retention risk
~250 kcal/day	~0.25 kg/week	Very low
~500 kcal/day	~0.5 kg/week	Low, with adequate protein
~750 kcal/day	~0.75 kg/week	Moderate — requires careful protein management
~1,000 kcal/day	~1.0 kg/week	High — appropriate only short-term

It’s worth retiring the old “3,500 kcal = 1 lb of fat” rule of thumb here. That conversion assumes a fixed, linear relationship between calories and weight change, but a 2013 modelling study showed it consistently overpredicts real-world weight loss, particularly over periods longer than a few weeks, because it ignores metabolic adaptation and shifts in body composition as the deficit progresses [4]. The same dynamic is described in the broader energy-balance literature: as you lose weight, your TDEE drops too, so a fixed deficit doesn’t keep producing the same rate of loss indefinitely [3][6].

Step 4 — Set your daily calorie target

With TDEE and your chosen deficit size in hand, the final calculation is simple subtraction:

Daily calorie target = TDEE − deficit

Worked example concluded: TDEE of 1,925 kcal, with a 500 kcal/day deficit.

Daily target = 1,925 − 500 = 1,425 kcal/day

Before locking that number in, check it against the safety floors that apply regardless of how the math works out: never go below 1,200 kcal/day for women or 1,500 kcal/day for men. Below these thresholds it becomes very difficult to meet protein and micronutrient needs simultaneously, even with careful food choices. If your calculated target falls under the floor, reduce the deficit size rather than the floor.

This number is a starting point, not a permanent fixture. As your body weight changes, your TDEE changes with it, and the same fixed deficit will produce smaller real-world results over time [6] — which is why the target needs to be revisited periodically rather than set once and forgotten.

Why manual calculation has limits

Every number above is an estimate, not a measurement. The Mifflin-St Jeor equation predicts BMR within roughly ±10% for most people — accurate enough to be useful, but not a lab-verified figure for any one individual [2]. Activity multipliers compress an enormous range of real-world movement into five buckets, and self-reported activity level is one of the most consistently overestimated inputs in the entire calculation [5]. On top of that, packaged food labels carry their own legally permitted error margins — typically up to ±20% in both the US and EU — which compounds with any imprecision in your logging.

None of this means the calculation is useless. It means the result should be treated as a working hypothesis, not a fixed prescription. The right way to use a calculated calorie target is to apply it consistently for 2–3 weeks, watch what your actual weight trend does, and adjust the number based on results rather than recalculating from scratch every time you feel uncertain.

Rather than doing this arithmetic by hand, you can run the same Mifflin-St Jeor calculation automatically with the NoGymLab Calorie Deficit Calculator — it applies the activity adjustment and checks your result against the safety floor in one step.

Worked example — full calculation from start to finish

Here’s the entire process applied to one person, start to finish.

Sophie, 34, female, 72 kg, 168 cm, moderately active (gym 3 days/week), goal: 0.5 kg/week loss.

1. BMR (Mifflin-St Jeor, women): BMR = (10 × 72) + (6.25 × 168) − (5 × 34) − 161 BMR = 720 + 1,050 − 170 − 161 = 1,439 kcal

2. TDEE (moderately active, ×1.55): TDEE = 1,439 × 1.55 = 2,230 kcal/day

3. Deficit for a 0.5 kg/week goal: 500 kcal/day

4. Daily calorie target: 2,230 − 500 = 1,730 kcal/day

5. Protein target, to protect lean mass during the deficit [7][8]: 72 kg × 1.8 g/kg = 130 g of protein per day

Protein intake matters as much as the calorie number itself during a sustained deficit — the position stand from the International Society of Sports Nutrition recommends 1.6–2.2 g/kg body weight for most people actively trying to lose fat while preserving muscle [7][8]. A dedicated protein intake calculator is in development for a future NoGymLab release; until then, multiplying your body weight in kg by 1.6–2.2 gives a solid working range.

How to track and adjust

A calculated target is only useful if you can tell whether it’s working. A simple tracking routine:

• Weigh yourself daily, ideally at the same time under similar conditions, and use a 7-day rolling average rather than any single reading — day-to-day fluctuations from water, sodium, and digestion are normal and not meaningful on their own.
• If you’re losing faster than 0.5 kg/week and that wasn’t the goal, increase your target by 100–150 kcal.
• If the average isn’t moving after 2–3 weeks of consistent adherence, reduce your target by 100–150 kcal, or audit your logging accuracy first — under-reporting is extremely common and often explains a stalled deficit better than metabolism does.
• Recalculate your BMR and TDEE every 4–5 kg lost. As body weight drops, so does TDEE, and the metabolic adaptation documented in dieting research means the same fixed deficit produces progressively smaller results the longer it’s maintained [6].
• Consider a diet break — 1–2 weeks back at maintenance intake every 8–12 weeks — which the metabolic-adaptation literature suggests can help partially restore hormonal and metabolic markers that drift during extended restriction, while also supporting longer-term adherence [6].

Use the calculator

The calculation above takes a few minutes by hand and carries several independent sources of error — BMR estimation, activity multiplier selection, and rounding. The NoGymLab Calorie Deficit Calculator runs the same Mifflin-St Jeor formula in under a minute, applies the activity adjustment automatically, checks your result against the safety floor, and gives you a personalised target you can save and revisit as your weight changes.

For the full formula documentation behind every NoGymLab tool, see our methodology.

Frequently asked questions

What is the formula for calculating a calorie deficit?

Your calorie deficit equals your Total Daily Energy Expenditure (TDEE) minus your daily calorie target. First calculate your BMR with the Mifflin-St Jeor equation, multiply it by an activity factor to get TDEE, then subtract 250–500 kcal to set your target. The gap between TDEE and your target is your deficit.

How accurate is the Mifflin-St Jeor equation?

Mifflin-St Jeor estimates resting metabolic rate within roughly ±10% for most healthy adults and consistently outperforms the older Harris-Benedict equation, especially for non-obese individuals [1][2]. No equation is exact — it gives a reliable starting estimate, not a measured value.

What activity multiplier should I use?

Choose from five levels: sedentary (×1.2), lightly active (×1.375), moderately active (×1.55), very active (×1.725), and extremely active (×1.9). Most people overestimate their own activity level [5], so when in doubt, pick the lower of two plausible options and adjust after 2–3 weeks of real results.

How do I know if my calorie target is working?

Track your weight daily and compare weekly averages rather than single readings, which are noisy day to day. A working calorie target produces roughly 0.25–0.5 kg of loss per week. If the average isn’t moving after two to three weeks, adjust your target down by 100–150 kcal.

Can I calculate my calorie deficit without weighing food?

Yes. Apps like MyFitnessPal estimate portions from descriptions, and visual portion guides (palm-sized protein, fist-sized vegetables, cupped-hand carbs) work reasonably well for most people. Weighing food gives more precision, but consistent estimation is enough to find and maintain a deficit.

Should I recalculate my deficit as I lose weight?

Yes. Your TDEE drops as body weight decreases, so a fixed calorie target produces a smaller deficit over time [6]. Recalculate your BMR and TDEE every 4–5 kg lost, or whenever your weekly weight-loss rate stalls for more than two to three weeks despite consistent adherence.

Sources

1. Mifflin, M. D., St Jeor, S. T., Hill, L. A., Scott, B. J., Daugherty, S. A., & Koh, Y. O. (1990). A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 51(2), 241–247. PubMed · DOI: 10.1093/ajcn/51.2.241

2. Frankenfield, D., Roth-Yousey, L., & Compher, C. (2005). Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association, 105(5), 775–789. PubMed · DOI: 10.1016/j.jada.2005.02.005

3. Hall, K. D., Sacks, G., Chandramohan, D., Chow, C. C., Wang, Y. C., Gortmaker, S. L., & Swinburn, B. A. (2011). Quantification of the effect of energy imbalance on bodyweight. Lancet, 378(9793), 826–837. PubMed · DOI: 10.1016/S0140-6736(11)60812-X

4. Thomas, D. M., Martin, C. K., Lettieri, S., Bredlau, C., Kaiser, K., Church, T., Bouchard, C., & Heymsfield, S. B. (2013). Can a weight loss of one pound per week be achieved with a 3500-kcal deficit? International Journal of Obesity, 37(12), 1611–1613. PubMed · DOI: 10.1038/ijo.2013.51

5. Tooze, J. A., Troiano, R. P., Carroll, R. J., Moshfegh, A. J., & Freedman, L. S. (2012). A measurement error model for physical activity level as measured by a questionnaire with application to the 1999–2006 NHANES questionnaire. American Journal of Epidemiology, 175(2), 111–122. PubMed · DOI: 10.1093/aje/kwr280

6. Trexler, E. T., Smith-Ryan, A. E., & Norton, L. E. (2014). Metabolic adaptation to weight loss: implications for the athlete. Journal of the International Society of Sports Nutrition, 11(1), 7. PubMed · DOI: 10.1186/1550-2783-11-7

7. Helms, E. R., Aragon, A. A., & Fitschen, P. J. (2014). Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. Journal of the International Society of Sports Nutrition, 11(1), 20. PubMed · DOI: 10.1186/1550-2783-11-20

8. Aragon, A. A., Schoenfeld, B. J., Wildman, R., Kleiner, S., VanDusseldorp, T., Taylor, L., Earnest, C. P., Arciero, P. J., Wilborn, C., Kalman, D. S., Stout, J. R., Willoughby, D. S., Campbell, B., Arent, S. M., Bannock, L., Smith-Ryan, A. E., & Antonio, J. (2017). International society of sports nutrition position stand: diets and body composition. Journal of the International Society of Sports Nutrition, 14, 16. PubMed · DOI: 10.1186/s12970-017-0174-y