How many calories do you need a day? And why your trend matters more
I’m Yema, and I’ll show you where your calorie number comes from. A heads-up: it’s a starting point, and what really counts is your trend, not one day.
YemaYour companion for eating better.
Updated · 2 min read
How your number is worked out
First, what you burn at rest is estimated with the Mifflin-St Jeor formula, which uses your age, height, weight and sex.1 That’s what your body burns even when you don’t move: breathing, thinking, keeping warm.
Then it’s multiplied by your activity. FAO and WHO use factors of 1.4 to 1.69 for a sedentary or light life, and 1.7 to 1.99 for an active one.2 That gives you what you burn on a normal day.
To lose weight, a little less, with a floor
If you want to lose weight, guidelines suggest eating 500 to 750 kcal a day less than you burn.3 A small deficit is easier to keep, protects your muscle and still lets you eat well.
And there’s a floor: without medical supervision, going below 1,200 kcal a day for women or 1,500 for men isn’t recommended.3 I never suggest a goal below that.
Why your trend matters more than one day
Your weight goes up and down from one day to the next with water, salt and digestion. In many women, the body holds more fluid around the first day of their period.4 That’s why a single number on the scale says little.
What does tell you something is the line over several weeks. I watch that trend to adjust your goal, and a day when you ate more doesn’t ruin anything: what counts is the week.
What about your watch’s calories?
Watches measure your heart rate well, but they’re off by about 28 % on calories.5 That’s why I don’t add everything your watch says to your goal: I adjust it with your real weight.
Frequently asked questions
How many calories should I eat a day?
It depends on your age, height, weight, sex and activity. What you burn at rest is estimated with the Mifflin-St Jeor formula and multiplied by your activity.1 Yema works it out for you and shows you where each number comes from.
How many fewer calories should I eat to lose weight?
Guidelines suggest 500 to 750 kcal a day less than you burn, without going below 1,200 kcal for women or 1,500 for men without medical supervision.3 A small deficit is easier to keep than a big one and still lets you eat well.
Why does my weight go up from one day to the next?
It’s almost always water: salt, digestion and the menstrual cycle change how much fluid your body holds.4 You don’t gain that much fat in a single day. That’s why it’s better to look at the trend over several weeks than at one number.
Sources
- Mifflin and St Jeor, 1990A new predictive equation for resting energy expenditure in healthy individuals
- FAO, WHO and UNU, 2004Human energy requirements. Report of a Joint FAO/WHO/UNU Expert Consultation (Table 5.3)
- AHA, ACC and TOS, 20132013 AHA/ACC/TOS Guideline for the Management of Overweight and Obesity in Adults
- White et al., 2011Fluid retention over the menstrual cycle: 1-year data from the Prospective Ovulation Cohort
- Choe and Kang, 2025Apple watch accuracy in monitoring health metrics: a systematic review and meta-analysis
Yema doesn’t replace your doctor or your dietitian. If you have a health condition, are expecting a baby or take medication, talk to a professional.