How we calculate your Brain Fitness Index
Your Brain Fitness Index — BFI for short — is a single number from 0 to 100 that summarises your whole brain training. It exists so you don't have to juggle six separate scores to know whether you're trending up. This page walks through exactly how it's built, because a number you can't see the working behind isn't worth much.
Step 1 — a rating for each skill
Every one of the six cognitive skills carries its own Elo rating that rises and falls as you play games in that skill. Ratings start at 1200 and move within a 500–2500 range — the same mechanism that drives the adaptive difficulty of your sessions. So before there's any single index, there are six honest, independent numbers underneath it.
Step 2 — put every skill on the same 0–100 scale
A raw Elo around 1200 doesn't mean much on its own, so each skill's rating is normalised onto a common 0–100 scale. That makes the six skills directly comparable and ready to be combined — a strong Memory and a developing Speed can sit side by side and add up sensibly.
Step 3 — combine them, with weights
The BFI is a weighted averageof those six normalised scores. The skills aren't weighted equally — Memory and Attention each count a little more, reflecting how central they are to everyday cognition. The weights are fixed and public:
Memory
20%
Attention
20%
Speed
15%
Flexibility
15%
Problem Solving
15%
Language
15%
100%
BFI = 0.20·Memory + 0.20·Attention + 0.15·Speed + 0.15·Flexibility + 0.15·Problem Solving + 0.15·LanguageEach term is that skill's rating normalised to 0–100. Multiply by its weight, add them up, and you have a single number from 0 to 100.A worked example
Suppose your six normalised skill scores are Memory 78, Attention 70, Speed 65, Flexibility 72, Problem Solving 60, and Language 80. Apply the weights — 0.20·78 + 0.20·70 + 0.15·65 + 0.15·72 + 0.15·60 + 0.15·80 — and your Brain Fitness Index works out to 71.6, which rounds to a BFI of 72. Lift any one skill and the index moves by that skill's weight — improving a 20%-weighted skill shifts your BFI more than the same gain in a 15%-weighted one.
Why one number at all
The BFI isn't there to flatten the detail — your six skill trends are always available underneath it. It's there to answer the everyday question, “am I keeping up my training and is it trending the right way?” at a glance. It's a summary of your performance on CogDojo's games, measured consistently over time — and that's the whole of what it claims to be.
One honest note: the BFI measures how you do on our games, not your real-world cognitive ability or health. It's a training metric, and it's most useful watched as a trend over weeks rather than read as a verdict on any single day.