What Is TPI (Threads Per Inch)?

A common term that’s used when talking about screws is threads per inch, or TPI, and you might be wondering what exactly TPI is and how it’s determined. Screw TPI defines the thread count in an inch of a bolt. When looking directly at a screw, you can distinctly see and count the number of threads, and to measure the screw, you can identify an inch-long section on the fastener and count how many peaks are on it. This process can typically be done by a user of the screw, as long as there are not too many threads.

Screw TPI is used for American fasteners only — you can see how #-series sizes correspond to fractional inches and TPI ranges in our U.S. screw size conversion chart. In general, smaller fasteners have finer threads, producing a higher screw thread count. Screws that have finer threads are known for being stronger in tension. Contrarily, larger fasteners have greater threads, producing a lower thread count and less strength in tension.

How Do I Calculate TPI?

Luckily, you can easily convert metric pitch into TPI using a formula. Follow this formula to accurately calculate the TPI of your screw: 

Threads per inch equals the number of threads divided by the length of the thread

What Is Thread Pitch?

On the other hand, the thread pitch is the distance between two threads. This measurement is typically used in place of TPI for metric fasteners. It is calculated by measuring the distance from one peak to the next and is often measured in millimeters. The calculation of thread pitch can also be used in a formula that determines the screw’s thread count.

How Do I Calculate Thread Pitch?

If you already have some American fasteners on hand for your project but are looking for more, you can get the right size by doing some calculations. Follow this formula to accurately measure your thread pitch:

Thread pitch equals the length divided by the number of threads

TPI vs. Thread Count

Something to notice when looking at thread pitch is that when fasteners have a lower thread pitch, they have a finer thread, which is the opposite when compared to the screw TPI.

While each offer different benefits to their applications, both screw TPI and thread pitch are used to measure the threading of a bolt or fastener to ensure that they can be secured properly. Without knowing these measurements, the bolt and nut could either seize or strip the threading and result in an unusable condition.

Threads Per Inch vs. Thread Pitch: What’s the Difference?

Threads per inch and thread pitch describe the same feature from opposite directions:

• TPI counts how many threads fall within one inch, so the number climbs as threads get finer.
• Thread pitch measures the distance from one thread to the next, so that number shrinks as threads get finer.

Inch-based fasteners are specified by TPI, while metric fasteners are specified by pitch in millimeters.

The two values convert directly. Divide 25.4 by the TPI to find the pitch in millimeters, or divide 25.4 by the pitch to find the TPI. A 100 TPI screw, for example, has a pitch of 0.254 mm. Knowing both figures lets you match an inch-spec adjuster to a metric drawing without guesswork.

Why Higher TPI Means Finer, More Precise Adjustment

Every full turn of a single-start screw advances it by the distance of one thread, so the travel per revolution equals one divided by the TPI. A coarse 20 TPI screw moves 0.050 inch per turn. A 100 TPI screw moves only 0.010 inch per turn, one-fifth the distance for the same rotation.

That smaller step is why high-TPI screws are used for precision positioning. The finer the thread, the smaller the movement a user can dial in, and the more resistant the setting is to drifting under load or vibration. Finer threads also increase mechanical advantage, so the screw holds its position with less back-driving. For optical mounts, vacuum feedthroughs, and instrument adjusters, that resolution is the entire point, which is why we cut our 1/4-100 TPI fine adjustment screws as fine as 100 threads per inch.

Threads Per Inch Chart

Standard inch fasteners come in a coarse (UNC) and a fine (UNF) thread series, each with a set TPI for a given diameter. The threads per inch chart below lists the common sizes for quick reference. Precision adjustment screws run finer still, often 80 to 100 TPI, well beyond the standard fine series.

Selecting Your TPI Screws

As you search for your TPI screws, know that we have a large selection to choose from. For our line of TPI screws, we have the following options:

• 1/4 Inch – 100 TPI Screws
• 1/4 Inch – 80 TPI Screws
• 3/16 Inch – 100 TPI Screws
• .137 Inch – 80 TPI Screws

All of our TPI screws listed above are available in different lengths to fit your needs. Upon request, we are also able to create custom tips for your screws, with blunt or coned tips available. Plus, we also offer screws supplied with specified drive variations upon request.

Due to our highly developed and custom-modified machinery, we can supply fine adjustment screws that exceed the industry standards. Please be aware that we do not guarantee that our TPI screws will be compatible with screws and bushings that are sold by alternative manufacturers. On this topic, it is also important to be careful when converting metric pitch to TPI in order to get the best fit for your project.

Our High-Vacuum Screw Options

Our high-vacuum compatible fine and ultra-adjustment screws are supplied with a ball tip which is made of hardened steel. These screws are able to have air escape from under the ball tip due to the design that causes a relief in the ball cavity. The options that we offer for your high-vacuum TPI screws include:

• 1/4 Inch – 80 TPI High-Vacuum Screws
• 1/4 Inch – 100 TPI High-Vacuum Screws
• 3/16 Inch – 100 TPI High-Vacuum Screws

If interested, we are able to create custom lengths and custom tips upon request. Our high-vacuum screws are able to deliver precision smooth movements, especially in the most demanding OEM applications.

Threads Per Inch FAQs

What does threads per inch (TPI) mean?

Threads per inch is the number of thread crests, or peaks, counted along one inch of a screw or bolt. A higher number means finer, more closely spaced threads.

How is TPI measured?

Line a ruler up with the threaded section, mark off one inch, and count the thread crests inside it. If you measure less than a full inch, divide the crest count by the length you measured. A thread pitch gauge gives a faster, more reliable reading.

What is the difference between TPI and thread pitch?

TPI counts threads per inch and is used for inch fasteners, while thread pitch measures the distance between threads in millimeters and is used for metric fasteners. Convert between them by dividing 25.4 by either value.

Is a higher TPI better?

A higher TPI is better for fine adjustment, holding power, and resistance to vibration, because each turn moves the screw a shorter distance. Coarse, lower-TPI threads assemble faster and resist stripping in softer materials. The right choice depends on the application.

What TPI do precision adjustment screws use?

Precision adjusters commonly use 80 to 100 TPI, far finer than standard fasteners. At 100 TPI, one full turn advances the screw just 0.010 inch. Our high-vacuum fine adjustment screws are available in these fine pitches for demanding OEM applications.

Choose Kozak Micro Adjusters for your TPI Screws

We have been providing quality screws and products for over 40 years. Our standards often exceed even those of our clients, and we stand by our superior craftsmanship in creating TPI screws and other products. We take pride in providing the highest quality products to meet the needs of your company.

Contact us today to learn why our TPI screws exceed industry standards.