Most beginners hear two pieces of advice that pull in opposite directions: "lift to build muscle" and "watch what you eat". Whether you need to eat more than usual to build muscle depends on one number, the calories your body uses on an ordinary day. That is your maintenance. Eat about that much and your weight stays roughly steady. Eat a little above it and your body has spare material to build with.

The good news for a first-year lifter: you do not need a big surplus, and you may not need one at all. This guide shows how to estimate your maintenance, what a small step up looks like, and how to check it against your own scale instead of trusting any formula blindly.

Where maintenance comes from

Your body burns energy in three main ways. Most of it goes on simply staying alive: breathing, circulation, keeping warm. This is your basal metabolic rate, or BMR. On top of that comes everyday movement (walking, fidgeting, chores) and deliberate exercise. Digesting food costs a little too, but it is a small slice.

The calculator below uses the Mifflin-St Jeor equation, which dietetic bodies generally prefer for healthy adults:

BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5     (men)
BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161   (women)

It then multiplies your BMR by an activity factor, from 1.2 for a desk-bound week to about 1.7 for a very active one, to estimate maintenance. Think of the answer as a starting point. Studies of the equation find individual errors of around 10%, so the real figure could sit a couple of hundred calories either side.

Notice how wide the shaded band is. That is the honest part of the answer: the formula cannot know whether your metabolism runs a little fast or slow. It only gives you somewhere to start.

How big should the step up be?

Muscle is slow to build. Even in the first year, a beginner gains perhaps half a kilo of muscle a month at best, and often less. Your body does not need a flood of extra food for that. A common suggestion is a surplus of about 5 to 10% above maintenance, which is roughly 100 to 250 calories for most people, and the higher end suits true beginners better than people who have trained for years.

Bigger is not better. Beyond what your body can use for building, the extra energy is stored as body fat, and you would just have more to deal with later. Start small and adjust.

Two things matter more than the exact calorie number:

• Protein. Calories give your body the energy, but protein supplies the building blocks. If you have not worked out your daily amount, our guide on whether you need protein powder shows how to find your gap.
• Training that gets harder over time. Food does not build muscle by itself. Without a reason to adapt, extra calories have nowhere useful to go.

Check it against the scale, not the formula

This is the step most calculators skip. Any estimate is a guess until you compare it with what really happens. Here is a simple way to test it:

1. Eat around your target for three to four weeks. Do not change much else.
2. Weigh yourself a few mornings a week, after the toilet and before breakfast, and average each week. Single days jump around with water and food in your gut.
3. Look at the trend. If your average weight is flat and you want to gain, add around 100 to 150 kcal a day. If it is climbing faster than roughly 0.1 to 0.25 kg a week, trim a little. These are rough guides, not rules.
4. Also watch the bar: are your lifts going up over the weeks? That is the real sign things are working.

Be patient. Weight can wobble by a kilo or more from day to day, so give each change a few weeks before judging it.

Who this is not for

This is a rough guide for healthy adults. It is not a fit if you are under 18, pregnant or breastfeeding, have a medical condition that affects eating or metabolism, or have ever had a difficult relationship with food or counting calories. In those cases, please speak with a doctor or a registered dietitian first. Counting does not suit everyone, and it is fine to skip it entirely and simply eat regular meals with enough protein.

Also remember the formula can underestimate people with unusually high muscle mass and is less reliable at the very ends of the body-size range.

What to take away

• Maintenance is the calories that keep your weight steady. Estimate it, then test it.
• Beginners often build muscle at or slightly above maintenance. A step of 5 to 10% is plenty.
• Protein and progressive training matter as much as the calorie figure.
• Trust a few weeks of your own scale readings over any formula.