Thread Pitch Calculator

Thread Pitch Calculator for TPI and Metric Threads

This thread pitch calculator helps you convert a thread count into the distance between thread crests. Enter threads per inch (TPI) to find pitch in inches and millimeters, or enter a metric pitch to find its TPI equivalent. You can also estimate thread lead for multi-start threads and see how many thread intervals or screw turns fit across a chosen length. The calculator is useful when checking a drawing, comparing fasteners, learning thread measurements, or preparing a preliminary machining estimate.

Thread pitch is a length measured parallel to the screw axis. It is not the same thing as the outside diameter. Diameter identifies how wide a screw is, while pitch tells you how far apart its threads are. A bolt diameter by itself cannot reveal its pitch: the same nominal diameter may be sold with coarse, fine, or special threads. Use a thread gauge, drawing, product specification, or a measured TPI count to identify an unknown fastener.

The results are mathematical conversions and layout estimates. They do not identify a complete thread standard, tolerance class, fit, or strength rating. Before ordering parts or cutting threads, confirm the full designation and dimensions against the engineering drawing, fastener marking, manufacturer data, and applicable standard.

What Thread Pitch Means

Thread pitch is the axial distance from a point on one thread to the matching point on the next thread. Most often, this is measured from crest to crest along the length of a screw. The measurement runs parallel to the centerline of the fastener. It is not the diagonal distance that follows the sloping thread helix around the screw.

Metric threads state pitch directly in millimeters. An M10 × 1.5 thread, for example, has a nominal diameter of 10 mm and a pitch of 1.5 mm. That means each thread interval advances 1.5 mm along the axis on a single-start thread. A smaller pitch places the crests closer together and is often called a finer pitch. A larger pitch is coarser.

Inch thread specifications commonly give threads per inch, shortened to TPI. A count of 20 TPI means there are 20 equal thread intervals in one inch. Because TPI is a count per unit length, pitch in inches is its reciprocal. A 20 TPI thread has a pitch of 1/20 inch, or 0.05 inch. Converting that distance to metric gives 1.27 mm.

The terms coarse and fine describe relative spacing within a particular size and thread series. They are not enough to specify a fastener by themselves. A complete designation may include nominal diameter, pitch or TPI, series, thread class, and other details. For example, a fractional inch diameter followed by 13 TPI still needs the correct thread series and fit information for a dependable match.

Thread Pitch Formula: Convert TPI to Pitch

To find thread pitch from threads per inch, divide one inch by the TPI value. In inch units, the formula is P(in) = 1 ÷ TPI. To express the result in millimeters, multiply the inch pitch by 25.4 mm per inch. The combined formula is P(mm) = 25.4 ÷ TPI. This calculator displays both results so you can compare imperial and metric spacing without doing the conversion by hand.

For example, consider a thread with 13 TPI. Its pitch is 1 ÷ 13 = 0.076923 inch. In metric units, 25.4 ÷ 13 = 1.953846 mm. A value displayed with fewer decimal places is rounded for readability. Keep additional precision when a calculation feeds another calculation, but use the required drawing precision for inspection or manufacturing.

A second example is 24 TPI. The pitch is 1 ÷ 24 = 0.041667 inch, or 25.4 ÷ 24 = 1.058333 mm. As the number of threads per inch increases, the spacing gets smaller. This inverse relationship is why a larger TPI value represents a finer thread, not a wider space between threads.

The inverse formula converts metric pitch to an equivalent threads-per-inch value: TPI = 25.4 ÷ P(mm). If a metric thread has a pitch of 1.25 mm, then 25.4 ÷ 1.25 = 20.32 TPI. That number is a unit conversion, not proof that the thread belongs to a specific Unified series. Metric and inch thread systems have their own nominal sizes, profiles, tolerances, and designations.

• TPI to inch pitch: P(in) = 1 ÷ TPI
• TPI to metric pitch: P(mm) = 25.4 ÷ TPI
• Metric pitch to TPI: TPI = 25.4 ÷ P(mm)
• Threads per centimeter: threads/cm = 10 ÷ P(mm)

How to Use the Thread Pitch Calculator

Choose the calculation direction that matches the information you have. Select TPI to pitch when a drawing, thread gauge, or product label gives threads per inch. Select metric pitch to TPI when you know the spacing in millimeters and want to compare it with inch thread counts. The calculator updates the converted pitch in both millimeters and inches, along with the equivalent thread count.

Enter the number of threads per inch or the metric pitch. Values must be greater than zero. If you know the nominal major diameter, you can add it as a reference. The diameter is included in the result summary for convenience, but it does not change the pitch calculation. A pitch conversion needs a thread count or a measured pitch; it cannot be calculated from diameter alone.

Enter the number of thread starts when you need the lead for a multi-start screw. Most ordinary bolts have one start. If the application has a specified axial travel length, enter it to estimate the number of pitch intervals across that length and the number of screw revolutions needed to travel it. Select the length unit carefully so that the input is interpreted correctly.

Review the result card before copying, saving, exporting, or printing the figures. The CSV report is handy for notes and shop calculations. Keep the original drawing dimensions and source measurements with the exported result so another person can verify how the estimate was made. The tool is designed to make routine conversions clear, not to replace a calibrated thread gauge or a controlled inspection procedure.

Thread Pitch, TPI, and Thread Spacing

Pitch and thread spacing describe the same axial interval for a conventional single-start screw thread. When a thread gauge fits into the grooves, its marked pitch or TPI helps identify the spacing. For metric gauges, the blades are marked in millimeters, such as 0.75 mm or 1.25 mm. Inch gauges are often marked with TPI values such as 18, 20, or 24.

The total distance from the first crest to the last crest across a set of threads depends on how many intervals lie between those crests. If you measure from the first crest to the eleventh crest, there are ten pitch intervals, not eleven. This distinction matters when measuring a short length with calipers. Counting the spaces between matching points can improve the estimate, especially for fine threads.

For a known axial length, dividing that length by pitch gives the number of pitch intervals across it. For instance, a 25.4 mm length contains 20 pitch intervals at a 1.27 mm pitch. The result can be a decimal if the selected length does not end exactly on a thread interval. It is a geometric count, not a count of complete thread crests at the endpoints.

A screw's outside diameter does not tell you the spacing. Nor does a matching diameter guarantee that two fasteners will mate. If one screw is 1/4 inch in diameter with 20 TPI and another is 1/4 inch with 28 TPI, their crests are spaced differently. They are not interchangeable with the same nut or tapped hole, even though their nominal diameters are the same.

Pitch and Lead on Single-Start and Multi-Start Threads

Pitch measures the distance between adjacent thread forms. Lead measures how far a screw advances in one full revolution. For a single-start thread, pitch and lead are equal. For a multi-start thread, lead is pitch multiplied by the number of starts. A two-start screw with a pitch of 2 mm therefore has a lead of 4 mm per revolution.

Multiple starts are separate helical paths arranged around the same screw. They allow greater linear movement in one turn while keeping a chosen distance between neighboring thread forms. Multi-start threads appear in motion and adjustment applications, such as some lead screws, caps, and mechanisms. They should not be confused with the ordinary fastening threads used on many bolts and nuts.

When you calculate travel over a selected length, the number of revolutions is axial travel divided by lead. A single-start screw with a 1.5 mm pitch needs 10 turns to advance 15 mm. A two-start screw at the same pitch has a 3 mm lead and needs 5 turns for the same travel. The calculator presents this as a geometric estimate, assuming ideal motion and no lost movement from backlash or deformation.

A pitch conversion alone cannot confirm whether a screw is single-start or multi-start. Use a drawing, product specification, or direct inspection to establish the start count. On a multi-start thread, identifying lead as if it were pitch can produce a factor-of-two or larger error, so keep the two terms separate in measurements and notes.

Measuring an Unknown Thread

Start by cleaning the fastener and checking for a part marking or drawing. Measure the major diameter across the thread crests with suitable calipers or a micrometer. Then use a thread pitch gauge to find a blade that seats along the thread without rocking or leaving visible gaps. Compare several candidate blades, especially if dirt, wear, plating, or damage makes the fit uncertain.

If a pitch gauge is unavailable, measure several adjacent intervals over a known axial distance. Align the measuring tool with the screw axis, identify matching points, and divide the distance by the number of intervals. Measuring over multiple threads reduces the effect of small reading errors. Be careful not to divide by the number of crests when the first and last crests create one fewer interval.

A diameter measurement and a pitch measurement together narrow down possible thread sizes, but they may not identify the full standard. You may also need thread series, handedness, tolerance class, form, and whether the thread is tapered. Pipe threads and other special forms follow different rules from common parallel metric and Unified screw threads. A visual match is not enough when a joint must seal, carry a load, or meet a safety requirement.

For a worn, damaged, coated, or safety-critical fastener, use an appropriate gauge and consult the manufacturer or a qualified inspection resource. This online calculator can convert a reliable TPI or pitch measurement; it cannot inspect a physical part, detect damage, or certify that a mating pair is compatible.

Common Thread Pitch Examples

Common inch thread counts vary with nominal diameter and series. Values such as 8, 10, 12, 13, 16, 20, 24, 28, and 32 TPI appear in many screw and fastener references, but the correct value depends on the size and specified thread series. These numbers are examples, not a complete standard table. Confirm the correct combination from a current reference or engineering drawing.

Metric pitch values commonly appear as decimal millimeters, including values such as 0.5, 0.7, 0.8, 1.0, 1.25, 1.5, and 2.0 mm. Which pitch is standard for a particular diameter depends on the metric series and whether the thread is coarse or fine. A calculator can convert any positive pitch, but it does not label a custom value as a standard size unless you verify that designation elsewhere.

A useful quick comparison is that 1.0 mm pitch is equivalent to 25.4 TPI, while 2.0 mm pitch equals 12.7 TPI. The converted TPI value may not be a whole number. This is normal: the metric spacing is a length, and its exact reciprocal in inch units does not have to match a common whole-number TPI designation.

Avoid rounding too early when comparing thread systems. For example, 1.25 mm converts to 20.32 TPI, not exactly 20 TPI. A 20 TPI thread has a pitch of 1.27 mm, so the two pitches differ by 0.02 mm per interval. A small difference can prevent correct engagement over multiple turns. Use the actual specified thread form rather than treating close converted values as interchangeable.

Choosing the Right Thread for a Job

The right thread depends on the mating part, material, load, environment, assembly method, and applicable design requirements. Coarse threads are often easier to start and can be useful in some materials or field conditions. Fine threads provide smaller axial movement per turn and can be useful where adjustment or a thin wall matters. These are general observations, not a rule that one thread type is always stronger or safer.

Check the full thread callout before buying a fastener. The callout may state nominal diameter, pitch or TPI, series, class, handedness, and length. Confirm that the nut, insert, tapped hole, or other mating part uses the same thread specification. Do not force a fastener that starts poorly; mismatched threads can cross-thread and damage both parts.

Thread pitch is one input to a mechanical design. It does not establish the load capacity of a bolt, the pullout strength of a tapped hole, the sealing ability of a connection, or the fatigue life of a joint. Those questions depend on more information, including material properties, engagement length, installation torque, temperature, vibration, and the geometry of the assembly.

For ordinary repair work, matching the existing fastener's exact size and thread form is often the simplest path. For new designs, use the specified standard and engineering requirements. If the connection is structural, pressure-containing, lifting-related, or otherwise safety-critical, follow the approved design and inspection process rather than relying on a pitch conversion by itself.

Accuracy, Rounding, and Limitations

The formulas in this tool are exact unit relationships: one inch is 25.4 millimeters, and TPI is the reciprocal of pitch in inches. Displayed results are rounded to a practical number of decimal places. The input measurement may be less accurate than the conversion itself, so extra displayed digits should not be mistaken for guaranteed measurement accuracy.

The optional diameter and length fields are references for your calculation record. Diameter does not alter pitch, and an entered travel distance does not account for backlash, elastic stretch, friction, wear, or manufacturing error. The number of revolutions assumes a screw advances by its theoretical lead on every turn.

This tool does not determine thread class, tolerance allowance, effective engagement, crest or root form, or compliance with a manufacturing standard. It does not replace a pitch gauge, thread micrometer, inspection drawing, or current standard tables. Use it as a quick converter and planning aid, then verify critical dimensions using the correct equipment and documentation.

When sharing a result, include the source value and its unit. A note such as ‘13 TPI converts to about 1.9538 mm pitch’ is easier to review than a pitch number with no context. Keep enough decimals for the comparison you need, and use the drawing's specified precision when documenting production or inspection results.