What gauge should I use for drop D?

Dropping the low string from E to D with no gauge change is the single most common retuning on an electric guitar, and the honest answer is: a stock set survives it, but it does not stay at the tension the set was built for. A .042" nickel-wound low string tuned to standard E2 at a 25.5" scale carries about 14.8 lbs of tension. Drop that same string to D2 with nothing else changed and it falls to about 11.7 lbs — roughly a fifth less pull on that one string, with the other five unchanged.

That is not dangerous, and plenty of players run exactly this setup happily. What it changes is feel and response, not safety: a looser string flops more under a hard pick attack, buzzes more readily against the frets at the same action height, and the pitch bends slightly flatter under the same finger pressure than it did in standard tuning. None of that is a defect — it is simply a different string doing a lower-tension job than it was gauged for.

If the goal is to get that string back to roughly the tension it carried in standard tuning — the "feels the same, just lower" goal most players actually want from drop D — the fix is a heavier single string, not a whole heavier set. Working through the same nickel-wound table at D2 and a 25.5" scale: a .046" reaches about 13.9 lbs, a .048" about 15.0 lbs, and a .049" about 15.6 lbs. The .048" is the closest single-gauge match to that standard-tuning .042" target of 14.8 lbs, which is why "drop-D sets" sold as a unit almost always ship a .048–.052" low string paired with a standard-gauge set for the rest — only the string that actually changed pitch gets a heavier gauge, because only that string's tension actually dropped.

The practical version of this: buying a single heavier low string (a .048" or .052" nickel-wound, sold individually by every major manufacturer) to add to an otherwise-normal set is cheaper than buying a dedicated "drop tuning" set, and it solves the exact problem — restoring the low string's tension — without changing anything about the five strings that never moved. A full heavier set only makes sense if every string is also being detuned (a half-step-down drop-D scheme, for instance), because then every string has lost tension, not just one.