How this instrument works
Losing body fat through exercise comes down to a calorie deficit — burning more energy than you take in, sustained over time. This calculator isolates the exercise side of that equation: it takes your workout's intensity (as a MET value), your body weight, and how long and how often you train, and converts that into a weekly calorie burn using the same MET-based formula exercise physiologists use to estimate energy expenditure — kcal per minute equals METs times 3.5 times body weight in kilograms, divided by 200.
That weekly calorie figure then gets compared against your weight-loss goal using the widely cited approximation that roughly 7700 kilocalories corresponds to one kilogram of body fat (the metric analogue of the familiar ~3,500 kcal per pound). Dividing the total calories needed by your weekly exercise burn gives an estimated number of weeks to reach the goal, assuming exercise is the only lever being pulled — no dietary changes factored in at all.
Both halves of this calculation are genuinely useful approximations, not precise biological laws. The 7,700-kcal figure traces back to a 1958 calculation by researcher Max Wishnofsky and assumes a straightforward, linear relationship between calorie deficit and fat loss; real bodies adapt to sustained deficits by slowing metabolism somewhat, which means this estimate tends to run optimistic over longer timeframes. Treat the weeks figure as a reasonable starting expectation to plan around, not a guaranteed countdown clock, and know that pairing exercise with even modest dietary changes will get you there meaningfully faster than exercise alone.
- Enter your activity's MET value (intensity) — brisk walking is around 3.5-4, moderate cycling around 6-8, running around 8-12.
- Enter your body weight in kilograms.
- Enter your typical session length in minutes and how many sessions per week.
- Enter how many kilograms of weight you're aiming to lose.
- Read the estimated number of weeks to reach that goal through exercise alone.
Worked example — MET 6.0 activity, four sessions a week
An 80kg person picks up a MET-6.0 activity — something like moderate cycling or a brisk aerobics class — for 45 minutes, four times a week, aiming to lose 5kg through exercise alone. Each session burns 6.0 × 3.5 × 80 / 200 × 45 = 378 kcal, and across four sessions that's 1,512 kcal burned per week.
The 5kg goal requires roughly 5 × 7700 = 38,500 kcal total, so dividing that by the weekly burn gives 38,500 / 1,512 ≈ 25.5 weeks — nearly six months of consistent training to hit the goal through exercise alone. Bumping the intensity to MET 8.0 and adding a fifth weekly session, while lowering the goal to 3kg, cuts that same kind of estimate down to roughly 15.7 weeks — a clear illustration of how much both intensity and frequency matter to the timeline.
Questions
Does this calculator factor in any changes to my diet?
No — this is an exercise-only estimate. It calculates how long reaching your goal would take if calorie burn from training were the sole source of your deficit, with food intake held constant. In practice, pairing even a modest dietary change with the same exercise routine will get you to the same goal considerably faster than exercise alone, since you'd be creating a deficit from both sides at once.
How accurate is the 7700-kcal-per-kg rule this calculator relies on?
It's a widely used approximation, not an exact biological constant. The figure traces back to a 1958 calculation by researcher Max Wishnofsky and assumes a simple, linear relationship between calorie deficit and fat loss. Modern research shows real weight loss slows over time as the body adapts metabolically to a sustained deficit, so this rule tends to be optimistic for longer timelines, even though it remains a reasonable starting-point estimate.
How do I know the MET value for my specific activity?
MET values are published for hundreds of specific activities in the Compendium of Physical Activities, the standard exercise-science reference — brisk walking sits around 3.5-4 METs, moderate cycling or swimming around 6-8, and running or vigorous training around 8-12 and up. If you're unsure, a slightly conservative MET estimate gives a more realistic (longer) timeline than an optimistic one.
Why does adding one more weekly session change the timeline so much?
Because the total weekly calorie burn scales directly with how many sessions you complete, and the weeks-needed figure is inversely proportional to that weekly total. Going from four sessions a week to five is a 25% increase in weekly burn, which shortens the estimated timeline by roughly the same proportion — small, consistent increases in frequency compound meaningfully over a multi-month goal.
Is losing weight through exercise alone actually a good strategy?
It works, but it's genuinely the slower and harder path compared to combining exercise with dietary changes, mainly because it's far easier to create a large calorie deficit by eating less than by exercising enough to burn an equivalent amount. Exercise brings real benefits beyond calorie burn — cardiovascular fitness, muscle preservation, mood — so it's worth doing regardless, but expect this exercise-only timeline to be a slow, patient one.
What's considered a safe rate of weight loss to aim for?
Most health guidance points to roughly 0.5 to 1 kg (about 1-2 lb) per week as a sustainable target for most adults, which corresponds to a daily deficit in the range of a few hundred to around a thousand calories. If this calculator's exercise-only timeline implies a much faster rate than that, it's a sign the underlying weekly deficit is unusually aggressive and worth sanity-checking against your actual calorie intake too.