About BJJMath
A dedicated, source-cited home for the numbers grapplers actually search for — built in the open, corrected in the open.
The problem
If you train Brazilian Jiu-Jitsu, you have searched some version of these questions: What weight class am I in? How do I cut to it safely? What size gi do I order? How long until my next belt? The answers are scattered across forum threads, half-remembered gym wisdom, Reddit comments and the occasional physical-therapy clinic blog. Some of it is right. A lot of it is wrong, out of date, or quietly mixing up gi and no-gi rules.
No one had built a single, careful place for it. So we did.
The standard we hold ourselves to
BJJMath is a hub of calculators that each answer one specific question, with the method written down and the sources cited. Two principles run through all of it:
If it is a published rule, we cite the rule. The IBJJF weight classes, points, and age divisions are not estimates — they come straight from the official rule book, cross-checked against current event listings, and labelled verified.
If it is an estimate, we say so plainly. Calorie targets, belt timelines, recovery windows and training load are built on the best available evidence, but they are starting points for a conversation with a coach, dietitian or clinician — never a substitute. Those tools carry a clear "review pending" status until a credentialed brown or black belt (and, where relevant, a sports-medicine professional) has signed off on the methodology.
Launching openly
This site is new. We are putting it in front of the BJJ community precisely because the community is the fastest, most honest error-checker there is. If a number looks wrong, tell us — see the methodology page for how.
How we verify every number
Legitimacy here doesn't come from claiming credentials — it comes from showing our work. Every calculator falls into one of three honestly-labelled tiers, and the methodology page documents the exact rule, formula or reference behind each one so you can check it yourself.
Verified against the official rules. The competition tools — weight class, points, and age divisions — are transcribed directly from the IBJJF Official Rule Book and cross-checked against current event listings. These are facts, not opinions: the adult gi divisions (Rooster at 57.5 kg through to Ultra Heavy), the points values, and the competition-year age rule are exactly as the federation publishes them. Anyone can hold our numbers up against ibjjf.com and confirm.
Built on published science. The health tools run on named, peer-reviewed work, not gym lore. The calorie tool uses the Mifflin–St Jeor equation (American Journal of Clinical Nutrition, 1990) — the best-validated of the common predictive formulas — with protein and energy ranges drawn from ACSM and sports-nutrition position stands. Each reference is cited by name on the methodology page.
Honestly marked when unverified. Where a tool is an estimate or hasn't yet been reviewed by a credentialed practitioner, it says so on its face — "review pending" — rather than borrowing authority it hasn't earned. We would rather under-claim and be trusted than over-claim and be caught.
How we make money — and how we don't
BJJMath is free. It is supported by affiliate links: when a calculator recommends a gi or an instructional and you buy through our link, we may earn a commission at no extra cost to you. We only link to gear and resources that genuinely fit the calculator's output, the recommendations are never paid placements, and every affiliate link is marked. You can read the full affiliate disclosure any time.
What we will never do is let a sponsor change a calculator's answer. The math is the product. The day people stop trusting it, there is no business.
Get in touch
Questions, corrections, or you're a brown/black belt or sports-medicine professional who wants to help review a tool? Email hello@bjjmath.com. We read everything.