Thread Pitch Calculator
Find the thread pitch and threads per inch for a bolt, from its diameter and thread standard.
Reviewed by the ToolNestr Editorial Team — July 2026
How the thread pitch calculator works
Inch-standard fasteners (UNC and UNF, under the Unified Thread Standard) describe thread spacing as threads per inch (TPI) — literally how many thread crests occur in one inch of length. Metric fasteners instead describe the same spacing as pitch, the linear distance between adjacent threads in millimeters. Since both measure the same physical spacing, they convert directly: pitch in millimeters equals 25.4 divided by TPI.
UNC (Unified Coarse) threads have fewer, larger threads per inch for a given bolt diameter, making them the more common general-purpose choice — coarser threads assemble faster, resist stripping better in softer materials, and are less prone to cross-threading. UNF (Unified Fine) threads pack more threads into the same inch, giving finer holding adjustment and marginally higher tensile strength at the same nominal diameter, at the cost of being more sensitive to cross-threading and slower to run down.
Pitch diameter is a third dimension worth knowing: it's the diameter measured at the imaginary cylinder where the thread width and the gap between threads are equal, sitting roughly between the bolt's major (outside) and minor (root) diameters. It's the dimension a thread gauge actually checks to confirm a fastener meets its stated size and class — this calculator gives an approximate pitch diameter using a common rule of thumb, not the precise value from the full thread standard tables.
Thread pitch, TPI, and pitch diameter conventions follow ASME B1.1 (Unified Inch Screw Threads) and ISO 68-1 for metric threads.
Pick a common size
Or enter a custom diameter and TPI.
Read pitch and TPI
Both units at once for cross-reference.
Check pitch diameter
An approximate value for quick reference.
The formula explained
Pitch from TPI
pitch(mm) = 25.4 ÷ TPI. A 3/8-16 bolt: 25.4 ÷ 16 = 1.588 mm.
Pitch in inches
= 1 ÷ TPI = 1 ÷ 16 = 0.0625 in between threads.
Approx. pitch diameter
≈ major dia − 0.65 × pitch(in) = 0.375 − 0.041 = 0.334 in.
Worked example
A 1/2-20 UNF bolt.
Real-world context
A 1/4-20 UNC bolt, one of the most common general-purpose fasteners in hardware store bins, computes to a pitch of 1.27 mm, a pitch spacing of 0.05 in, and an approximate pitch diameter of 0.2175 in. That coarse 20-TPI spacing is exactly why 1/4-20 is the go-to choice for wood and sheet-metal assembly work — it's forgiving of slightly misaligned starts and resists cross-threading better than a finer pitch would.
Step up to a 5/16-18 UNC bolt, common in automotive and machinery brackets, and the numbers become pitch = 1.41 mm, pitch spacing = 0.0556 in, and pitch diameter ≈ 0.2764 in. Notice the TPI actually drops slightly (18 vs 20) even though the diameter increased — coarse-thread series don't scale TPI in a fixed ratio to diameter, which is why a thread chart, not a formula guess, is the reliable way to identify an unlabeled bolt.
A larger 1/2-13 UNC bolt, typical of structural connections and larger machine frames, works out to pitch = 1.95 mm, pitch spacing = 0.0769 in, and pitch diameter = 0.45 in. The pattern across all three sizes — coarser threads (lower TPI) on larger diameters — reflects how UNC standardizes for practical assembly speed and thread strength rather than a strict mathematical progression.
Common misconceptions
"A higher TPI number means a coarser thread." It's the opposite — higher TPI means finer threads. More threads packed into one inch means each thread is smaller and more closely spaced, which is a finer thread (like UNF). Fewer, larger threads per inch (like UNC) is the coarse option, even though its TPI number is lower.
"Metric and inch bolts of similar size are interchangeable." Even close diameters rarely share a compatible thread form or pitch. A metric M6 bolt and a 1/4-inch UNC bolt are close in diameter but have different major diameters and completely different pitch, so a nut or tapped hole for one will not properly thread the other — always match both diameter and thread standard.
"Pitch diameter is the same as the bolt's measured outer diameter." Pitch diameter is always smaller than the major diameter. The major diameter is what a caliper measures across the outer thread crests, while pitch diameter sits at the imaginary midpoint where thread and gap are equal width — it's a functional gauging dimension, not the number stamped on the bolt or read off a tape measure.
Related calculators
Frequently asked questions
What is the difference between thread pitch and threads per inch?
Threads per inch (TPI) is used for inch (UNC/UNF) fasteners and counts how many thread crests fit in one inch. Metric thread pitch is the distance between adjacent threads in millimeters. The two are reciprocals of related units: pitch (mm) = 25.4 ÷ TPI.
What is the difference between UNC and UNF threads?
UNC (Unified Coarse) has fewer, larger threads per inch and is the more common general-purpose choice, offering easier assembly and better resistance to stripping. UNF (Unified Fine) has more threads per inch, giving finer adjustment and slightly higher tensile strength in the same nominal diameter, but is more prone to cross-threading.
How do I identify a bolt's thread size?
Measure the nominal outside diameter, then count the threads per inch (for inch bolts) with a thread gauge or by counting threads over a measured length, or measure the distance between threads in millimeters for metric bolts. Compare against a standard thread chart to identify the exact size and pitch.
What is pitch diameter?
Pitch diameter is the diameter measured at the imaginary cylinder where the thread's width and the gap between threads are equal — roughly midway between the major (outer) and minor (root) diameters. It's the dimension thread gauges actually measure to verify a fastener meets spec.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.