Know the numbers behind the numbers.
Each calculator on FitCalc is built on an established formula. These guides explain where those formulas come from, what they get right, where they fall short, and how to actually use the result.
Every guide here follows the same shape. It opens with a working version of the relevant calculator so you can get your number immediately, then explains the equation behind it — who published it, what data it was fitted to, and why it was chosen over the alternatives. After that comes the part most calculator sites skip: where the method stops being reliable, and what to use alongside it when it does.
They're written to be read by someone with no background in nutrition or exercise science, and they're deliberately non-prescriptive. The aim is to explain what a number means and how it's normally used, not to tell you what to do with your body. Where a method has a well-known weakness — and most of them do — the guide says so plainly rather than burying it. If you want the reasoning behind how these formulas were selected and verified, that's set out on the about page.
TDEE Explained: How to Calculate the Calories You Actually Burn
What TDEE means, how the Mifflin–St Jeor equation gets there, and how to use your number without obsessing over it.
Read the guide →BMI Explained: What It Gets Right, What It Misses
Where BMI came from, what it's actually useful for, and when you should look at other measurements instead.
Read the guide →How to Set Your Macros: A Practical Guide
What protein, carbs, and fat each actually do, and how to pick a split that fits your goal and your life.
Read the guide →How to Use Your One-Rep Max to Plan Smarter Training
What 1RM is for, how the Epley formula estimates it, and how to turn that number into an actual training plan.
Read the guide →How Big Should Your Calorie Deficit Be?
The maths behind losing 1kg a week, why bigger deficits usually backfire, and how to pick a rate that's actually sustainable.
Read the guide →Body Fat Percentage: How to Measure It Without a DEXA Scan
The U.S. Navy tape-measure method, how to measure correctly, and how accurate it really is.
Read the guide →How Much Water Should You Actually Drink?
Why "8 glasses a day" is mostly a myth, and a formula that actually accounts for your body, activity, and climate.
Read the guide →Ideal Weight Calculators: Why the Formulas Disagree
Devine, Robinson, Miller, and Hamwi compared — where each came from, and why body composition matters more than any one of them.
Read the guide →Lean Body Mass Explained: What It Is and Why It Matters
The Boer formula, how LBM differs from body fat percentage, and why it's a better basis for protein targets than total weight.
Read the guide →Heart Rate Training Zones Explained
Tanaka vs. the outdated "220 minus age" rule, the Karvonen method, and what each of the five training zones is actually for.
Read the guide →TDEE vs BMR vs Calorie Deficit: What's the Difference?
The terms that get confused most often in nutrition content, defined plainly and shown as one connected chain.
Read the guide →Steps to Calories: How Many Calories Does Walking Actually Burn?
Why step count alone isn't enough, the MET-based formula that converts steps to calories, and how accurate it really is.
Read the guide →Pregnancy Calorie Needs: What the Guidance Actually Says
NHS trimester guidance, why this calculator only ever estimates maintenance, and where a calculator's usefulness ends.
Read the guide →Start with the calculator, or start with the guide
If you already know which number you're after, the calculators are the faster route — all eight sit on a single page with no sign-up and nothing to install. If you're less sure which measure answers your question, the guides are the better starting point, because choosing the wrong metric is a more common problem than calculating the right one badly.
A rough map: use TDEE to work out what you burn, then calorie deficit to decide how far below it to eat and macros to split that intake up. For body composition, BMI is the quick screen and body fat percentage the more informative follow-up. One-rep max covers strength programming, and hydration covers the one most people guess at.
Spotted something wrong, or want a guide that isn't here yet? Get in touch — corrections get priority.