ℹ️ This article is educational and not medical or fitness advice. Estimates are approximate. Consult a professional for guidance tailored to you.
The problem with “calories burned”
Every treadmill and fitness app shows a calories-burned number, but where does it come from? Your body doesn’t have a built-in calorie meter. Instead, these figures are estimates built from decades of research that measured how much oxygen people consume during different activities. The unit that captures that research is the MET, and understanding it is the key to reading any calories-burned figure with the right amount of skepticism.
The Calories Burned calculator applies this research to your own weight and workout time to produce a personalised estimate.
What a MET is
MET stands for Metabolic Equivalent of Task. One MET is defined as the rate of energy you use sitting quietly at rest — about 3.5 millilitres of oxygen per kilogram of body weight per minute, which works out to roughly 1 kcal per kilogram of body weight per hour.
Every activity is then expressed as a multiple of that resting rate:
- Sitting: 1 MET
- Slow walking: ~2.5 METs
- Brisk walking: ~3.5 METs
- Moderate cycling: ~7.5 METs
- Running: ~9.8 METs
- Jumping rope: ~11 METs
A 10-MET activity burns energy about ten times faster than sitting still. Researchers have catalogued thousands of these values in the Compendium of Physical Activities, which is the source most calculators draw on.
The formula
Knowing the MET, body weight, and duration is enough to estimate calories. The standard equation from the American College of Sports Medicine (ACSM) is:
calories per minute = MET × 3.5 × weight(kg) ÷ 200
total calories = calories per minute × duration(minutes)The 3.5 is the resting oxygen rate per kilogram, and dividing by 200 converts millilitres of oxygen into kilocalories (since roughly 5 kcal are released per litre of oxygen consumed). For a 70 kg person running (9.8 METs) for 30 minutes:
per minute = 9.8 × 3.5 × 70 ÷ 200 ≈ 12 kcal/min
total = 12 × 30 ≈ 360 kcalA simpler approximation you’ll sometimes see is calories ≈ MET × weight(kg) × hours, which gives a similar ballpark. The ACSM version is slightly more precise, and it’s what the Calories Burned calculator uses.
Why weight and duration drive the result
Two variables do most of the work in the formula:
- Weight. Moving more mass requires more energy, so the burn scales directly with body weight. Double the weight, double the calories for the same activity and time. This is exactly why a generic “running burns 300 calories” claim is nearly useless — it ignores who’s running.
- Duration. Calories scale linearly with time. Thirty minutes burns twice what fifteen does at the same intensity.
Intensity is folded into the MET value itself: a faster run simply has a higher MET than a jog.
How accurate is this, really?
Honest answer: it’s a reasonable estimate, not a measurement. Published MET values are population averages, and individual energy expenditure can differ by 15–30% because of:
- Fitness level. A trained athlete is more efficient and may burn slightly less for the same task.
- Intensity within an activity. “Cycling” spans a gentle commute and a sprint; one MET value can’t capture both.
- Body composition. Muscle and fat have different metabolic costs.
- Conditions. Terrain, wind, temperature, and technique all matter.
This is why fitness trackers, even with heart-rate data, disagree with each other and with formula-based estimates. Use the number to compare workouts and track trends, not to balance your diet to the calorie.
Using estimates wisely
A few practical guidelines:
- Personalise it. Always enter your real weight — it’s the biggest lever in the formula.
- Don’t “eat back” every calorie. Because estimates run high as often as low, treating the figure as license to consume more can erase a deficit.
- Compare, don’t obsess. The relative difference between activities is more reliable than the absolute number.
- Use a custom MET when needed. If your activity isn’t listed, look it up in the Compendium of Physical Activities and enter the value directly.
Burn versus daily needs
A workout’s calorie burn is only one side of the energy-balance equation. The other, much larger side is your Total Daily Energy Expenditure — the calories your body uses across the whole day, most of which come from your resting metabolism rather than exercise. For perspective, a 30-minute run might burn 300–400 calories, while your body quietly spends two to three thousand keeping you alive and moving through an ordinary day.
That contrast matters when you set goals. Exercise is excellent for health, fitness, and mood, but it’s a smaller lever on body weight than many people expect, and it’s easy to “eat back” a workout’s burn in minutes. The most reliable approach is to know both numbers: your daily needs as a baseline and your workout burn as a bonus on top. If you want to understand how resting metabolism and total expenditure relate, BMR vs TDEE, Explained breaks the relationship down clearly.
Putting it together
MET values turn decades of metabolic research into a single, usable number, and the ACSM formula combines it with your weight and time to estimate calories burned. Knowing the math — and its limits — lets you use those figures sensibly rather than treating them as exact. Estimate your workout burn with the Calories Burned calculator, then pair it with your daily needs from the Calorie Calculator for the full energy-balance picture.