What is BMR and why does it matter?

BMR (Basal Metabolic Rate) is the number of calories your body needs just to sustain basic functions — breathing, circulation, cell repair — at complete rest. It's the floor of your calorie needs. Everyone burns at least their BMR every day regardless of activity. The Mifflin-St Jeor equation calculates BMR from your weight, height, age, and sex. It's more accurate than the older Harris-Benedict formula and is the standard used in most clinical and research settings.

How accurate is the 3,500 calories per pound rule?

It's a useful approximation, not a precise law. The actual number varies based on your body composition, metabolic adaptation, and the source of the deficit. In practice, most people lose slightly less than predicted because metabolism adapts downward as you lose weight — your TDEE decreases as you get lighter. Recalculating your TDEE every 10–15 pounds of weight loss keeps your target accurate. The rule is accurate enough for planning purposes.

Why is 1,200 calories a floor?

Below approximately 1,200 calories, it becomes very difficult to get adequate protein, vitamins, and minerals. Severe deficits also trigger metabolic adaptation — your body reduces TDEE to compensate — and accelerate muscle loss. Very low calorie diets (under 800 calories) exist as medical interventions but require supervision. For most people, aggressive deficits are counterproductive: they're harder to sustain, cause more muscle loss, and often lead to rebound weight gain.

How often should I recalculate?

Recalculate every 10–15 pounds of weight loss. As your weight decreases, your BMR and TDEE decrease too — meaning the same calorie intake produces a smaller deficit over time. This is the most common reason people plateau after initial success. The calculator uses your current weight, not your goal weight, so updating it as you progress keeps the target accurate.

Does exercise count toward the deficit?

Yes — but it's already factored in through your activity multiplier. When you select 'moderately active (3–5 days/week)', that raises your TDEE to account for those calories burned, which gives you more to eat while maintaining the same deficit. Avoid double-counting by not selecting a higher activity level than your actual average. If you have a particularly intense week, don't increase your food intake assuming you burned extra — calorie burn estimates from exercise are notoriously imprecise.

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Calorie Deficit Calculator

How big a calorie deficit to lose weight?

What This Does

Weight loss has one non-negotiable requirement: consuming fewer calories than you burn. Everything else — what you eat, when you eat, which diet you follow — is a strategy for how to achieve that deficit. This calculator tells you the number. It uses the Mifflin-St Jeor equation, the most accurate BMR formula for most people, to estimate your Basal Metabolic Rate — the calories your body burns at rest. That gets multiplied by your activity factor to calculate your Total Daily Energy Expenditure (TDEE), which is your maintenance calorie level. Subtracting your chosen deficit from TDEE gives your daily calorie target. One pound of fat is approximately 3,500 calories. A 500-calorie daily deficit produces roughly 1 pound of weight loss per week — the rate most research supports as sustainable without significant muscle loss or metabolic adaptation. More aggressive deficits are possible but come with trade-offs. This calculator applies a floor of 1,200 calories for women and 1,500 for men (represented as 1,200 here) — going below these levels is counterproductive for most people and not recommended without medical supervision. The goal date projection gives you a realistic timeline. Most people find that a concrete date is more motivating than an abstract target weight.

When Should You Use This?

→Starting a weight loss effort — get a specific daily calorie number, not generic advice
→Stalled on a diet — your TDEE changes as you lose weight, recalculate every 10–15 lbs
→Comparing different loss rates — see the timeline difference between 0.5, 1, and 1.5 lbs/week
→Checking whether a diet plan's calorie target is appropriate for your size and activity level
→Setting a realistic goal date before an event or deadline

Example Scenario

Rachel is 34, female, 5'6" (66 inches), 172 pounds, moderately active. She wants to reach 148 pounds. Her BMR is 1,582 calories; TDEE at her activity level is 2,452. At a 1 lb/week loss rate, her daily target is 1,952 calories — a 500-calorie daily deficit. The calculator projects she'll reach her goal in 24 weeks — about 6 months — with a specific date. She also runs it at 1.5 lbs/week: target drops to 1,702 calories and goal date shrinks to 16 weeks. She decides the middle path — tracking at 1,952 — is sustainable without feeling restrictive.

Calorie Deficit Calculator

Daily Target · BMR/TDEE · Weight Timeline · Rate & Activity Comparison

Results update in real time as you adjust any input.

Sex

Age

Current Weight (lbs)

Height (inches)

5′10″ = 70 · 6′0″ = 72 · 5′6″ = 66

Goal Weight (lbs)

Loss Rate

Activity Level

About This Calculator

This calorie deficit calculator uses the Mifflin-St Jeor equation (the most validated BMR formula for adults) to compute Basal Metabolic Rate from weight, height, age, and sex. TDEE is computed as BMR × activity factor (5 levels from sedentary to extra active). Daily calorie target = TDEE − (loss rate × 3,500 ÷ 7). Safety floors of 1,500 cal (male) and 1,200 cal (female) are applied if the calculated target would fall below these thresholds. All results update in real time. Metric and imperial units are fully supported.

The Timeline tab renders a line chart of projected weight from current to goal weight (week by week, smoothed to up to 26 data points), with a `ReferenceLine` at the goal weight and a month-by-month milestones table with a progress bar for each checkpoint. An energy balance horizontal bar chart (BMR, TDEE, Target) is always visible above the tabs. The Rates tab shows a bar chart of weeks to goal at each of the 4 loss rates (0.5, 1, 1.5, 2 lbs/week), colour-coded by muscle loss risk, and a second bar chart of daily calorie targets at each rate with a TDEE reference line. The Activity tab shows a grouped bar chart (TDEE vs calorie target) across all 5 activity levels, with current level highlighted.

Plan score (0–100) rewards conservative loss rates (35 points), higher activity levels (25 points), and absence of calorie floor application (20 points), plus a 20-point base. Dynamic accent: emerald (Sustainable Plan ≥70), amber (Moderate Plan ≥45), red (Aggressive — Review). Two risk flags auto-trigger: the calorie floor warning (when the requested rate would drop below 1,200/1,500 cal) and the aggressive rate warning (at 1.5+ lbs/week). All four insights and four What To Do Next steps adapt to three plan tiers (sustainable, moderate, aggressive).

Results are estimates only and do not constitute medical advice. Consult a qualified healthcare professional before making health decisions.