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The BondingCurve contract address can be obtained from MemeFactory.bondingCurve(). See Contract Overview for details.

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Constant Product Formula

KasFun uses a constant-product bonding curve (similar to Uniswap V2), enhanced with virtual reserves: 
(tokenReserves + virtualToken) * (kasReserves + virtualKas) = k
The virtual reserves set the initial price and curve shape without requiring seed liquidity.

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Getting Curve Parameters

Before calculating prices, fetch the token’s current state and curve parameters: 
// 1. Current reserves
const info = await factory.getTokenInfo(tokenAddress);
const { tokenReserves, kasReserves } = info;

// 2. Virtual parameters
const params = await factory.getTokenParamVersion(tokenAddress);
const { virtualToken, virtualKas, graduationThreshold } = params;

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Price Calculation Functions

The BondingCurve contract provides three pure calculation functions:

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Calculate Buy Return

How many tokens will I get for a given KAS amount? 
function calculateBuyReturnWithParams(
    uint256 kasIn,
    uint256 tokenReserves,
    uint256 kasReserves,
    uint256 vToken,
    uint256 vKas
) pure returns (uint256 tokensOut)

const kasIn = ethers.parseEther("100"); // 100 KAS
const tokensOut = await bondingCurve.calculateBuyReturnWithParams(
  kasIn,
  info.tokenReserves,
  info.kasReserves,
  params.virtualToken,
  params.virtualKas
);
console.log("Tokens out:", ethers.formatEther(tokensOut));

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Calculate Sell Return

How much KAS will I get for selling a given amount of tokens? 
function calculateSellReturnWithParams(
    uint256 tokensIn,
    uint256 tokenReserves,
    uint256 kasReserves,
    uint256 vToken,
    uint256 vKas
) pure returns (uint256 kasOut)

const tokensIn = ethers.parseEther("1000000"); // 1M tokens
const kasOut = await bondingCurve.calculateSellReturnWithParams(
  tokensIn,
  info.tokenReserves,
  info.kasReserves,
  params.virtualToken,
  params.virtualKas
);
console.log("KAS out:", ethers.formatEther(kasOut));

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Calculate Buy Cost

How much KAS do I need to spend to get a specific amount of tokens? 
function calculateBuyCostWithParams(
    uint256 tokensOut,
    uint256 tokenReserves,
    uint256 kasReserves,
    uint256 vToken,
    uint256 vKas
) pure returns (uint256 kasIn)

const desiredTokens = ethers.parseEther("1000000"); // I want 1M tokens
const kasNeeded = await bondingCurve.calculateBuyCostWithParams(
  desiredTokens,
  info.tokenReserves,
  info.kasReserves,
  params.virtualToken,
  params.virtualKas
);
console.log("KAS needed:", ethers.formatEther(kasNeeded));

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Current Price

Get the current token price (KAS per token): 
// Via MemeFactory (convenience)
const price = await factory.getCurrentPrice(tokenAddress);

// Via BondingCurve (from reserves)
const price = await bondingCurve.getCurrentPrice(
  info.tokenReserves,
  info.kasReserves
);

console.log("Price:", ethers.formatEther(price), "KAS per token");

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Price Impact

For large trades, the price impact can be significant. Calculate it by comparing the effective price with the current price: 
const kasIn = ethers.parseEther("1000"); // 1000 KAS
const tokensOut = await bondingCurve.calculateBuyReturnWithParams(
  kasIn, tokenReserves, kasReserves, vToken, vKas
);

const effectivePrice = kasIn * BigInt(1e18) / tokensOut; // KAS per token
const currentPrice = await factory.getCurrentPrice(tokenAddress);
const priceImpact = Number(effectivePrice - currentPrice) / Number(currentPrice) * 100;

console.log(`Price impact: ${priceImpact.toFixed(2)}%`);
When kasIn is very small relative to kasReserves, the price impact approaches zero and the effective price approaches the spot price.

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Graduation Threshold

Each token has a graduationThreshold in its curve parameters. When kasReserves >= graduationThreshold, the token graduates: 
const params = await factory.getTokenParamVersion(tokenAddress);
const info = await factory.getTokenInfo(tokenAddress);

const remaining = params.graduationThreshold - info.kasReserves;
console.log("KAS until graduation:", ethers.formatEther(remaining));