Guides
Short, sourced explanations of the numbers the calculators produce - and where they stop being reliable.
Why these guides exist
A calculator gives you a number. A guide tells you what to do with it, how far to trust it and when to ignore it. Each guide below is written to be read once and referred back to: it names the formula or threshold in plain English, shows where it comes from, and says what it cannot tell you.
Everything here is general information for healthy adults, not medical advice. If you have a condition, take medication or are pregnant, the numbers in these guides need a clinician's judgement alongside them.
Start here if you are new
If you are setting up a plan for the first time, read these four in order: how accurate a TDEE estimate is, how to choose an activity level honestly, what a calorie deficit actually does, and how much protein you need. That covers the four decisions that determine almost everything else.
- How accurate is TDEE? - the size of the error you are working with
- Activity level guide - the input people get wrong most often
- Calorie deficit explained - what a deficit is and how large it should be
- How much protein per day - the one macro with a real minimum
Reference guides
The remaining guides are reference material: the origin and limits of the most-quoted equations, how macro splits compare, how body fat methods differ, how to program with percentages of a one-rep max, and how the classic ideal-weight formulas were derived.
What each guide answers
Twelve guides cover the recurring questions. If you only read one, read the accuracy guide; if you only read two, add the activity level guide, because that is the input most people get wrong.
| Guide | Question it answers | Read it with |
|---|---|---|
| How accurate is TDEE? | How wide are the error bars? | TDEE calculator |
| Activity level guide | Which multiplier is mine? | TDEE calculator |
| BMR vs TDEE | Which number do I eat to? | BMR calculator |
| Calorie deficit explained | How big should the deficit be? | Calorie deficit calculator |
| Mifflin-St Jeor equation | Where does the formula come from? | BMR calculator |
| How much protein per day | What is the minimum I need? | Protein calculator |
| Macro splits guide | How do I divide my calories? | Macro calculator |
| Water intake guide | How much should I drink? | Water intake calculator |
| Body fat measurement methods | Which method should I trust? | Body fat calculator |
| One-rep max training guide | How do I train with percentages? | One-rep max calculator |
| Ideal weight formulas | Why do the formulas disagree? | Ideal weight calculator |
| Why am I not losing weight? | What do I do when it stalls? | Weight loss calculator |
How to use a guide with a calculator
Run the calculator first, then read the guide that matches the number you were given. The order matters because the articles explain the limits of a specific figure, and reading them before you have that figure makes the caveats abstract.
Every guide names the formula or threshold it describes and lists its sources at the bottom. Where the evidence is thin, the text says so rather than presenting a guess as a standard.
What the guides deliberately leave out
Three topics are excluded on purpose. Medical conditions, because general guidance stops being safe once a diagnosis is involved. Supplement and fat-burner advice, because the evidence for most products is weak and the incentives behind the content are not. And meal plans, because a plan that works for one person rarely transfers, and the useful part of a diet is the calorie and protein target rather than the specific foods.
Where a guide touches on a clinical threshold, such as the waist bands or the BMI categories, it attributes the threshold to the body that set it and repeats that it is a screening tool rather than a diagnosis.
Frequently asked questions
Are these guides medical advice?
No. They explain published formulas and thresholds for healthy adults. Anything clinical - medication, pregnancy, eating disorders, diabetes, kidney disease - needs a clinician who knows your history.
How often are they updated?
Guides carry a review date. When a formula, threshold or source changes, the page is dated with the change rather than silently edited.
Where do the numbers in the formulas come from?
From published research and public health guidance: validation studies for the calorie equations and WHO, NHS or CDC material for thresholds. Each guide lists its sources at the foot of the page.
Can I cite these guides?
Yes, with a link back to the page. They summarise primary sources rather than replacing them, so for academic work cite the original study, which is named in the reference list.
Do the guides cover medical conditions?
No. They describe healthy adults. Pregnancy, eating disorders, diabetes, kidney disease and thyroid conditions change the numbers enough that general guidance stops being useful, and those cases need a clinician.
Which order should I read them in?
Start with the accuracy guide, then the activity level guide, then the calorie deficit and protein guides. That covers the four decisions that set up a plan. Everything else is reference material to dip into when a specific number looks wrong.
Sources
Formulas and thresholds are taken from the sources above. They are a starting point for adults, not a diagnosis.
Last reviewed September 12, 2026. How we calculate · Editorial policy