About this tool
The Pokémon Stat Calculator reproduces the exact stat formula used from Generation III onward (Ruby/Sapphire through Scarlet/Violet), so the number you get matches what the game shows. Choose the stat you want (HP, Attack, Defense, Sp. Atk, Sp. Def or Speed), enter the Pokémon's base stat, its Individual Value (IV, 0–31), its Effort Values (EV, 0–252), the level, and the nature, and it returns the final value. It all runs in your browser with integer floor arithmetic — the same rounding the games use.
The HP stat is floor(0.01 × (2 × Base + IV + floor(EV / 4)) × Level) + Level + 10. Every other stat starts the same way — floor(0.01 × (2 × Base + IV + floor(EV / 4)) × Level) + 5 — and is then multiplied by the nature modifier and floored again. The nature modifier is ×1.1 for the stat a nature raises, ×0.9 for the stat it lowers, and ×1.0 otherwise; the full 25-nature table is built in, so picking Adamant applies +10% Attack and −10% Sp. Atk automatically. HP is never affected by nature (Shedinja is the lone exception, with a fixed 1 HP).
The tool also shows the neutral value (before nature) and the nature effect on the chosen stat, which is handy for planning competitive spreads: at level 50 with 31 IVs and 252 EVs a base-100 attacking stat reaches 152 neutral, or 167 with a boosting nature. Values are floored at each step exactly as the games do, so results are exact rather than rounded estimates.