Sending Bitcoin

Last updated: 2026-04-13

Sending bitcoin with an air-gapped COLDCARD works in three steps: build the transaction on your computer or phone, sign it on the COLDCARD, broadcast. Your private keys never leave the device.

The PSBT Signing Flow

PSBT (Partially Signed Bitcoin Transaction) is an open standard (BIP-174) for passing unsigned transactions between devices safely.

Step 1: Build the Transaction

In your wallet app (Sparrow, Cove, Nunchuk, or similar):

  1. Enter the destination address
  2. Enter the amount
  3. Set the fee (higher fee = faster confirmation)
  4. The app creates an unsigned PSBT

Step 2: Transfer to COLDCARD

Via MicroSD (Mk5 and Q): Save the PSBT file to a MicroSD card, insert into your COLDCARD.

Via QR code (Q only): The wallet app displays the PSBT as an animated QR code. Scan it with the COLDCARD Q's camera. Faster than MicroSD for routine transactions.

Step 3: Verify and Sign

On the COLDCARD screen, verify:

  • Destination address — does it match what you intended?
  • Amount — is it correct?
  • Fee — is it reasonable?
  • Change — is change going back to your own wallet?

Always verify on the COLDCARD screen, not your computer. Your computer could be compromised. The COLDCARD screen is the source of truth.

Approve the transaction. The COLDCARD signs the PSBT and saves the signed version to MicroSD or displays it as a QR code.

Step 4: Broadcast

Transfer the signed PSBT back to your wallet app and broadcast to the Bitcoin network.

Address Verification

Before sending bitcoin to your wallet, verify the receive address on your COLDCARD's screen. Your computer could show you an attacker's address — clipboard malware is common. The COLDCARD always shows the correct address derived from your seed.

In Sparrow: right-click the receive address and select "Show on Hardware Wallet." The COLDCARD will display the address for you to confirm.

Coin Control

Every time you receive bitcoin, it creates a UTXO — an unspent transaction output. Think of UTXOs as individual coins in your wallet, each with its own history and amount.

When you send bitcoin, your wallet app selects which UTXOs to spend. This is coin control, and it matters for two reasons:

  1. Privacy: Merging UTXOs from different sources lets observers link those transactions to the same owner. Keep UTXOs from different contexts separate.
  2. Fees: More UTXOs in a transaction means a larger transaction and higher fees. Consolidate small UTXOs when fees are low.

Sparrow Wallet has excellent coin control — you can see, label, and select individual UTXOs for each transaction. Use labeling to track where each UTXO came from.

Transaction Fees

Bitcoin fees are based on transaction size in bytes, not the dollar amount. A 0.01 BTC transfer costs the same as 100 BTC if the transaction is the same size.

Fees fluctuate with network demand. Check the mempool for current rates. Non-urgent? A low fee works fine. Time-sensitive? Pay more.

Your wallet app shows the estimated fee before you sign. If it looks wrong, cancel and rebuild.