Generation 5 (Pokémon Black, White, B2, W2) utilizes a specific modification of the core catching formula. The key change from previous generations is the buff to the Sleep and Freeze status conditions, moving from a 2.0x multiplier to 2.5x.
The Catch Rate Formula (a)
The game first calculates a value 'a'. If 'a' is 255 or higher, the Pokémon is caught automatically. If not, the game proceeds to the "shake checks."
Critical Catches: Introduced in Gen 5, this mechanic gives a small chance for the ball to shake only once. The probability depends on how many Pokémon species you have already caught.
Dark Grass: In Unova, if you encounter Pokémon in Dark Grass where double battles can occur, the catch rate is halved if you haven't caught enough Pokémon to offset the difficulty.
Level Adjustment: Unlike Gen 4, Gen 5 does not use the level of the Pokémon directly in the core catch formula, though specific balls like the Level Ball (available via transfer) still use it.
Common Base Catch Rates
Pokémon Type
Base Rate
Common (Patrat, Purrloin)
255
Common Unbound (Pidove)
255
Starters / Rare (Snivy, Tepig, Oshawott)
45
Legendaries (Reshiram, Zekrom)
45 (Special for Gen 5)
Most Box Legendaries (Kyurem)
3
function calculateCatchRate() {
var maxHp = parseFloat(document.getElementById('maxHp').value);
var currHp = parseFloat(document.getElementById('currHp').value);
var baseRate = parseFloat(document.getElementById('baseRate').value);
var ballMod = parseFloat(document.getElementById('ballType').value);
var statusMod = parseFloat(document.getElementById('statusType').value);
var dexMod = parseFloat(document.getElementById('pokedexCount').value);
var grassMod = parseFloat(document.getElementById('darkGrass').value);
if (isNaN(maxHp) || isNaN(currHp) || isNaN(baseRate) || currHp > maxHp) {
alert("Please enter valid HP and Catch Rate values. Current HP cannot exceed Max HP.");
return;
}
// Step 1: Calculate 'a'
// Formula: a = (((3 * MaxHP – 2 * CurrHP) * BaseRate * BallMod) / (3 * MaxHP)) * StatusMod
var a = (((3 * maxHp – 2 * currHp) * baseRate * ballMod) / (3 * maxHp));
a = a * statusMod * grassMod;
var captureProb = 0;
var shakeProb = 0;
if (ballMod === 255 || a >= 255) {
captureProb = 100;
} else {
// Step 2: Calculate 'b' (Shake Probability)
// b = 65536 / (255/a)^0.25
var b = 65535 * Math.pow((a / 255), 0.25);
// Probability of 4 successful shakes
// (b/65536)^4
captureProb = Math.pow((b / 65535), 4) * 100;
shakeProb = b;
}
// Critical Catch Calculation (Gen 5)
// Critical catches are separate from normal capture probability.
// Simplified for display:
var critChance = (Math.min(255, a) * dexMod) / 6;
var critPercent = (critChance / 256) * 100;
// Output results
document.getElementById('resultBox').style.display = "block";
document.getElementById('catchPercentage').innerHTML = captureProb.toFixed(2) + "% Catch Chance";
var details = "Calculated 'a' value: " + Math.floor(a) + "";
details += "Shake Probability (b): " + Math.floor(shakeProb) + " / 65535″;
details += "Critical Catch Chance: ~" + critPercent.toFixed(2) + "%";
if (captureProb < 5) {
details += "Tip: Use a status effect like Sleep or a better Ball (Ultra/Dusk) to increase your odds!";
} else if (captureProb > 90) {
details += "Tip: Excellent odds! You should be able to catch it easily.";
}
document.getElementById('catchDetails').innerHTML = details;
}