Aperture Diagonal & Drill Size Calculator
Get the corner-to-corner diagonal of a rectangular aperture plus the stocked tool that covers it — the drill that clears the diagonal, or the cutter that fits the short side and the corner radius it leaves.
Formula
Reference: Geometry; tooling grid is stocked metric PCB tooling in 0.05 mm steps. 1 in = 25.4 mm exactly.
How It Works
A rectangular opening in a board can be made two ways, and the two have opposite constraints.
Drilling it means finding a round hole that fully contains the rectangle. A circle contains a rectangle only if its diameter reaches corner to corner, so the requirement is the Pythagorean diagonal of the two sides. Tooling is stocked on a grid, so the answer is the next size at or above that diagonal — rounding down would leave the corners uncut, which is not a small error but a hole that does not do its job.
Routing it reverses the constraint. The cutter has to fit inside the opening, so it is bounded by the short side, and the answer is the largest stocked tool at or below it. That produces a second consequence that catches people out: a routed corner can never be sharper than the cutter radius. The opening comes back with corners of half the cutter diameter whatever the drawing says, and no amount of CAM programming removes it.
When the corner radius becomes a large fraction of the short side, the routed opening stops reading as a rectangle at all and starts to look like an oval. For a slot that has to clear a rectangular part, that matters.
Board shops stock metric tooling on a 0.05 mm grid, and this calculator reports against that grid. A callout that lands between sizes will be quoted against the next one up, so knowing which tool your dimension actually implies is worth more than the exact decimal.
Where the diagonal exceeds the largest stocked drill, the opening is a routed feature rather than a drilled one, and where the short side is narrower than the smallest cutter it cannot be routed at all. Both are real answers and the calculator gives them rather than naming a tool that does not exist.
Worked Example
Take a 2.0 mm by 1.0 mm rectangular aperture.
The corner-to-corner diagonal is:
√(2.0² + 1.0²) = √5 = 2.2361 mm
To drill it as a single round hole you need a tool at least that large. On the 0.05 mm grid the next size up is 2.25 mm, which is 0.0139 mm larger than strictly required — the price of a stocked grid.
To rout it instead, the cutter must fit within the 1.0 mm short side. The largest stocked tool at or below that is 1.0 mm exactly, which leaves a corner radius of:
1.0 / 2 = 0.5 mm
That radius is half the short side, so the routed opening will be a 2.0 mm by 1.0 mm slot with fully rounded ends — an obround, not a rectangle. The calculator raises that as a warning, because a drawing showing square corners and a part expecting them will not agree with what arrives.
If the rectangle were 2.0 mm by 0.08 mm instead, the short side would be narrower than the smallest stocked cutter and the opening could not be routed at all — which is the answer, not a failure to compute one.
Practical Tips
- ✓For small openings, a single drill hit is far cheaper than a routed slot. If the part clears the round hole, the shape does not matter.
- ✓If a square corner is functionally required, design it as a corner relief rather than expecting the router to produce it.
- ✓Quote the tool size on the drawing rather than the theoretical dimension — it removes an ambiguity the fabricator would otherwise resolve for you.
- ✓Where the corner radius is more than about a quarter of the short side, redraw the feature as an obround; that is what you will get anyway.
- ✓For a slot that must clear a rectangular part, add the corner radius to the clearance check, not just the nominal dimensions.
- ✓Very large openings are routed, not drilled — this calculator says when you have crossed that line.
Common Mistakes
- ✗Sizing the drill from the long side rather than the diagonal. A hole the length of the rectangle still leaves two corners uncovered.
- ✗Expecting square corners from a routed opening. The cutter is round and always leaves a radius of half its diameter.
- ✗Rounding the drill size down to the nearest stocked tool. Down is the one direction that cannot work.
- ✗Specifying an exact decimal that falls between stocked sizes and assuming the shop will hit it. They will quote the next size up.
- ✗Forgetting that the corner radius scales with the cutter. Choosing a smaller cutter gives sharper corners at the cost of a slower, more expensive rout.
Frequently Asked Questions
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