Can the selected sample coins cover the payment?
How does adding an input change the result?
Select sample coins A and B, worth 30,000 and 50,000 satoshis. Their total is 80,000. For the 60,000-satoshi payment, the classroom size estimate is 11 + 68 × 2 + 31 × 2 = 209 vbytes. At 3 sat/vB the fee is 627 satoshis and change is 19,373 satoshis.
Select coin C as well. You add 80,000 satoshis, but also one assumed input costing 68 vbytes. The estimate becomes 277 vbytes and the fee becomes 831 satoshis at the same rate. Change becomes 99,169 satoshis. The exercise shows why input selection affects both available value and fee arithmetic.
Which assumptions explain the estimate?
The model always reserves two outputs and uses 11 vbytes of overhead, 68 per input and 31 per output. These are explicit classroom approximations for common native witness key-hash shapes, not consensus constants. Final size depends on real serialization and signatures. No signing takes place here.
A zero remainder still keeps that two-output estimate. A small positive remainder is shown arithmetically without applying a dust threshold. Actual local policy may reject an uneconomic output. The tool deliberately does not silently add a small remainder to the fee or change the payment amount.
What happens with an impossible payment?
A shortfall is shown instead of negative change. Empty selections, fractional satoshis and invalid rates produce a readable error. Editing any field clears the earlier result so it cannot be mistaken for the new entries. Values remain in the page only; this calculator code does not transmit or save them.
Where do the accounting and weight rules come from?
Sources: Input and output serialization; Witness weight and virtual size. All selected amounts and fee rates in these tools are invented examples, not network observations.