Bitcoin is a decentralised payment network and the name of its native cryptocurrency, traded under the ticker BTC. For a quant, a BTC position needs two separate records: how many units can be spent, and what those units are worth in a chosen reporting currency. Network validation establishes neither a market price nor a provider's ability to honour withdrawals.
Bitcoin is one cryptocurrency, with its own transaction and issuance rules. Its blockchain is a public transaction history. The base accounting unit is the satoshi: 1 BTC = 100,000,000 satoshis, as defined in Bitcoin Core's amount constants.
A payment consumes an output, not part of an account
Bitcoin tracks unspent transaction outputs (UTXOs). Each identifies an amount and conditions that must be satisfied to spend it. A wallet's balance aggregates relevant outputs; the wallet manages the keys and transaction construction rather than storing coins inside itself.
An ordinary transaction references previous outputs as inputs and consumes each selected output in full. It creates new outputs, often including a payment and change controlled by the spender. A signature provides cryptographic spending authorisation where required by the output's conditions; it does not disclose the private key. Output conditions can be more complex than one signature. The developer transaction guide explains this model.
Consider a fictional on-chain payment, assuming the input is spendable and all authorisation and other validity conditions are met:
| Transaction component | Amount in BTC | Meaning |
|---|---|---|
| Input consumed | 1.000 | A previous UTXO is spent in full |
| Payment output | 0.600 | A new UTXO for the recipient |
| Change output | 0.399 | A new UTXO controlled by the spender |
| Transaction fee | 0.001 | The input value left after both outputs |
The new outputs total 0.999 BTC. The 0.001 BTC fee is not a third output of this transaction; it is available for the block's miner to claim through the separate coinbase transaction. No new bitcoin is issued by the ordinary payment. Bitcoin Core checks that inputs are available and that their value covers the outputs.
Original fictional Bitcoin transaction ledger, all amounts in BTC. The fee illustrates value accounting; it is neither a current network quote nor a recommended fee.
Payment size alone does not determine a real transaction's fee. Transaction weight, demand for block space and the sender's inclusion objective matter. Bitcoin Core's fee estimator uses virtual transaction size and a confirmation target; the chosen fee does not guarantee a confirmation time.
Signing, validation and mining do different jobs
Signing satisfies relevant spending conditions. Full-node validation checks transactions and blocks against consensus rules, including protection against spending an already-spent output. Mining uses proof of work to propose new blocks. A successful proof of work does not make an otherwise invalid block acceptable to a validating node.
Among valid competing histories, nodes follow the chain with the greatest cumulative proof of work. A transaction gains its first confirmation when included in the accepted chain; subsequent blocks add confirmations. Reorganisations can replace recent blocks, so confirmation depth provides probabilistic settlement assurance under assumptions about network connectivity and mining power, rather than an absolute guarantee. Block discovery and transaction inclusion do not run on a guaranteed timetable. These distinctions are described in the developer blockchain guide and Bitcoin's payment cautions.
The separate coinbase transaction allows the miner to collect a protocol-limited block subsidy, which introduces new BTC, plus eligible transaction fees. Fees redistribute existing value; they are not the subsidy. Mining also cannot supply a missing spending signature for someone else's output.
The BTC exposure on a trading screen
A directly held BTC balance and a custodial exchange balance involve different control arrangements. With a custodian, the provider controls the keys and the customer relies on the custody agreement and withdrawal process. The SEC staff's custody bulletin explains that distinction.
For valuation, quantity in BTC multiplied by a price in USD per BTC gives a USD mark. That mark can change while every UTXO remains unchanged. Record the venue, timestamp and quote currency: neither a confirmed transaction nor control of a private key removes market risk, liquidity risk or transaction costs.
Sources
- Bitcoin Developer Guide, Transactions: inputs, UTXOs, change and spending conditions.
- Bitcoin Developer Guide, Block Chain: independent validation, proof of work, competing histories and coinbase transactions.
- Bitcoin Core, consensus amount definitions and input-value validation: satoshi units and transaction value accounting. Current source inspected 27 October 2026.
- Bitcoin Core, estimatesmartfee RPC: virtual transaction size and confirmation-target fee estimates.
- Bitcoin Project, Some things you need to know: custody, confirmations and uncertain inclusion timing.
- SEC Office of Investor Education and Assistance, Crypto Asset Custody Basics for Retail Investors, 12 December 2025. Staff educational bulletin.
- John C. Hull, Options, Futures, and Other Derivatives, ninth edition, Global Edition, Pearson, 2018, §1.8, pp. 36–37: quantity and quoted-price cash-position valuation. Protocol claims use the Bitcoin sources above.