Metal Removal Rate Calculator
Calculate CNC milling and turning Metal Removal Rate (MRR in cm³/min and in³/min), estimated spindle power (kW / HP), and chip load.
Open toolTap Drill Size Calculator (Metric ISO & Unified Inch Cutting vs. Forming)
Compute exact pre-tapping hole diameters (`mm` and `inches`) for **ISO Metric (`M`)** and **Unified Inch (`UNC`/`UNF`)** 60° threads: compare 75% thread height vs. the `D − P` machinist rule and **Cutting Tap** vs. **Cold-Forming Roll Tap** bore sizes.
Free to useNo sign-upRuns in your browser
Enter your details
Enter the inputs, then select Calculate Tap Drill Size.
Your result
Enter your details
Result will appear here
How Cutting & Cold-Forming Tap Drill Formulas Work
The tap drill diameter sets the minor diameter of the internal thread, determining how much material the tap flutes must shear (or displace) and how much flank area engages the mating bolt.
Cutting Tap: D_drill = D - (P * Pct) / 76.9800; Forming Tap: D_drill = D - (P * Pct) / 147.0588; Unified Pitch P_in = 1 / TPI (P_mm = 25.4 / TPI).
In Unified Inch mode, the calculator automatically converts `D` (`in`) and `TPI` into both decimal inches and millimeters.
Standard Metric & Unified Inch Tap Drill Quick-Reference Table (75% Thread)
Exact 75% theoretical drill sizes, standard shop `D − P` drill bits, and cold-forming tap drill sizes for common coarse threads.
| Thread Size & Pitch | Cutting Tap (75% Exact) | Standard Shop Drill (D − P) | Forming / Roll Tap (75%) |
|---|---|---|---|
| `M6 × 1.0` (Coarse) | `5.03 mm` (`0.1979 in`) | `5.00 mm` (`76.98%` thread) | `5.49 mm` (`5.50 mm` drill) |
| `M8 × 1.25` (Coarse) | `6.78 mm` (`0.2670 in`) | `6.80 mm` (`75.89%` thread) | `7.36 mm` (`7.40 mm` drill) |
| **`M10 × 1.5` (Default)** | **`8.54 mm` (`0.3362 in`)** | **`8.50 mm` (`76.98%` thread)** | **`9.24 mm` (`9.25 mm` drill)** |
| `1/4"-20 UNC` (`0.25 in`, `20 TPI`) | `0.2013 in` (`5.11 mm`) | `#7 Drill` (`0.2010 in`) | `0.2245 in` (`#1` or `7/32"`) |
How to Calculate Your Tap Drill Diameter in 3 Steps
Select your thread system, enter diameter and pitch/TPI, and pick your tap geometry.
Choose Metric ISO (mm) or Unified Inch (TPI)
Pick `Metric ISO` when working with millimeter pitch threads (`M3` through `M68`) or `Unified Inch` when working with threads per inch (`#4-40`, `1/4-20`, `3/8-16`).
Enter nominal major diameter D and pitch P (or TPI)
Enter major diameter `D` (`10` for `M10`, `0.25` for `1/4"`) and pitch (`1.5` mm or `20` TPI). Note that pitch must be strictly smaller than half the major diameter.
Select Cutting Tap vs. Forming Tap and target thread %
Choose `75%` for general aluminum, brass, and mild steel, or lower to `60%–65%` for work-hardening stainless steels and titanium.
Worked Machining & CNC Tapping Examples
See how thread percentage and tap type change hole diameter across Metric and Unified threads.
M10 × 1.5 Cutting Tap at 75% Thread (Default Benchmark)
With `D = 10 mm` and `P = 1.5 mm`, the exact 75% cutting tap drill is `10 - (1.5 * 75 / 76.98) = 8.54 mm` (`0.3362 in`), while the standard `D - P` metric drill bit is `8.50 mm` (`76.98%` thread engagement).
M10 × 1.5 Cold-Forming Roll Tap in 6061-T6 Aluminum
Switching `tapType` to `Forming / Roll Tap` at `75%` thread changes the required hole diameter to `10 - (1.5 * 75 / 147.06) = 9.24 mm` (`0.3636 in`), matching a standard `9.25 mm` carbide drill.
1/4"-20 UNC in 316 Stainless Steel (65% Thread Relief)
In `Unified Inch` mode (`D = 0.25 in`, `20 TPI`, `threadPct = 65%`), the cutting tap drill increases from `0.2013 in` (`#7` drill at 75%) to `0.2078 in` (`#5` or `#4` drill), reducing tap torque in 316 stainless.
How to Choose Between 60%, 75%, and 85% Thread Height
More thread height does not mean a proportionally stronger bolted joint.
Fastener pull-out tests show that when a steel screw engages at least `1.5x` its major diameter in depth, a `60%` thread height already develops enough shear area to break the bolt shank in tension before stripping the internal threads. Raising thread percentage above `75%` sharply increases tap tooth chipping.
If you are also programming roughing passes before drilling and tapping, pair this tool with our Metal Removal Rate Calculator (/tools/metal-removal-rate-calculator) to balance spindle load.
Thread Geometry Checks & Shop Notes
Verify these geometric limits before running a rigid tapping cycle.
Cutting Tap: D_drill = D - (P * Pct) / 76.9800; Forming Tap: D_drill = D - (P * Pct) / 147.0588; Unified Pitch P_in = 1 / TPI (P_mm = 25.4 / TPI). In Unified Inch mode, the calculator automatically converts `D` (`in`) and `TPI` into both decimal inches and millimeters.
STL Volume Calculator
Compute signed tetrahedral volume, surface area, bounding box dimensions, watertight edge warnings, and estimated 3D printing part weight from ASCII STL mesh geometry.
Open tool
Paper Size & Sheet Cut Calculator
Calculate how many smaller sheets or cards fit on a parent paper sheet (ISO A0-A4, SRA3, US Tabloid, or custom mm/in). Compare portrait, landscape, and mixed cut yields and waste %.
Open toolMore free tools to try
Choose another tool for your next calculation, conversion, or text task.
Number Line Generator
Generate printable SVG and text number lines with custom start/end values, step increments, tick labels, and plotted coordinate markers.
Open tool
STL Surface Area Calculator
Calculate 3D mesh total surface area, enclosed volume, bounding box dimensions, and facet count from ASCII STL meshes in mm², cm², and in².
Open tool
Lawn Mowing Time Calculator (Deck Width, Overlap, Speed & Acres/Hr)
Calculate how long it takes to mow any lawn size by deck width, mowing speed, pass overlap, and turn/obstacle field efficiency.
Open tool
Ring Size Calculator & International Converter (mm, US, UK, EU, JP)
Convert ring inner diameter (mm) or finger circumference (mm) to US/Canada, UK/Australia letter, EU/ISO, and Japanese ring sizes with wide-band sizing guidance.
Open toolFrequently Asked Questions About Tap Drill Sizes
Answers about using Tap Drill Size Calculator (Metric ISO & Unified Inch Cutting vs. Forming) and understanding its results.
Where does the 76.98 constant in the tap drill formula come from?
For standard 60-degree ISO Metric and Unified National threads, the fundamental triangle height is H = (sqrt(3) / 2) * P = 0.866025 * P, and the standard internal thread height is (5/8) * H = 0.541266 * P. Because thread depth applies to both sides of the hole diameter, 100% full thread engagement reduces the hole diameter by 2 * 0.541266 * P = 1.082532 * P = P / 0.92376, or P * (100 / 76.9800) when using the traditional machinist 76.98 divisor.
Why does the simple 'Major Diameter minus Pitch' (D - P) rule work for metric cutting taps?
When you drill at D - P (for example, 10.0 mm - 1.5 mm = 8.50 mm for an M10 × 1.5 thread), the resulting percentage of thread is 1.0 / 1.2990 = 76.98% of full theoretical thread height. That sits right at the 75% workshop standard.
Why do cold-forming (roll) taps require a much larger drill bit than cutting taps?
A cold-forming tap has no cutting edges and produces zero chips; instead, its lobes plastically displace metal from the hole wall inward to form the thread crests. Using a cutting-tap drill size with a roll tap will seize and snap the tap immediately. For M10 × 1.5 at 75% thread, a cutting tap uses an 8.54 mm (8.50 mm standard) drill, whereas a forming tap requires a 9.24 mm (9.25 mm standard) drill.
What thread percentage should I use for stainless steel, titanium, or deep holes?
Use 60% to 65% thread height for work-hardening alloys (304/316 stainless steel, Inconel, titanium) and blind holes deeper than 2 diameters. Machinery's Handbook tests show that 60% thread height provides over 95% of the pull-out strength of a 75% thread when thread engagement length is at least 1.5x major diameter, while cutting tapping torque by ~40%.
Try your own values
Choose another tool for your next calculation, conversion, or text task.