Nutrition

BMR Calculator

Estimate basal metabolic rate and maintenance calories using Mifflin-St Jeor assumptions and activity scaling.

Decision Summary

Estimated maintenance calories

2,709 kcal

Mifflin-St Jeor BMR scaled by selected activity factor.

Outcome Comparison

Primary result and the most important supporting metrics.

Estimated maintenance calories

2,709 kcal

BMR

1,748 kcal

Fat-loss target

2,209 kcal

Lean-gain target

2,959 kcal

Key Metrics

BMR

1,748 kcal

Fat-loss target

2,209 kcal

Lean-gain target

2,959 kcal

Protein baseline

140.4 g

How to use it

Enter height, weight, age, and sex — the Mifflin-St Jeor formula requires all four. Double-check you entered height in the correct unit: 68 inches vs 68 cm produces a 30% BMR difference.
BMR is the calories your body burns at complete rest — organs, circulation, cell repair. It is your absolute floor. Eating below BMR long-term triggers metabolic adaptation and accelerates muscle loss.
BMR is not a calorie target. Multiply it by your activity factor (1.2 sedentary → 1.9 twice-daily training) to get actual daily burn. Use the TDEE Calculator for this step.
BMR drops roughly 2–3% per decade of age and decreases as you lose weight. A 20 lb loss typically reduces BMR by 100–150 calories — recalculate whenever weight changes significantly.
If your calculated BMR seems surprisingly low for your body weight, check for unit errors. A person who is 5 ft 10 in has a BMR ~15% higher than someone who is 5 ft 5 in.

AI Integrations

Contract, discovery endpoints, and developer notes for agent use.

Always available for agents

Tool contract JSON

https://aifithub.io/contracts/bmr-calculator.json

Stable input and output contract for this exact tool.

Human review

People can use the browser page to sense-check outputs and charts, but agents should still execute against the contract and discovery endpoints.

/agent-tools.json - machine-readable tool index.
/llms.txt - human-readable model discovery guide.
/.well-known/webmcp.json - WebMCP capabilities manifest.
/.well-known/ai-plugin.json - plugin-style discovery manifest.

{
  "tool": "bmr_calculator",
  "sex": "male",
  "age": 30,
  "weight_kg": 78,
  "height_cm": 178,
  "activity_level": "moderate"
}

Open tool contract JSON Open AI integrations guide

Expand developer notes

Agent playbook

Resolve BMR Calculator from /agent-tools.json and open its contract before execution.
Validate inputs against the contract schema instead of scraping labels from the page UI.
Open the browser page only when a person wants to review charts, assumptions, or related tools.

Agent FAQ

Should ChatGPT, Claude, or another agent click through the UI?

No. Start with /agent-tools.json, then follow the tool's contract URL. The page UI is for human review, not parameter discovery.

When do tools show Quick and Advanced?

Every tool opens in Quick Start first. Advanced Controls keeps the same scenario, reveals more assumptions or diagnostics, and every tool keeps AI integrations inline below the instructions.

When should an agent still open the browser page?

Open it when a human wants to sense-check the output, review the chart, or keep exploring related tools after the calculation finishes.

Questions people usually ask

What is BMR and why does it matter?

Basal Metabolic Rate is the minimum calories your body needs to sustain vital functions at complete rest — breathing, circulation, cell repair. It accounts for 60-75% of total daily calorie burn for most people, making it the largest component of your energy budget.

Which BMR formula is most accurate?

The Mifflin-St Jeor formula (used here) outperforms the older Harris-Benedict formula in most studies, with mean error under 10% vs measured metabolic rate. The Katch-McArdle formula is more accurate if you know your lean body mass, since it accounts for muscle mass directly.

Does muscle mass affect BMR?

Yes significantly. Skeletal muscle burns approximately 6 calories per pound per day at rest, compared to 2 calories per pound for fat. A person with 140 lb of lean mass has a meaningfully higher BMR than someone of identical weight with 115 lb of lean mass.

What happens to BMR during a calorie deficit?

BMR decreases. Severe restriction (below 25% of TDEE) can trigger adaptive thermogenesis — metabolic adaptation that reduces BMR by 5-15% beyond what weight loss alone predicts. This is why gradual deficits and diet breaks help long-term fat loss.

Can I increase my BMR?

Yes. Building muscle through resistance training is the most effective method — each pound of muscle raises daily BMR by approximately 6 calories. Adequate protein intake (0.7-1g per lb of body weight) also prevents BMR-reducing muscle loss during weight loss.

Related Resources

Learn the decision before you act

Every link here is tied directly to BMR Calculator. Use the explanation, formula, examples, and benchmarks to pressure-test the calculator output from first principles.

Browse all 13 resources

Learn · 2 Formulas · 1 Examples · 1 How To Use · 9

How To Use

5 STEPS

How to Use BMR Calculator

reveal your body's baseline calorie needs with our BMR calculator. Learn to accurately input data, interpret results, and strategize weight loss, gain, or maintenance goals.

Read

How To Use

5 STEPS

How to Use Creatine Intake Calculator

Optimize your creatine supplementation with our intake calculator. Learn to determine personalized loading and maintenance dosages based on your body weight and fitness goals for enhanced performance and muscle growth.

Read

How To Use

5 STEPS

How to Use Food-to-Exercise Converter

Convert food calories into equivalent exercise minutes. Understand the physical effort needed to burn off your meals and snacks, empowering smarter dietary choices and fitness motivation.

Read

How To Use

5 STEPS

How to Use Intermittent Fasting Window Planner

Plan your ideal intermittent fasting schedule with our easy-to-use calculator. Find your perfect eating and fasting windows based on your lifestyle and goals. Optimize your IF journey.

Read

How To Use

5 STEPS

How to Use Macro Calculator

reveal your fitness potential by mastering our Macro Calculator. Customize your protein, carb, and fat intake for weight loss, muscle gain, or maintenance. Optimize your diet today!

Read

How To Use

4 STEPS

How to Use Meal Macro Split Planner

Optimize your daily nutrition! Learn how to use the AI Fit Hub's Meal Macro Split Planner to perfectly distribute your protein, carbs, and fats across your meals for any fitness goal.

Read