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For a common metric cutting tap, the rule of thumb is: tap drill diameter ≈ nominal thread diameter − thread pitch. For an M10 × 1.5 thread, that gives 10 mm − 1.5 mm = 8.5 mm. Treat this as a quick estimate. It applies to metric cutting taps only, and it does not replace a thread-specific chart or the tap manufacturer’s recommendation for inch threads, special profiles, cold-form taps, or critical holes.
How to calculate a metric tap drill size
The shortcut works because a tapped thread needs a pilot hole slightly smaller than the thread’s major diameter. Subtracting the pitch leaves enough material in the hole for the tap to cut the thread teeth. To apply it:
- Read the thread designation. In M10 × 1.5, the M10 is the nominal major diameter (10 mm) and 1.5 is the pitch in millimetres.
- Subtract the pitch from the nominal diameter: 10 − 1.5 = 8.5 mm.
- Select the nearest standard drill that matches the chart value. For M10 × 1.5, the drill is 8.5 mm.
- Cross-check against a metric tap-drill table. Machinery’s Handbook Pocket Companion, hosted by IEM Canada, lists 8.50 mm for M10 in its metric tapping table, so the shortcut and the reference table agree for this size.
The shortcut is a cutting-tap estimate. Confirm the tap type on the tap itself before drilling, because the same thread can call for a different hole if the tap forms rather than cuts the thread.
When the rule does not apply
The formula is specific to common metric cutting taps. Use the table below to decide which source governs the hole.
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12Pcs Metric Drill and Tap Set with Tap Wrench for Metal, from M2 to M6
- 【Perfect 12Pcs Metric Tap and Drill Set With Tap Wrench】Include 6 Size of Taps: M2x0.4, M2.5x0.45, M3x0.5, M4x0.7, M5x0.8,& M6x1；6 Size of Drills: 1.6mm, 2.05mm, 2.5mm, 3.3mm, 4.2mm, & 5mm；and 1pc Tap Wrench.This tap and drill tool kit will meet all your tapping needs.
- 【Top Quality】All the taps and drill bits in our tap tool sets are made of top-grade M2 steel and are titanium-plated to ensure its high hardness, rust proof, sturdy and durable.Fits most drill driver brands on the market. The taps in this set can be driven by hand and machine.
- 【Widely Applications】This drill tap set are designed to Create new threads or re-thread damaged or jammed threads. Thread tap can process through holes or shallow blind holes, with downward chip removal, right-hand cutting, and fast and accurate tapping. Suitable for most metals, stainless steel, cast iron, aluminum, aluminum alloy, bronze and hard plastics.
- 【Exquisite Storage Box】All taps and drill bits are stored in an orderly manner in this storage box, which is very convenient for accessing and storing tap tools and convenient for carrying around.
- 【Warm Tips】Drill holes with a twist drill bit first, then tap with a thread tap. Tapping and drilling angle should be 90 degrees vertical and stable. The higher the hardness, the slower the speed. The tapping depth should not exceed 1.5 times the diameter of the tap (For example, the diameter M6x1 is about 6mm, and the depth should not exceed 9mm.)
| Situation | What the reviewed sources establish | What to use |
|---|---|---|
| Common metric cutting tap | Nominal diameter minus pitch gives a quick estimate; M10 × 1.5 gives 8.5 mm. | The shortcut, checked against a metric tap-drill table. |
| Inch thread families | The shortcut is metric-only. Allied Machine & Engineering’s technical guide includes inch and metric formulas and tables. | A tap-drill chart for the matching inch series. |
| Cold-form (form) taps | Machinery’s Handbook Pocket Companion gives a separate cold-form table, distinct from fluted-tap sizes. Its sizes are calculated to give 60 to 75 percent full thread. | The cold-form table, or the tap maker’s recommended core hole. |
| Special thread profiles | The rule is not a universal formula for all thread types. | A chart matched to the exact thread profile. |
| Unusual materials or critical holes | The reviewed sources do not establish one drill adjustment that works for every material or blind-hole condition. | The tap manufacturer’s recommended drill. |
Thread engagement and why drill size matters
Thread engagement is the percentage of a full thread the tapped hole actually produces. A hole drilled to the rule’s size is a starting point for that percentage, not a guarantee of it.
The 75 percent convention
The FM Carbide tap drill chart describes about 75 percent thread engagement as a common general-purpose convention. Its publication date is not confirmed in the reviewed material, so treat the figure as a guideline. A given standard drill, material, and cutting condition can produce a different result.
Rank #2
Drill Tap Set 6Pcs, Titanium Plated Combination Taps Drill Bit Set
- Metric Tap Drill Set: Set of 6 Drill Tap Bits - M3x0.5, M4x0.7, M5x0.8, M6x1, M8x1.25, M10x1.5. Not easy to shake and drill accurately. Suitable for processing soft metals (such as aluminum alloy, zinc alloy, tin alloy), thin boards and plastics
- Easy to Use: The quick-change composite cone adopts the drill edge principle, drilling and tapping in one step, and can be used for drilling, tapping, deburring and countersunk holes in wood, copper, brass, aluminum, sheets and plastics, greatly Save processing steps and time costs, and improve work efficiency
- High Versatility: Each tap bit is engraved with size markings, making it easy for you to choose the size that suits you. The tapered end on the hex shank prevents over-tapping, and the back taper over the tap prevents over-drilling and damaging the threads
- Easy Quick Change: The hex handle fits all power tools more easily and securely, especially for locking quick change chucks. These drill taps are a must if you need to fabricate in large quantities; you can use aluminum, plastic, etc.; they are easy to use
- Portable: This drill bit and tap set comes with a portable storage case for easy use and storage. If you encounter any problems when using the product, please do not hesitate to ask us and get satisfactory customer service
Theoretical and probable engagement
Allied Machine & Engineering’s Recommended Cutting Data: Technical Guide (2025) gives formulas for both theoretical and probable percentage of full thread, in inch and metric systems. The guide notes that probable results can drop under less ideal cutting conditions, so the theoretical figure is an upper reference rather than an expected result.
How drill size shifts the cut
- Larger pilot hole: leaves less material for the tap, so the thread engagement is lower.
- Smaller pilot hole: leaves more material for the tap to cut, which raises the engagement and the cutting load on the tap.
- Nominal drill size: stay with the chart value unless the tap maker or a thread-specific table calls for a different one.
Checklist before you drill
- Confirm the full thread designation, including nominal diameter and pitch (metric) or threads per inch (inch).
- Confirm the tap type: cutting or cold-form. Only the cutting-tap shortcut is covered above.
- Check the exact drill size against a chart for that thread series. A metric tap drill set covers the common sizes, but the size you need still has to match the table.
- For a critical, blind, or unusual-material hole, follow the tap manufacturer’s recommended drill rather than the shortcut.
What the sources establish, and their dates
- Allied Machine & Engineering, Recommended Cutting Data: Technical Guide (2025): inch and metric tap-drill formulas, with probable and theoretical engagement.
- Machinery’s Handbook Pocket Companion (hosted by IEM Canada; publication year not confirmed): metric tapping table listing 8.50 mm for M10, and the separate cold-form table.
- CGTK, “Tap Drill Sizes” (copyright 2020): the metric cutting-tap rule with the M10 × 1.5 example.
- FM Carbide, “Tap Drill Chart — Inch & Metric” (publication date not confirmed): the approximately 75 percent engagement convention and the manufacturer-recommendation caveat.
Because the reviewed sources do not establish a single drill adjustment for every material or blind-hole situation, the rule should be read as a starting estimate for common metric cutting taps and nothing wider.
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18Pcs Metric Drill and Tap Set with Tap Wrench for Metal, from M2 to M12
- 【Perfect 18Pcs Metric Tap and Drill Set With Tap Wrench】Include 9 Size of Taps: M2x0.4, M2.5x0.45, M3x0.5, M4x0.7, M5x0.8, M6x1, M8x1.25, M10x1.5, & M12x1.75；9 Size of Drills: 1.6mm, 2.05mm, 2.5mm, 3.3mm, 4.2mm, 5mm, 6.75mm, 8.5mm, & 10.2mm；and 1pc Tap Wrench.This tap and drill tool kit will meet all your tapping needs.
- 【Top Quality】All the taps and drill bits in our tap tool sets are made of top-grade M2 steel and are titanium-plated to ensure its high hardness, rust proof, sturdy and durable.Fits most drill driver brands on the market. The taps in this set can be driven by hand and machine.
- 【Widely Applications】This drill tap set are designed to Create new threads or re-thread damaged or jammed threads. Thread tap can process through holes or shallow blind holes, with downward chip removal, right-hand cutting, and fast and accurate tapping. Suitable for most metals, stainless steel, cast iron, aluminum, aluminum alloy, bronze and hard plastics.
- 【Exquisite Ultra-Hard Storage Box】All taps and drill bits are stored in an orderly manner in this exquisite ultra-hard Indexed storage box, which is very convenient for accessing and storing tap tools and convenient for carrying around. 【Note】The drill bit is very sharp and is tightly fixed to the drill bit holder. Please wear gloves or use tools to remove it from the holder.
- 【Warm Tips】Drill holes with a twist drill bit first, then tap with a thread tap. Tapping and drilling angle should be 90 degrees vertical and stable. The higher the hardness, the slower the speed. The tapping depth should not exceed 1.5 times the diameter of the tap (For example, the diameter M6x1 is about 6mm, and the depth should not exceed 9mm.)
Rank #4
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Kraftek Metric(M3 - M10) 6pcs Combination Drill and Tap Bit Set, Ultra TiAlN Coating M2 Drill Tap Set for One-Step Drilling, Tapping, and Deburring
- 1-STEP DRILL&TAP SET - Allows you to complete hole drilling, tapping, and deburring/countersinking in one simple operation using a power drill to save time and minimize labor
- TAPS A VARIETY OF MATERIAL - Simplifies the process of tapping up to 10-gauge metal and creates holes in a variety of materials including stainless steel, aluminum, brass, copper, plastic, and mild steel
- HEAVY-DUTY CONSTRUCTION - Hardened high-speed steel withstands heavy-duty use on the jobsite and offers a longer life than carbon-steel tools
- TiAlN COATING - With 89HRC surface hardness, harder materials can be cut and higher cutting speeds can be achieved.
- BACK-TAPERED AND HEX SHANK: Unique back-tapered design protects against thread damage from over-drilling, while the sturdy hex shank ensures a strong connection to the drill chuck
Rank #3
Accusize Industrial Tools HSS Tap and Drill Set, Metric, 18 Pc, 0001-0052
- 18 Pcs/Set H.S.S. Tap & Drill Set, Metric
- Size of Taps: M2.5X0.45, M3.0X0.50, M3.5X0.60, M4.0X0.70, M5.0X0.80, M6.0X1.00, M8.0X1.25, M10.0X1.50, & M12.0X1.75
- Size of Drills: 2.05mm, 2.50mm, 2.90mm, 3.30mm, 4.20mm, 5.00mm, 6.70mm, 8.50mm, & 10.20mm
- HSS Fully Ground
- Drills and taps are matched, improving work efficiency
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