Three Numbers, One Goal
Most calorie calculators give you a single figure and leave you to guess what to do with it. This one is built around a simpler idea: your maintenance calories keep your weight stable, eating consistently below that number leads to weight loss, and eating consistently above it leads to weight gain — so rather than showing you one target, it shows you all three at once, so you can pick the one that matches where you actually are right now, whether that's cutting, maintaining, or building.
How This Calculator Works
BMR (Mifflin-St Jeor) then × Activity Multiplier = Maintenance Calories
±500 kcal/day ≈ ±0.5 kg/week
The calculation starts with your Basal Metabolic Rate, the energy your body needs at complete rest, calculated using the Mifflin-St Jeor equation — widely regarded as the most reliable formula for typical adults without access to lab testing. That number is then multiplied by an activity factor reflecting your typical week, from largely sedentary through very active, to produce your maintenance calories. From there, the calculator applies a standard ±500 calorie adjustment to generate cutting and bulking targets, based on the well-established relationship between a sustained calorie deficit or surplus and the rate of body weight change over time.
Reading Your Three Targets
For example, imagine your maintenance calories work out to 2,200 per day. Your deficit target would land around 1,700, designed to produce roughly 0.5 kg of fat loss per week — a pace widely considered sustainable and more likely to preserve muscle mass than a much more aggressive deficit. Your surplus target would land around 2,700, supporting weight gain, though for people prioritising muscle over fat gain, a smaller surplus of 200-300 calories combined with resistance training is often a better fit.
Why the Same 500-Calorie Rule Doesn't Fit Everyone Equally
A typical mistake we often see is applying the same flat 500-calorie deficit to everyone, regardless of their maintenance level. For someone with a low maintenance number — a smaller, less active person — a 500 calorie deficit represents a much bigger percentage cut than it does for someone with a high maintenance number, which can affect how sustainable that deficit actually feels day to day. In those cases, a percentage-based deficit, commonly 15-25% below maintenance, often produces a more comfortable and sustainable outcome than a flat 500-calorie rule applied across very different body sizes.
Picking the Right Target for Your Actual Goal
If your primary goal is fat loss and you're not overly concerned with preserving every bit of muscle, the deficit target is the straightforward choice. Let's say you're newer to strength training and want to build muscle while minimising fat gain — sometimes called "body recomposition." Eating closer to maintenance while training consistently with progressive resistance work can produce simultaneous, if slower, improvements in both body fat and muscle mass, particularly for beginners. If pure muscle and strength gain is the priority, the surplus target paired with a structured resistance programme tends to produce the best combination of muscle gain relative to fat gain.
Adjusting Over Time
None of these three numbers are fixed for life. As your weight changes, your BMR — and therefore all three targets — shifts with it, generally by a modest amount for typical changes, but enough to matter over a longer cutting or bulking phase. Many freelancers and busy professionals set a target once and never revisit it. Recalculating every few weeks during an active phase keeps your targets aligned with where your body actually is, rather than where it was when you first ran the numbers.