What this converter does that the chart cannot
The drill bit size chart is a lookup table of 354 fixed, real drill sizes. It's exhaustive for what actually exists as a standard drill, but it can't answer "what's the nearest drill to 0.2000 inch exactly" the way a live tool can, because 0.2000" itself isn't one of the 354 rows. This converter accepts any value, standard size or not, computes its exact conversion to every unit, and then separately reports the nearest real drill in each of the four overlapping systems (number, letter, fractional, metric), with the exact gap to each one.
Why drill systems overlap in the first place
Number, letter, fractional, and metric drills were each standardized independently, at different points in manufacturing history, and none of them ever replaced another. A single physical hole size can usually be called out in two or three of these systems at once, and the "closest" drill in each system is rarely the exact same physical size as the closest in another. That's why this tool reports all four instead of collapsing them into one answer: the right one depends on which drill index you actually have in the drawer.
Picking a drill when your target falls between two sizes
Real-world targets, a reamer's recommended pilot, a fit calculated from a tolerance chart, a metric print converted to inches, rarely land exactly on a standard drill. When that happens, the two bracketing drills shown here (the nearest one below your target and the nearest one above) are your real options. Going to the smaller, lower drill leaves the hole tight; going to the larger, upper drill leaves it loose. Which direction is safer depends entirely on what the hole is for: a press fit or a hole that will be tapped usually wants to stay on the tight side, while a clearance hole for a bolt shank usually tolerates going slightly loose more easily than tight.
Tolerance implications of "nearest"
The offset shown next to each nearest drill is not a rounding error to ignore, it's the actual dimensional difference you'd be accepting by using that drill instead of your exact target. For a rough clearance hole, a few thousandths either way rarely matters. For a press fit, a slip fit with a specified tolerance class, or a hole that feeds into a reamer afterward, that same offset can be the difference between a part that assembles correctly and one that doesn't. Treat the offset as data to check against your actual tolerance requirement, not as a number to ignore because the tool called it "nearest."
How the conversions are computed
Inch-to-millimeter conversion uses the exact definition, 1 inch equals 25.4mm, with no rounding beyond display precision. The nearest-64th figure rounds your exact decimal value to the closest 1/64" increment, the finest graduation typically used in shop fractional measurement, clamped to a minimum of 1/64" the same way the drill bit size chart handles its smallest sizes. Both of those are pure computation from your input. The nearest-real-drill results are a separate lookup against the actual verified dataset, not a computed guess, which is why they can only return a size that genuinely exists as a standard drill.