Thread Calculator

Q: What is the tap drill size for M10x1.5?

Use an 8.5 mm drill for M10x1.5. That gives about 75% thread engagement. The quick rule for metric coarse threads is: Drill size = major diameter − pitch So 10 − 1.5 = 8.5 mm. This rule works well for most metric coarse and fine sizes.

Q: How do I convert TPI to pitch in mm?

Divide 25.4 by the TPI. Pitch (mm) = 25.4 ÷ TPI 20 TPI → 25.4 ÷ 20 = 1.27 mm 13 TPI → 25.4 ÷ 13 = 1.954 mm 8 TPI → 25.4 ÷ 8 = 3.175 mm To go the other way, divide 25.4 by the pitch in mm.

Thread Calculator

Updated Sep 8, 2026 By Jehan Wadia

Introduction

This Thread Calculator gives you the exact numbers for any screw thread. Pick a standard, choose a size, and it shows the pitch diameter, minor diameter, thread depth, tolerances, tap drill size, lead angle, and over-wire measurements right away.

It covers the thread types used most in shops and design work:

Metric (M), coarse and fine, per ISO 68-1, ISO 261, and ISO 965-2
Unified Inch (UN), UNC, UNF, UNEF, and constant-pitch series, per ASME B1.1
NPT and BSPT, tapered pipe threads that seal on the threads
BSPP (G), straight pipe threads that seal with a gasket or O-ring
ACME, general purpose and centralizing power threads, per ASME B1.5

You can set the tolerance class (like 6g, 6H, 2A, or 3G), pick internal or external threads, add multiple starts, and enter a plating thickness. If you add plating, the tool also shows the machined size to cut before coating. Results can be shown in mm or inches with one click.

Every answer comes with a step-by-step solution, so you can check the math instead of trusting a black box. A chart draws the thread profile so you can see the crest, root, and flanks. There is also a tap drill table with 50%, 65%, and 75% thread engagement, plus the nearest real drill size you can actually buy.

Use it to size a tapped hole, set up a lathe, check a part on a comparator, or measure pitch diameter with three wires. Type a thread name like M10x1.5-6g or 1/4-20 UNC-2A into the designation box and the calculator will fill in the rest for you.

How to use our Thread Calculator

Pick your thread standard, size, and pitch, then set the fit and form you need. The calculator gives you the full thread profile, tolerance limits, tap drill sizes, lead and helix angles, and over-wire measurements.

Thread Standard: Choose the thread system you are working with, such as Metric (M), Unified Inch (UN), NPT, BSPT, BSPP, or ACME. This sets the thread shape, angle, and unit used for all results.

Thread Series / Type: Pick coarse or fine for metric threads, or UNC, UNF, UNEF, or a constant-pitch series for inch threads. This box only shows up for standards that have more than one series.

Nominal Size: Select the thread size from the list, like M10 or 1/4-20. Choose "Custom" if your size is not listed.

Custom Nominal Major Diameter: Only shows when you pick "Custom." Type the outside diameter of the thread in mm or inches.

Pitch or TPI: Enter the pitch in mm for metric and BSP threads, or threads per inch for inch threads. Picking a standard size fills this in for you.

Number of Starts: Enter 1 for a normal single-start thread. Use 2 or more for multi-start threads, which makes the lead longer than the pitch.

Thread Form: Choose External for a bolt or shaft, or Internal for a nut or tapped hole. The limits and measuring method change with your pick.

Tolerance Class: Choose the fit class, like 6g or 6H for metric, 2A or 2B for inch, or 2G to 5G for ACME. Lower numbers in metric and higher numbers in ACME give a tighter fit.

Length of Engagement: Enter how deep the threads mesh. The default is 5 times the pitch, which works for most jobs.

Coating / Plating Thickness: Enter the thickness added to each surface by plating. Leave it at 0 for bare threads. Any other value adds a pre-coating machining target to each diameter.

Output Unit Display: Pick Native to see results in the standard's own unit, or Alternate to flip between mm and inches. This changes results only, not your inputs.

Thread Designation: Type a full thread call-out like M10x1.5-6g or 1/4-20 UNC-2A and press the wand button. The calculator fills in every field for you.

Target Engagement: In the tap drill table, choose 75%, 65%, or 50% thread engagement. Lower engagement makes tapping easier; higher engagement makes the joint stronger.

Measuring Wire Diameter: On the Over-Wire tab, pick a wire size, enter a custom one, or click "Use Ideal Wire" to get the best-size wire for your pitch.

Screw Threads Explained

A screw thread is a spiral ridge cut around a bolt, screw, rod, or hole. It turns spinning motion into straight motion. That is how a bolt pulls two parts together and how a lead screw moves a machine table. Every thread has a shape (profile), a size, and a set of limits that decide if two parts will fit.

Key Thread Terms

Major diameter is the biggest diameter, measured across the crests of a bolt.
Minor diameter is the smallest diameter, measured at the roots of a bolt or the crests of a nut.
Pitch diameter is the imaginary diameter where the ridge and the groove are the same width. This is the number that really controls fit and strength.
Pitch is the distance from one crest to the next. Metric threads use millimeters. Inch threads use TPI (threads per inch), which is 1 ÷ pitch.
Lead is how far the part moves in one full turn. On a single-start thread, lead equals pitch. On a two-start thread, lead is double the pitch.
Lead angle is the slope of the thread at the pitch diameter. Small lead angles hold tight and resist backing off. Large ones move fast but can back-drive.
Thread angle is the V-angle of the form: 60° for metric and Unified, 55° for British pipe, 29° for ACME.

Common Thread Standards

Metric (M), ISO 68-1 and ISO 965. Written like M10×1.5-6g. Coarse and fine pitches are both common.
Unified Inch (UN), ASME B1.1. Series are UNC (coarse), UNF (fine), and UNEF (extra fine), such as 1/4-20 UNC-2A.
NPT, ASME B1.20.1 tapered pipe thread. The taper is 1:16, so the threads wedge together and seal on the flanks.
BSPT (R), ISO 7-1 tapered pipe thread with a 55° form. It also seals on the threads.
BSPP (G), ISO 228-1 straight pipe thread. It does not seal by itself and needs a washer, gasket, or O-ring.
ACME, ASME B1.5 power thread with a 29° trapezoid shape. Used on vises, jacks, and lead screws because it carries heavy loads and is easy to cut.

Tolerance Classes and Fit

No thread is made at its exact basic size, so standards give a tolerance band and an allowance (a small gap). Metric uses a number and a letter, like 6g for a bolt and 6H for a nut. Lower-case letters mean external threads, upper-case mean internal. Unified uses 1A, 2A, 3A for bolts and 1B, 2B, 3B for nuts. Bigger class numbers mean a tighter fit. A loose class (1A, 8g) is easy to assemble and leaves room for plating. A tight class (3A, 4g) gives better centering and strength. Threads are checked with GO and NO-GO gauges: the GO gauge must run on, and the NO-GO gauge must stop within about two turns.

Tap Drill Size and Percent Engagement

Before you cut an internal thread you drill a hole. The hole size sets the thread engagement, which is how much of the full thread depth is actually there. About 75% engagement is the usual target, but 50% to 65% is often smarter in hard metals and deep holes. Going from 100% to 75% engagement gives up only a little strength while cutting tap breakage and torque a lot. A rough rule for metric: drill size ≈ major diameter − pitch.

Measuring Over Wires

You cannot touch the pitch diameter with a micrometer directly. So machinists lay three equal wires in the grooves (two on one side, one on the other) and measure across them. The "best-size" wire touches the flanks right at the pitch line, which cancels out small angle errors. The measured value M is then converted back to pitch diameter with a simple formula that depends on the pitch and the thread half-angle.

Coating and Plating

Zinc, anodizing, and other coatings add thickness to every surface. On a bolt, the plating grows outward on the crest and both flanks, so the pitch diameter grows by about four times the coating thickness measured on the flank. In practice the machined size is cut smaller before plating. On a tapped hole, the part must be cut larger. This is why coarse classes like 6g and 2A are often chosen for parts that will be plated.

Where It Matters

Getting thread numbers right prevents cross-threading, stripped nuts, leaking pipe joints, and tap breakage. Machinists, designers, maintenance techs, and hobbyists all use these values to pick tap drills, set up single-point turning, order gauges, and check parts on the bench.

Formulas used

ISO metric pitch diameter

d_2 = D - 0.649519\,P

ISO metric minor diameters (basic and external root)

d_1 = D - 1.082532\,P, \qquad d_3 = D - 1.226869\,P

Fundamental triangle height (60 degree V-form)

H = 0.866025\,P

Lead and lead angle at pitch diameter

L = P \times n_{starts}, \qquad \lambda = \arctan\!\left(\frac{L}{\pi d_2}\right)

Best-size (ideal) measuring wire diameter

d_w = \frac{P}{2\cos\alpha}

Measurement over/between wires

M_{ext} = E + d_w\left(1 + \frac{1}{\sin\alpha}\right) - \frac{P}{2}\cot\alpha, \qquad M_{int} = E + \frac{P}{2}\cot\alpha - d_w\left(\frac{1}{\sin\alpha} - 1\right)

Tap drill diameter for a target percent thread engagement

d_{drill} = D - \frac{\%_{eng}}{100}\,K\,P, \qquad \%_{eng} = \frac{D - d_{drill}}{K\,P}\times 100

NPT taper pipe thread pitch diameters

E_0 = D - (0.05D + 1.1)P, \qquad E_1 = E_0 + \frac{L_1}{16}, \qquad L_2 = (0.8D + 6.8)P

Frequently asked questions

What is the tap drill size for M10x1.5?

Use an 8.5 mm drill for M10x1.5. That gives about 75% thread engagement.

The quick rule for metric coarse threads is:

Drill size = major diameter − pitch

So 10 − 1.5 = 8.5 mm. This rule works well for most metric coarse and fine sizes.

How do I convert TPI to pitch in mm?

Divide 25.4 by the TPI.

Pitch (mm) = 25.4 ÷ TPI

20 TPI → 25.4 ÷ 20 = 1.27 mm
13 TPI → 25.4 ÷ 13 = 1.954 mm
8 TPI → 25.4 ÷ 8 = 3.175 mm

To go the other way, divide 25.4 by the pitch in mm.

What does 6g mean on a metric thread?

It is the tolerance class for an external thread (a bolt).

The 6 is the tolerance grade, or how wide the size band is. Smaller numbers are tighter.
The g is the position, or how much gap is left below the basic size.

Lower-case letters mean external threads. Upper-case letters like 6H mean internal threads (nuts). A 6g bolt in a 6H nut is the normal general-purpose fit.

What is the difference between 2A and 3A threads?

Both are external inch threads, but 3A is tighter.

2A is the standard commercial fit. It has an allowance (a small gap), so it assembles easily and leaves room for plating.
3A has no allowance and a smaller tolerance band. It centers better and holds more load, but costs more to make.

Most bolts you buy are 2A. Use 3A for precision or high-stress parts.

What is pitch diameter and why does it matter?

Pitch diameter is the imaginary diameter where the thread ridge and the groove are the same width.

It matters because it controls the actual fit and strength of a thread. Two parts can have perfect major diameters and still not fit if the pitch diameters are wrong. GO and NO-GO gauges mostly check pitch diameter, not the outside size.

For metric threads: pitch diameter = major diameter − 0.6495 × pitch.

Can NPT and BSPT threads be used together?

No. Do not mix them.

Both are tapered 1:16 pipe threads, but the thread angles are different: NPT is 60° and BSPT is 55°. The thread counts per inch are also different in most sizes.

If you force them together, only the crests touch. The joint will leak and can crack the fitting. Use an adapter fitting instead.

What is the difference between BSPP and BSPT?

Both use the 55° Whitworth form, but they seal in different ways.

BSPT (R) is tapered. The threads wedge together and seal on the flanks, usually with sealant or PTFE tape.
BSPP (G) is parallel (straight). It does not seal on the threads. You need a bonded washer, gasket, or O-ring at the face.

A BSPT male can screw into a BSPP female port, but not the other way around.

Why is 75% thread engagement the standard target?

It is the best trade-off between strength and easy tapping.

Going from 75% to 100% engagement adds only about 5% more strength, but it needs far more tapping torque and breaks taps often.

Dropping to 50% or 65% cuts torque a lot and only loses a little strength. In hard metals, deep holes, or blind holes, 50% to 65% is usually the smarter choice.

How do you measure pitch diameter with three wires?

Lay three equal wires in the thread grooves (two on one side, one on the other) and measure across them with a micrometer.

For a 60° external thread, use:

M = E + 3d − 0.86603 × P

Where M is the reading, E is the pitch diameter, d is the wire diameter, and P is the pitch.

The best-size wire is d = P ÷ (2 × cos 30°) = 0.57735 × P. It touches the flanks right at the pitch line, so small thread angle errors cancel out.

What is a multi-start thread used for?

A multi-start thread has two or more separate ridges wound side by side. It moves farther per turn.

Lead = pitch × number of starts. A 2-start thread moves twice as far in one turn as a single-start thread with the same pitch.

They are used for bottle caps, camera lens mounts, clamps, and lead screws, anywhere you want fast travel or quick assembly with a shallow, strong thread form.

When is a thread self-locking?

A thread is self-locking when its lead angle is smaller than the friction angle.

For dry steel on steel (friction about 0.10 to 0.15), the friction angle is roughly 5.7° to 8.5°. Almost every standard bolt has a lead angle around 2° to 4°, so bolts do not spin loose from load alone.

ACME lead screws with a coarse pitch or several starts can go above that angle. Those will back-drive and need a brake or a worm gear.

Why is ACME used for lead screws instead of a V thread?

ACME has a 29° trapezoid shape with a wide, flat root instead of a sharp V.

The thick root resists bending and shear, so it carries heavy axial loads.
The flat flanks spread the load and wear more evenly.
It is easier to cut, easier to inspect, and works with a split nut.

Sharp 60° V threads concentrate stress at the root and wear fast under repeated motion.

How much does plating change thread size?

A lot more than people expect. On the pitch diameter, plating adds about 4 times the coating thickness.

That is because the coating builds on both flanks, and the 60° geometry multiplies it. So 0.005 mm of zinc adds roughly 0.020 mm to the pitch diameter.

This is why bolts that will be plated are cut to a loose class like 6g or 2A. The allowance leaves room for the coating.

How many turns of NPT thread engagement do I need?

Hand-tight is normally 2 to 3 turns, then add 2 to 3 more turns with a wrench.

Total useful engagement is usually 4 to 6 turns for small sizes. Bigger pipe sizes take fewer turns because the pitch is coarser.

Do not over-tighten. The 1:16 taper means each extra turn wedges the fitting harder and can split a cast female port.

What is the difference between coarse and fine threads?

Coarse threads have a bigger pitch and fewer threads per inch. Fine threads have a smaller pitch and more threads.

Coarse assembles faster, tolerates dirt and damage better, taps easier, and works better in soft metals like aluminum.
Fine has more tensile area, a smaller lead angle so it resists loosening, and gives finer adjustment. It is better in thin walls and hard materials.

Coarse is the default choice unless you have a reason to go fine.

What is thread allowance?

Allowance is the intentional gap built in between a bolt and a nut at their maximum-material sizes.

It is not the same as tolerance. Tolerance is how much a size may vary. Allowance is a fixed minimum clearance so the parts always assemble.

Metric class 6g and inch class 2A both have an allowance. Class 6H, 2B, and 3A have zero allowance, so their basic size is the limit.

What do GO and NO-GO thread gauges check?

They check the pitch diameter and the overall thread form together.

The GO gauge must screw on or in over the full length. It proves the part is not too big (for a bolt) or too small (for a hole).
The NO-GO gauge must not enter more than about 2 turns. It proves the part is not too loose.

If GO fails, the thread is oversize or has form errors. If NO-GO goes on, the thread is worn or cut too deep.

How deep should a tapped hole be?

A common rule is 1.5 × the bolt diameter in steel and 2 to 2.5 × diameter in aluminum or cast iron.

The goal is to make the threads stronger than the bolt, so the bolt breaks before the threads strip. Soft metals need more depth to reach that.

For blind holes, drill about 3 to 5 more pitches deeper than the thread so the tap has room for chips and the lead taper.