How this instrument works
This calculator answers one specific question: if your activity level moves from one tier to another, how many daily calories does that shift add or remove from your maintenance number? It computes basal metabolic rate once with the Mifflin-St Jeor equation, then multiplies that same resting figure by two different activity multipliers — one for before the change, one for after — and reports both maintenance totals plus the difference between them.
The name reflects one common trigger for this kind of shift — a period of quarantine or isolation that suddenly drops someone from their usual activity level to near-sedentary — but the underlying math has nothing quarantine-specific in it. The identical calculation applies to recovering from an injury, starting a desk job after years of physical work, retiring from an active career, or ramping training up rather than down; it's simply the same maintenance-calorie formula evaluated twice and differenced.
Both maintenance figures carry the same population-average caveats as any Mifflin-St Jeor estimate: they predict a typical person of your size, sex and age, not a lab measurement of you specifically. The delta between them is more useful as a directional signal — eat roughly this much less after becoming less active, or this much more after becoming more active — than as a number to hit exactly, since the individual uncertainty in each maintenance estimate carries through to their difference.
- Set Sex, Weight, Height and Age — these feed a single basal metabolic rate shared by both calculations.
- Pick Activity level before the change from the five available tiers.
- Pick Activity level after the change, the tier you're moving to.
- Read Maintenance calories before and Maintenance calories after side by side.
- Read Change in maintenance calories — negative means eat less to hold weight steady, positive means eat more.
Worked example — moderately active to sedentary, 70 kg man
Enter 70 kilograms, 175 centimetres, age 30, Sex male, Activity before set to Moderate and Activity after set to Sedentary. BMR = 10 × 70 + 6.25 × 175 − 5 × 30 + 5 = 1,648.75 kcal per day, unaffected by either activity choice since it depends only on body size, age and sex. Before the change, maintenance calories = 1,648.75 × 1.55 = 2,555.5625 kcal/day; after, 1,648.75 × 1.2 = 1,978.5 kcal/day — a change of −577.06 kcal/day, meaning roughly 577 fewer calories a day are needed to avoid gaining weight once activity drops to sedentary.
The same instrument works in the opposite direction just as cleanly. A 45-year-old man, 90 kg, 180 cm, moving from Sedentary to Moderate activity: BMR = 1,805 kcal/day, before = 1,805 × 1.2 = 2,166 kcal/day, after = 1,805 × 1.55 = 2,797.75 kcal/day, a change of +631.75 kcal/day — proof this is a general before/after activity-change tool, not a one-way calculator for becoming less active.
Questions
Is this calculator only useful during a quarantine or lockdown?
No — the name reflects one common trigger for a sudden activity-level change, but the underlying formula has nothing quarantine-specific in it. The same before/after calculation applies to recovering from an injury, switching to a desk job, retiring from physically demanding work, or increasing training volume. Any two activity tiers work as inputs, in either direction.
What does a negative Change in maintenance calories mean?
A negative number means the after-activity tier burns fewer calories than the before-activity tier, so eating the same amount as before the change would tend to add weight over time. The magnitude tells you roughly how many fewer daily calories are needed to hold weight steady at the new, lower activity level — not an exact target, but a reasonable adjustment size to start from.
Why does basal metabolic rate stay identical before and after the change?
Because BMR in the Mifflin-St Jeor equation depends only on weight, height, age and sex — none of which this calculator assumes changes between the before and after periods. Activity level enters the calculation only through the multiplier applied on top of that fixed resting figure, which is why one BMR value feeds both the before and after maintenance totals.
Can the before and after activity levels be the same tier?
Yes, and doing so is a useful way to confirm the calculator is working as expected — with identical before and after tiers, both maintenance totals match and the change comes out to exactly zero, correctly showing that no calorie adjustment is needed when activity level hasn't actually changed.
How is this different from the site's Maintenance Calorie or TDEE calculators?
Those calculators run the identical Mifflin-St Jeor formula once, for a single activity level, and report one maintenance figure. This one runs the same formula twice, at two different activity levels, and reports the difference between them — useful specifically when you want to know how much a change in activity moves your number, rather than what your number is at a single fixed activity level.
Should I change my eating immediately when my activity level changes?
This calculator's delta is a reasonable starting adjustment, not a rule to follow to the day. Real maintenance calories shift gradually as your body adapts to a new activity pattern, and the estimate itself carries the same population-average uncertainty as any Mifflin-St Jeor figure — treat the direction and rough size of the change as guidance, then confirm against how your own weight actually trends over the following weeks.
References
- Mifflin et al. 1990, Am J Clin Nutr — original BMR equation (PubMed)
- NIH NIDDK — Body Weight Planner
- WHO — Physical activity fact sheet
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.