“Subtract the pitch” leaves 92% of the thread, not 100% — the same 92% for every size. And a #7 drill is neither a fraction nor a letter.
Calculator
I want to:
Major diameter 6 mm, pitch 1 mm. Minor diameter 4.9175 mm.
75% is the usual target. A higher figure is stronger by very little and much harder to cut; a lower one is easier and weaker.
Try:
Drill5.1881 mm
Nearest bit5.2 mm (+0.0119 mm → 73.9% thread)
For a M6 × 1.0 at 75% thread engagement. The exact figure is theoretical; you drill with the bit you have, so the real engagement is the one shown beside it.
Against the rule everyone is taught
Diameter minus pitch
5 mm → 92.38% thread
Your 75% target
5.1881 mm
The rule of thumb does not give a full thread — it gives 92.38%. And it is 92.38% for every size, metric or imperial, because the pitch cancels out of the arithmetic. That is well above the 75% normally recommended: the extra thread adds almost nothing to pull-out strength and needs about a quarter more torque to cut, which is how taps get broken in blind holes.
The nearest bit in each series
Metric5.2 mm — 5.2 mm (+0.0119 mm → 73.9% thread)
Fractional inch13/64" — 5.1594 mm (-0.0287 mm → 77.65% thread)
Number gauge#6 — 5.1816 mm (-0.0065 mm → 75.6% thread)
Letter gaugeA — 5.9436 mm (+0.7555 mm → 5.21% thread)
Four incompatible series, all sold in the same box. A bit that is slightly large gives less thread and taps more easily; one that is slightly small gives more thread and much more torque. When in doubt, err large.
If the bolt passes through instead
Close fit6.4 mm
Medium fit6.6 mm
Coarse fit7 mm
Clearance holes from ISO 273 — the hole the bolt passes through, not the one it threads into. Note how much larger these are than the tap drill above.
What this converter covers
The hole for any thread at any engagement, the nearest real bit in all four drill series with the thread each would actually leave, and the clearance hole for when the bolt passes through instead.
Tap drills for metric coarse and fine, UNC and UNF
Any thread engagement, not just the one figure a chart picked
The nearest bit in fractional, number, letter and metric series at once
What each of those bits would actually leave, as a percentage
ISO 273 clearance holes, flagged where they are derived rather than standard
Derived from ISO 68-1 The 92% rule exposed All four series Clearance holes too
Free, no signup — exact by definition, not an estimate.
Updated 7 September 2026
At a glance
Formula shown
full depth = 1.082532 × pitch · tap drill = major − (engagement ÷ 100) × 1.082532 × pitch
Scenario support
M6 at 75% = 5.19 mm, not the 5.0 the rule gives · 1/4-20 at 75% = 5.32 mm
Educational estimate
Planning support from the values you enter — not professional advice.
Subtract the pitch is not a full thread
Every workshop teaches the same rule. For a metric thread, the tap drill is the diameter minus the pitch: M6 × 1.0 gets a 5 mm hole. For an imperial thread, it is the diameter minus one over the threads per inch.
It is a good rule and it is widely misunderstood. Most people believe it gives a complete thread. It does not — it leaves 92.4% of one, and it does so for every size in both systems.
The reason is in the thread profile. ISO 68-1 builds the metric thread on a fundamental triangle of height H = P × √3 ⁄ 2, of which the internal thread occupies 5H/8 on each flank. Across the diameter that is 2 × 5/8 × √3/2 = 1.082532 pitches. So a full thread needs the hole cut down by 1.0825 × P, and subtracting a plain P gets you 1 ÷ 1.082532 = 92.4% of the way there.
The pitch cancels, which is why the answer is the same figure every time — M3, M12, 1/4-20 or 1/2-13, the rule of thumb lands on 92.4% of the thread. It is a genuinely useful constant to know, and it is not 100%.
Why 75% is the right target
Given that, the obvious reaction is to drill smaller and get the missing 7.6%. That is exactly backwards. The recommended target for most work is 75%, which means drilling larger than the rule of thumb, not smaller — 5.19 mm for an M6 rather than 5.00.
The trade is heavily lopsided. Going from 75% to full engagement adds only a few percent to the thread’s pull-out strength, because a properly engaged fastener fails in the bolt long before it strips the threads. But the torque needed to cut the thread rises with roughly the square of the depth of material being removed, so that last quarter of the thread is where most of the effort — and most of the broken taps — actually live. Cutting at 92.4% instead of 75% takes around a quarter more torque for a gain you will never measure.
This matters most in exactly the situations where a broken tap is worst: blind holes, small sizes where the tap is fragile, tough materials, and hand tapping where you cannot feel the load building until it is too late. A snapped tap in a part you have already spent an hour on is a much bigger problem than a thread that is 3% weaker.
Where higher engagement genuinely earns its keep is thin material, where there are only a couple of threads engaged and every one counts, and soft materials like aluminium and plastics where the thread strips more readily. Where lower engagement — 60 to 65% — is normal is tough alloys and stainless, where tapping torque is the binding constraint. The calculator takes the percentage as an input for precisely that reason: no single column of a chart is right for every material.
Four drill series, all in one box
A drill index contains sizes from four incompatible numbering systems, and the order they sit in follows none of them.
Fractional inch sizes step in 64ths and are the easiest to read. Number gauges run #1 to #80 and run backwards: #1 is the largest at 5.79 mm and #80 the smallest at 0.34 mm, for the same historical reason wire gauges do. Letter gauges run A to Z and run forwards, A being the smallest at 5.94 mm — which means letter A is larger than number #1, and the two series do not overlap so much as sit end to end. And metric sizes step in tenths of a millimetre through the useful range.
Every drill between 4.9 and 5.6 mm, in ascending order
Diameter
Size
Series
4.915 mm
#10
Number
4.978 mm
#9
Number
5.000 mm
5 mm
Metric
5.055 mm
#8
Number
5.100 mm
5.1 mm
Metric
5.105 mm
#7
Number
5.159 mm
13/64″
Fraction
5.182 mm
#6
Number
5.200 mm
5.2 mm
Metric
5.220 mm
#5
Number
5.300 mm
5.3 mm
Metric
5.309 mm
#4
Number
5.400 mm
5.4 mm
Metric
5.410 mm
#3
Number
5.500 mm
5.5 mm
Metric
5.556 mm
7/32″
Fraction
Sixteen bits inside seven tenths of a millimetre, from three different series, in an order that only makes sense once they are all converted to one unit. This is why a chart that names “#7” is unhelpful if your index is metric, and why the calculator above gives the nearest size in all four at once along with the thread each would really leave.
Tap drill against clearance hole
Two completely different holes get drilled for the same bolt, and mixing them up is a common and expensive mistake.
The tap drill is for the part the bolt threads into. It is smaller than the bolt — 5.19 mm for an M6 — because the thread has to be cut into the material that is left.
The clearance hole is for the part the bolt passes through. It is larger than the bolt — 6.4 to 7.0 mm for an M6, depending on the fit — so the bolt slides through freely and the joint is clamped by the head rather than by the shank binding in the hole.
The gap between them is bigger than people expect: for an M6 it is 5.19 against 6.4 mm, a difference of over a millimetre. Drill the clearance size into the part you meant to tap and there is nothing left to cut a thread in. Drill the tap size into the part the bolt should pass through and the bolt binds, the joint never pulls up tight, and the fastener carries load in shear it was not meant to.
ISO 273 gives three clearance fits — fine, medium and coarse. Fine is for accurate location, medium is the general-purpose default, and coarse is for rough work or where several holes have to line up despite tolerance stack-up. Those are a published table rather than a formula, which is why this page stores them as one and says explicitly when a figure has been derived instead.
Practical notes on not breaking taps
The arithmetic above is the easy part. A few things about the physical job change what the right number actually is.
The hole comes out bigger than the drill. A twist drill in a hand-held tool routinely cuts a tenth of a millimetre oversize or more, and that is enough to move the engagement by several percent on a small thread. If you want the engagement you asked for, drill on a press with the work clamped — otherwise treat the target as approximate and lean towards the larger bit rather than the smaller.
Err large, not small. When the exact size is not in the box, the bigger neighbour gives slightly less thread and taps much more easily. The smaller neighbour gives a few percent more thread and a great deal more torque. Given how little strength is at stake, the choice is not close.
Blind holes need the tap to reach. A taper tap does not cut a full thread to its tip, so a blind hole has to be drilled deeper than the thread is needed — and the drill point itself adds roughly a third of the drill diameter of depth that has no full-diameter hole in it.
Back the tap off. Turning half a turn back for every turn or so forward breaks the chip, which is what stops it packing into the flutes and jamming — the mechanism that actually snaps most taps in blind holes. Use cutting fluid appropriate to the material, and if the torque suddenly rises, stop and clear the hole rather than pushing through it.
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The thread arithmetic here is derived from the ISO basic profile rather than read off a chart, which is what lets the engagement be an input instead of a fixed column — the 1.0825 that appears in every reference is computed on this page as 2 × 5/8 × √3/2. The drill diameters are the opposite case: the number and letter gauges are defined by table and follow no formula at all, so they are stored as tables. So are the ISO 273 clearance holes, after an earlier draft of this page computed them from a plausible ratio and got an M6 wrong in both directions.
ISO 68-1 — ISO general purpose screw threads: basic profile, metricInternational Organization for Standardization · verified 2026-09-07 · The fundamental triangle of the metric thread, from which the 5H/8 internal thread height and therefore the 1.082532 × pitch full engagement depth used throughout this page are derived rather than copied
ISO 273 — Fasteners: clearance holes for bolts and screwsInternational Organization for Standardization · verified 2026-09-07 · The fine, medium and coarse clearance hole diameters tabulated on this page for the metric sizes — a table, not a formula, which is why they are stored as one
These are theoretical figures from the nominal thread geometry, and a workshop is not a geometry problem. Real holes come out larger than the drill that made them — a twist drill in a hand-held tool can oversize by a tenth of a millimetre or more, and that alone moves the thread engagement by several percent — so the engagement you get is not the one you asked for. Material matters too: the recommended engagement is lower in tough or gummy materials and higher in soft ones, and thin sheet, castings and plastics all behave differently again from the bar stock these figures assume. Nothing here covers tapered pipe threads, thread-forming taps, helical inserts or thread milling, all of which use different hole sizes entirely. For anything structural, anything that will be loaded in fatigue, or anything where failure has consequences, use the fastener manufacturer's own data and a current engineering handbook rather than a converter — and where a drawing or specification names a tap drill, that number wins over this page.
Published the Tap Drill and Drill Size Converter: tap drills for metric coarse and fine, UNC and UNF at any thread engagement, plus the nearest bit in all four drill series.
Derives the thread depth from the ISO 68-1 basic profile rather than reading a chart, which is what allows engagement to be an input instead of a fixed column -- the 1.0825 in every reference is computed here as 2 x 5/8 x root 3 over 2.
Shows that the universal subtract-the-pitch rule leaves 92.4 percent of the thread, not 100, and that it is the same 92.4 percent for every size because the pitch cancels out of the arithmetic.
Explains why the recommended 75 percent target means drilling LARGER than the rule of thumb: the extra thread adds almost nothing to pull-out strength, while costing about a quarter more tapping torque, which is where broken taps in blind holes come from.
Gives the nearest bit in all four series at once with the engagement each would really leave, because the series interleave -- sixteen bits from three different series sit inside seven tenths of a millimetre around 5 mm, and letter A is larger than number 1.
Separates the tap drill from the clearance hole, which for an M6 differ by over a millimetre, and states which fits are read from ISO 273 and which are derived because that standard covers the metric series only.
Verified by 103 automated cases, including full monotonicity sweeps of the number and letter series and exact inverses between hole size and engagement.
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