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Learn how to estimate fees for different operations in your Spark wallet. This guide covers Lightning payments, cooperative exits, and swap operations.

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Fee Structure Overview

Below is a breakdown of fees for different transaction types on Spark:
For Bitcoin
Transaction TypeFee structure
L1 to SparkOn-chain fee paid by the user
Spark to SparkFree. Small flat fee coming in 6-12 months
Spark to Lightning0.25% + routing fee
Lightning to Spark0.15%
Exit to L1L1 broadcast fee x 2
Unilateral ExitOn-chain fee paid by the user

For BTKN assets
Transaction TypeFee structure
L1 to SparkOn-chain fee paid by the user
Spark to SparkFree. Small flat fee coming in 6-12 months
Unilateral ExitOn-chain fee + bond locked by user
Some of the fees are sourced directly from the Lightspark SSP, specifically for Spark–Lightning interactions and exits back to L1. Lightspark is the first SSP on Spark, but the system is open. Anyone can run an SSP (SSP specs coming soon). If you’re an SSP, reach out, and we’ll include your fee structure.

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Fee Estimation Flow

The complete process for estimating fees before making transactions:
1

Choose Operation Type

Determine which type of operation you want to estimate fees for.
// Lightning payment
const lightningInvoice = "lnbc...";

// Cooperative exit (withdrawal to L1)
const withdrawalAmount = 10000;
const withdrawalAddress = "bc1p...";

// Swap operation
const swapAmount = 5000;
2

Get Fee Estimate

Call the appropriate fee estimation method.
// Lightning send fee estimate
const lightningFee = await wallet.getLightningSendFeeEstimate({
  encodedInvoice: lightningInvoice
});

// Cooperative exit fee estimate
const exitFee = await wallet.getWithdrawalFeeQuote({
  amountSats: withdrawalAmount,
  withdrawalAddress: withdrawalAddress
});

// Swap fee estimate
const swapFee = await wallet.getSwapFeeEstimate(swapAmount);
3

Review Fee Details

Examine the fee estimate response to understand costs.
console.log("Fee estimate:", feeEstimate.feeEstimate);
console.log("Original value:", feeEstimate.feeEstimate.originalValue, "sats");
console.log("USD equivalent:", feeEstimate.feeEstimate.preferredCurrencyValueRounded, "USD");
4

Proceed with Transaction

Use the fee estimate to set appropriate limits for your transaction.
// Use fee estimate for Lightning payment
const payment = await wallet.payLightningInvoice({
  invoice: lightningInvoice,
  maxFeeSats: lightningFee.feeEstimate.originalValue + 5 // Add buffer
});

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Lightning Send Fee Estimates

Estimate fees for sending Lightning payments before making the transaction. getLightningSendFeeEstimate(params) Gets an estimated fee for sending a Lightning payment. Note: the actual fee assessed may be different from the fee estimate as it will be determined by the actual Lightning node routing. 
const feeEstimate = await wallet.getLightningSendFeeEstimate({
  encodedInvoice: "lnbcrt1u1pnm7ammpp4v84f05tl0kzt6g95g056athdpp8f8azvg6d7epz74z562ymer9jqsp5nc50gazvp0e98u42jlu653rw0eutcl067nqq924hf89q4la4kd9sxq9z0rgqnp4qdnmwu8v22cvq9xsv2l05cn9rre7xlcgdtntxawf8m0zxq3qemgzqrzjqtr2vd60g57hu63rdqk87u3clac6jlfhej4kldrrjvfcw3mphcw8sqqqqrj0q7ew45qqqqqqqqqqqqqq9qcqzpgdq5w3jhxapqd9h8vmmfvdjs9qyyssqj7lf2w4m587g04n4t0ferdv0vnwftzca0xuc9yxycng78cnhrvmyw2mzaa8t76jskpypqnwqhp9xh0vnwxz90jytd34vrmhcngsnl8qplz7ylk"
});

console.log("Lightning send fee estimate:", feeEstimate);

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Example Response

{
  feeEstimate: {
    originalValue: 100,
    originalUnit: 'SATOSHI',
    preferredCurrencyUnit: 'USD',
    preferredCurrencyValueRounded: 0.05,
    preferredCurrencyValueApprox: 0.048
  }
}

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Cooperative Exit Fee Estimates

Estimate fees for withdrawing funds back to the Bitcoin network (cooperative exit). getWithdrawalFeeQuote(params) Gets a fee quote for a cooperative exit (on-chain withdrawal). The quote includes options for different speeds and an expiry time and must be passed to withdraw before it expires. 
const exitFeeEstimate = await wallet.getWithdrawalFeeQuote({
  amountSats: 10000,
  withdrawalAddress: "bc1p5d7rjq7g6rdk2yhzks9smtbqtedr4dekq08ge8ztwac72sfr9rusxg3297"
});

console.log("Cooperative exit fee estimate:", exitFeeEstimate);

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Example Response

{
  feeEstimate: {
    originalValue: 1000,
    originalUnit: 'SATOSHI',
    preferredCurrencyUnit: 'USD',
    preferredCurrencyValueRounded: 0.50,
    preferredCurrencyValueApprox: 0.483
  }
}

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Swap Fee Estimates

Estimate fees for swap operations between different token types. getSwapFeeEstimate(amountSats) Gets the estimated fee for a swap of leaves. 
const swapFeeEstimate = await wallet.getSwapFeeEstimate(5000);
console.log("Swap fee estimate:", swapFeeEstimate);

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Understanding Fee Components

All fee estimates return a consistent structure with the following fields:

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Fee Estimate Fields

originalValue

The fee amount in the original unit (SATOSHI)

originalUnit

The unit of the original value (always ‘SATOSHI’)

preferredCurrencyUnit

The converted currency unit (e.g., ‘USD’)

preferredCurrencyValueRounded

The rounded value in preferred currency

preferredCurrencyValueApprox

The approximate value in preferred currency

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When Fees Apply

Understanding when different types of fees are charged: Lightning Payments:
  • Routing fees for Lightning Network transactions
  • Fees vary based on network conditions and routing path
  • Estimated fees may differ from actual fees charged
Cooperative Exits (L1 Withdrawals):
  • Bitcoin network transaction fees
  • Different speeds available (fast, medium, slow)
  • Fees increase with urgency
Swap Operations:
  • Fees for converting between different token types
  • Based on current market conditions
  • May include liquidity provider fees

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Fee Deduction Options

For cooperative exits, you can choose how fees are handled:

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Deduct Fee from Withdrawal Amount

When deductFeeFromWithdrawalAmount is set to true: 
const withdrawal = await wallet.withdraw({
  onchainAddress: "bc1p...",
  exitSpeed: "medium",
  amountSats: 10000, // This is the net amount you'll receive
  feeQuote: feeQuote,
  deductFeeFromWithdrawalAmount: true // Fee deducted from amount
});
// You receive: 10000 sats - fee

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Pay Fee Separately

When deductFeeFromWithdrawalAmount is set to false: 
const withdrawal = await wallet.withdraw({
  onchainAddress: "bc1p...",
  exitSpeed: "medium",
  amountSats: 10000, // This is the gross amount you'll receive
  feeQuote: feeQuote,
  deductFeeFromWithdrawalAmount: false // Fee paid separately
});
// You receive: 10000 sats (fee paid from wallet balance)

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Best Practices

  • Monitor fee variations - Fees can change based on network conditions
  • Include fee estimates in planning - Always factor in estimated fees when planning transactions
  • Add buffer to estimates - Add a small buffer to fee estimates for Lightning payments
  • Compare fee options - For withdrawals, compare different speed options
  • Check fee quotes regularly - Fee quotes expire, get fresh ones before transactions

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Complete Example

import { SparkWallet } from "@buildonspark/spark-sdk";

async function estimateAllFees() {
  const { wallet } = await SparkWallet.initialize({
    options: { network: "REGTEST" }
  });

  try {
    // 1. Lightning send fee estimate
    const lightningInvoice = "lnbcrt1u1pnm7ammpp4v84f05tl0kzt6g95g056athdpp8f8azvg6d7epz74z562ymer9jqsp5nc50gazvp0e98u42jlu653rw0eutcl067nqq924hf89q4la4kd9sxq9z0rgqnp4qdnmwu8v22cvq9xsv2l05cn9rre7xlcgdtntxawf8m0zxq3qemgzqrzjqtr2vd60g57hu63rdqk87u3clac6jlfhej4kldrrjvfcw3mphcw8sqqqqrj0q7ew45qqqqqqqqqqqqqq9qcqzpgdq5w3jhxapqd9h8vmmfvdjs9qyyssqj7lf2w4m587g04n4t0ferdv0vnwftzca0xuc9yxycng78cnhrvmyw2mzaa8t76jskpypqnwqhp9xh0vnwxz90jytd34vrmhcngsnl8qplz7ylk";
    
    const lightningFee = await wallet.getLightningSendFeeEstimate({
      encodedInvoice: lightningInvoice
    });
    console.log("Lightning fee estimate:", lightningFee.feeEstimate.originalValue, "sats");

    // 2. Cooperative exit fee estimate
    const exitFee = await wallet.getWithdrawalFeeQuote({
      amountSats: 10000,
      withdrawalAddress: "bc1p5d7rjq7g6rdk2yhzks9smtbqtedr4dekq08ge8ztwac72sfr9rusxg3297"
    });
    
    if (exitFee) {
      console.log("Exit fee estimate:", exitFee.fast, "sats (fast)");
      console.log("Exit fee estimate:", exitFee.medium, "sats (medium)");
      console.log("Exit fee estimate:", exitFee.slow, "sats (slow)");
    }

    // 3. Swap fee estimate
    const swapFee = await wallet.getSwapFeeEstimate(5000);
    console.log("Swap fee estimate:", swapFee);

    // 4. Use estimates for actual transactions
    const payment = await wallet.payLightningInvoice({
      invoice: lightningInvoice,
      maxFeeSats: lightningFee.feeEstimate.originalValue + 5 // Add 5 sats buffer
    });
    console.log("Lightning payment initiated with fee buffer");

  } catch (error) {
    console.error("Fee estimation failed:", error);
  }
}