Why this is a money question, not a code question
Voltage drop lives in informational notes, not in enforceable text. 210.19(A) and 215.2(A) recommend holding a branch circuit to 3 percent and the whole run, feeder plus branch, to 5 percent. Nobody red tags you for 4 percent.
What happens instead is the motor runs hot, the lights dim when the compressor starts, the heater never reaches its rating, and the customer calls you back. You go out for free, you check everything twice, and the answer is that the wire was always too small. That callback costs more than the upsize would have.
Put the upsize on the quote
This is the part most calculators skip. When the run is too long for the wire you planned, the tool tells you the size it needs to be and how many feet of it you have to buy, counting both legs or all three. Put your own cost per foot in and you have the material number for the estimate. I am not going to guess your wire price for you, because copper moves weekly and your supply house is not mine.
Quoted up front, a conductor upsize on a long run is a line item the customer understands. Discovered after the fact, it is either a change order you have to justify or a loss you absorb. Same wire, different conversation, and the only variable is whether you ran the numbers before you wrote the price down.
The run that catches everybody
Detached garages and barns. A 100 foot trench feels short standing in the yard, and 100 feet of trench is 100 feet of conductor each way. At 40 amps on 240 volts, 10 AWG copper to a garage subpanel 150 feet out drops past 3 percent, and most people pull it anyway because 10 AWG is what a 30 amp circuit takes.
The other one is a well pump. Long run, inductive load, and a motor that will keep trying to start on low voltage until the windings give up. If you only run this calculator on one kind of job, run it on those.
What this does not do
This calculates drop from conductor resistance, which is the dominant term on the runs most residential and light commercial work involves. It does not account for conductor reactance, which starts to matter on large conductors carrying heavy current over long distances, or for power factor on strongly inductive loads. For those, Chapter 9 Table 9 has the reactance columns and the impedance figures you want.
It also does not size the conductor for ampacity. Drop and ampacity are two separate checks and the conductor has to pass both, so the size this page recommends is a floor from the drop side only.