Thread Pitch Chart: TPI to Metric Conversion (Printable)
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Open Identifier →Thread Pitch vs TPI: The Core Difference
Two systems, two units, zero compatibility. You need to know which one you're looking at before you pick up a replacement bolt.
Metric thread pitch is measured in millimeters — the distance from one thread crest to the next. An M10 × 1.5 bolt has threads spaced 1.5 mm apart. Smaller number = finer thread.
TPI (Threads Per Inch) is the US/Imperial system. Count the peaks in a one-inch span. A 1/4-20 bolt has 20 thread peaks per inch. Higher number = finer thread.
The conversion formula is simple:
TPI = 25.4 ÷ Pitch (mm)
Metric Thread Pitch Chart (M1.6 – M36 Coarse)
The values below are ISO metric coarse series — the standard you'll find on most fasteners unless "fine" is explicitly marked.
| Size | Pitch (mm) | Equiv. TPI | Common Use |
|---|---|---|---|
| M1.6 | 0.35 | 72.6 | Electronics, miniatures |
| M2 | 0.40 | 63.5 | PC fans, small electronics |
| M2.5 | 0.45 | 56.4 | PC motherboards |
| M3 | 0.50 | 50.8 | Small appliances, electronics |
| M4 | 0.70 | 36.3 | Bicycle components, fixtures |
| M5 | 0.80 | 31.8 | Machinery, automotive |
| M6 | 1.00 | 25.4 | Furniture, appliances — very common |
| M7 | 1.00 | 25.4 | Rare — mostly automotive specialty |
| M8 | 1.25 | 20.3 | Very common — general machinery |
| M10 | 1.50 | 16.9 | Very common — structural, automotive |
| M12 | 1.75 | 14.5 | Structural bolts, machinery |
| M14 | 2.00 | 12.7 | Automotive wheel bolts (some) |
| M16 | 2.00 | 12.7 | Heavy structural, flange bolts |
| M18 | 2.50 | 10.2 | Heavy equipment |
| M20 | 2.50 | 10.2 | Structural steel connections |
| M22 | 2.50 | 10.2 | Heavy machinery |
| M24 | 3.00 | 8.47 | Steel structures, cranes |
| M30 | 3.50 | 7.26 | Heavy structural |
| M36 | 4.00 | 6.35 | Bridge & industrial anchor bolts |
Highlighted rows (cyan) = most frequently encountered sizes. Bold = standard coarse pitch per ISO 261.
Metric Fine Thread Pitches
Fine threads are stronger in tension and resist vibration loosening better — but they strip easier in soft materials and require more turns to tighten. You'll find them most often in automotive and precision applications. Always confirm "fine" when ordering or re-tapping.
| Size | Coarse Pitch | Fine Pitch(es) | Fine Use Case |
|---|---|---|---|
| M6 | 1.00 | 0.75 | Precision instruments |
| M8 | 1.25 | 1.00 | Automotive, brake calipers |
| M10 | 1.50 | 1.00 / 1.25 | Engine bolts, spark plugs |
| M12 | 1.75 | 1.25 / 1.50 | Cylinder heads, structural |
| M14 | 2.00 | 1.50 | Wheel bolts, studs |
| M16 | 2.00 | 1.50 | Machinery, heavy structural |
| M20 | 2.50 | 1.50 / 2.00 | Precision machinery |
TPI to Metric Pitch Conversion Chart (UNC/UNF)
These are the most common US Unified National thread sizes. The pitch (mm) column lets you quickly compare against metric equivalents and confirm nothing in your parts bin will work as a substitute.
| Designation | TPI | Pitch (mm) | Diameter | Series |
|---|---|---|---|---|
| #4-40 | 40 | 0.635 | 2.84 mm | UNC |
| #6-32 | 32 | 0.794 | 3.50 mm | UNC |
| #8-32 | 32 | 0.794 | 4.17 mm | UNC |
| #10-24 | 24 | 1.058 | 4.83 mm | UNC |
| #10-32 | 32 | 0.794 | 4.83 mm | UNF |
| 1/4-20 | 20 | 1.270 | 6.35 mm | UNC |
| 1/4-28 | 28 | 0.907 | 6.35 mm | UNF |
| 5/16-18 | 18 | 1.411 | 7.94 mm | UNC |
| 3/8-16 | 16 | 1.588 | 9.525 mm | UNC |
| 7/16-14 | 14 | 1.814 | 11.11 mm | UNC |
| 1/2-13 | 13 | 1.954 | 12.70 mm | UNC |
| 5/8-11 | 11 | 2.309 | 15.88 mm | UNC |
| 3/4-10 | 10 | 2.540 | 19.05 mm | UNC |
| 7/8-9 | 9 | 2.822 | 22.23 mm | UNC |
| 1"-8 | 8 | 3.175 | 25.40 mm | UNC |
Cyan rows = most common sizes in hardware stores and home repair. UNC = Unified National Coarse. UNF = Unified National Fine.
How to Measure Thread Pitch Without a Gauge
You've got a loose bolt with no label. Here's the field method — no pitch gauge required, just calipers or a ruler.
Method 1: Mark 10 Threads (Metric)
Press the threaded shank against a piece of paper and roll it slightly to leave an impression of the threads, or simply lay it flat and mark the centers of the first and eleventh crest. Measure the distance between marks in millimeters with calipers, then divide by 10. That's your pitch in mm.
Example: 10 thread spans measure 12.5 mm → pitch = 12.5 ÷ 10 = 1.25 mm. That's M8 coarse.
Method 2: Count Threads Per Inch (Imperial)
Lay the bolt against a ruler marked in inches. Count the number of thread crests visible within exactly one inch. That number is the TPI. Then confirm the diameter with calipers to get the full designation.
Method 3: Thread Pitch Gauge (Best)
A thread pitch gauge is a small fan of metal leaves, each stamped with a pitch value. Press a leaf against the thread — when it meshes perfectly with zero wobble, you've found your pitch. A decent set costs $12–$20 and handles both metric and imperial. It's the fastest method by far.
The Closest Metric / Imperial Near-Matches (And Why They Fail)
These pairs look close enough that people try to mix them. None are interchangeable.
| Metric | Imperial Near-Match | Diameter Gap | Pitch Gap |
|---|---|---|---|
| M6 × 1.0 | 1/4-20 | −0.35 mm | 0.27 mm mismatch |
| M8 × 1.25 | 5/16-18 | +0.06 mm | 0.16 mm mismatch |
| M10 × 1.5 | 3/8-16 | +0.475 mm | 0.088 mm mismatch |
| M12 × 1.75 | 1/2-13 | −0.30 mm | 0.20 mm mismatch |
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Open Free Identifier →Recommended Tool
Thread Pitch Gauge Set (Metric + Imperial)
A dual-system thread pitch gauge is the single most useful tool for identifying unknown fasteners. A good set covers M0.5–M6 fine and 4–80 TPI, with individual stainless-steel leaves that mesh cleanly against thread crests. Costs $12–$20 and eliminates guesswork permanently.
View Options on Amazon →Frequently Asked Questions
What is thread pitch in mm?
Thread pitch in metric is the distance in millimeters between adjacent thread crests. An M8 × 1.25 bolt has threads spaced 1.25 mm apart. Smaller pitch numbers mean finer, more tightly packed threads. The pitch is always the second number after the "×" in a metric designation.
How do you convert TPI to mm pitch?
Divide 25.4 by the TPI number: Pitch (mm) = 25.4 ÷ TPI. Example: 20 TPI = 25.4 ÷ 20 = 1.270 mm. To go the other direction: TPI = 25.4 ÷ Pitch (mm). So M8 coarse at 1.25 mm converts to 25.4 ÷ 1.25 = 20.3 TPI — close to 1/4-20 but not the same diameter.
What is the standard thread pitch for M8?
The standard (coarse) thread pitch for M8 is 1.25 mm. The fine variant is M8 × 1.0. Always confirm before re-tapping: automotive parts often use fine thread while general hardware uses coarse.
How do I measure thread pitch without a gauge?
Mark 10 consecutive thread crests on the bolt shank, measure the total span in mm, then divide by 10. For imperial, count crests in one inch — that's the TPI. A pitch gauge is faster and more reliable for frequent use.
Are M10 × 1.5 and 3/8-16 the same thread?
No. M10 × 1.5 has a 10.0 mm major diameter and 1.5 mm pitch. 3/8-16 has a 9.525 mm diameter and 1.588 mm pitch (16 TPI). The 0.475 mm diameter gap and pitch mismatch make them incompatible. Never substitute one for the other.