We are a small team obsessed with one simple idea: the body composition tools available online should be honest, accurate, and free. So we built them.
FFMI Calculator is a free fitness tool platform created to help athletes, lifters, coaches, and everyday people understand their bodies better. We exist because the internet is full of body composition calculators that get the math wrong, hide their formulas, demand signups for basic results, or push misleading interpretations that confuse more than they help.
What you get here is the opposite. Every calculator on this site uses peer-reviewed formulas, runs entirely in your browser, requires no account, and stores none of your data. We show you the math, explain the limits, and give you the verdict in plain language. If a metric is not useful, we tell you. If a tool has flaws, we explain them.
The site started as an internal resource for our coaching clients who kept getting misled by mainstream calculators. It grew because the same problem affects millions of people who train seriously and deserve better numbers.
To give every athlete, lifter, and health-aware person free access to body composition tools that are accurate, transparent, and useful. We exist to replace the bad calculators that misclassify muscular athletes, hide their math, and demand signups for results that should always have been free.
A world where scale weight stops being the default health metric and where every person who trains has access to the same quality of body composition data that elite athletes get. Lean mass, fat mass, FFMI, and metabolic rate should be common knowledge, not gated behind clinics and apps.
FFMI Calculator was founded by Katherine Henshall, a personal trainer with a decade of experience helping clients build stronger, healthier, more confident bodies. Her coaching practice grew the site from a private spreadsheet into the public platform it is today.
Hi, I am Katherine. I am an experienced personal trainer and the founder of FFMI Calculator. What started as a tool I built for my own coaching clients has grown into the platform you are using right now.
My background is a little non-traditional. I have a degree in Dance, and I spent the early part of my career teaching fitness and dance classes. Over the past decade, I have moved into one-on-one personal training and coaching, where I have worked with more than 100 clients on body transformation, strength building, and sustainable health.
I specialise in creating tailored fitness programmes that support fat loss, strength gain, and long-term physical health. My coaching style is built on three pillars: education, consistency, and sustainability. I do not believe in quick fixes, extreme diets, or punishing routines. I believe in giving people the knowledge and tools to make decisions that work for their lives, year after year.
Here is the problem I kept running into. Every time a client checked their progress on a popular online calculator, the numbers either flattered them dishonestly or punished them for doing exactly what their training was supposed to do. A muscular athlete getting flagged "obese" by BMI. A natural lifter being told their FFMI was suspicious when the calculator was using outdated formulas. Body fat readings from bathroom scales that swung 8% in a single day.
So I built better ones. I researched the formulas, validated them against published studies, and asked a developer friend to help me put them on the internet. The first version of FFMI Calculator was a spreadsheet. The second was a single page. Now it is a full platform with multiple calculators, all free, all honest, all running entirely in your browser.
While the technical side of these tools matters, I have always cared most about the human side of fitness. Numbers are meaningless without context, and progress is meaningless without confidence. I built this site to give people both: the right numbers, plus the explanation of what those numbers actually mean for their training, their health, and their goals.
When I am not training clients or improving the calculators on this site, you will find me writing about body composition science, sharing fitness content on Instagram, and trying to convince every lifter I know that BMI is the worst metric ever invented. Connect with me on the platforms below if you want to follow along or have questions.
Three frustrations turned into a website. They are still the reasons we keep building.
Athletes were being told they were obese. Lifters were being told their natural results were suspicious. Beginners were being given numbers they could not interpret. The cost of bad calculator design is not abstract. It changes how people think about their bodies, their progress, and their potential.
The math behind every calculator on this site is public domain. There is no reason a basic FFMI or body fat estimate should be locked behind an app subscription, an email signup, or an upsell. We made the call that these tools should be free, full stop.
Your weight, height, and body fat are personal data. We have no interest in collecting, storing, or selling them. Every calculator runs in your browser. We have no database holding user calculator inputs. We have no analytics tracking individual results. What you put in, only you see.
Five principles guide every decision on this site, from which calculators we build to how we write the explanations beneath them.
Every formula is sourced from peer-reviewed research and validated against published examples. We show you the math, every time.
We tell you when a metric does not apply to your situation. BMI is not for athletes. Bathroom scale body fat readings are unreliable. We say so.
No paywalls, no signups, no premium tiers. The basic body composition tools you need should not require a credit card.
Calculators run in your browser. Your inputs never leave your device. We do not track individual results.
Body composition science is full of jargon. We explain everything in language a smart 16-year-old could understand without dumbing it down.
Most calculators were built for sedentary adults. Ours are built for people who train. The difference matters in every formula and every interpretation.
Every calculator on the site goes through the same four-step process before it ships. No exceptions.
The same beliefs that shape Katherine's coaching practice shape every page on this site. They are simple and they are non-negotiable.
We are still in the early years of what this site will become. Here is what we are working on now and what we plan to build next.
We add 1 to 2 new calculators per month based on user requests. Currently in development: a one-rep max calculator with multiple formulas, a strength standards comparison tool, and a macro calculator with goal-specific presets for cutting, bulking, and maintenance.
The blog at ffmicalculatore.com/blog is expanding into a full library of body composition science articles. We are publishing deep dives on FFMI history, body fat measurement accuracy, muscle gain timelines, and the difference between population health metrics and individual fitness metrics.
The next major feature is a free progress tracker that lets you record your calculator results over time, all stored in your browser only. No accounts, no servers, just a private timeline of your composition changes.
For personal trainers and coaches who use these calculators with their clients, we are building printable client report templates and a glossary suitable for sharing in coaching contexts.
Start with the FFMI Calculator. Eight tools in total are live and free, with more rolling out monthly.