Manufacturing · Fasteners
Thread Reverse Engineering Calculator
Identify unknown threads from measured diameter and pitch. Derive major, pitch, and minor diameters, mating thread geometry, tap drill size, and closest standard matches.
Thread form
Linear pitch in millimetres — standard for metric threads
Internal thread cross-section
Result
Derived thread
P = 1.25 mm · 20.32 TPI
Likely M8 × 1.25
Likely M8 × 1.25. Use the mating dimensions below to design the opposite gender thread.
Measured thread
Mating external (bolt)
Ø 6.80 mm
From standard table (M8 × 1.25)
Likely standard matches
- M8 × 1.25metric
Δ major +0.0002 mm · Δ pitch +0 mm
- 5/16"-18 UNCunc
Δ major +0.0622 mm · Δ pitch +0.1610 mm
- 1/4"-20 UNCunc
Δ major +1.65 mm · Δ pitch +0.0200 mm
- 3/8"-16 UNCunc
Δ major +1.52 mm · Δ pitch +0.3380 mm
- M10 × 1.5metric
Δ major +2.00 mm · Δ pitch +0.2500 mm
When you have an unknown threaded part — a tapped hole, bolt, or legacy fitting — you can often identify it by measuring one diameter and the pitch. This calculator works backward from those shop measurements to recover nominal major diameter, pitch diameter, minor diameter, and the basic dimensions of the mating thread.
For a symmetric V-thread basic profile, diameter offsets are proportional to pitch P:
60°: D = D₁ + 1.0825 P
D₂ = D − 0.6495 P
55° Whitworth uses BS 84 constants (≈ 1.2807 P / 0.6403 P)
- D / d — major (nominal) diameter
- D₂ / d₂ — pitch diameter
- D₁ / d₁ — minor diameter
- P — pitch (mm) or 25.4 / TPI
You measure an internal thread: minor ID ≈ 4.92 mm, pitch 1.0 mm, 60° form. Then D ≈ 4.92 + 1.0825 × 1.0 ≈ 6.0 mm — likely M6 × 1.0. Tap drill for a matching hole ≈ 5.0 mm. Mating bolt major ≈ 6.0 mm basic.
What should I measure on an internal thread?
Use calipers on the smallest inside diameter (minor diameter, at the thread roots) and measure pitch with a thread pitch gauge or by counting threads over a known length.
What is the difference between 60° and 55° thread forms?
60° V-threads cover ISO metric and Unified (UNC/UNF) fasteners. 55° Whitworth threads are common on older British equipment and some pipe fittings. The flank angle changes the depth and diameter relationships.
Why might my result not match a standard designation exactly?
This tool uses the basic thread profile without tolerance allowances. Wear, plating, and manufacturing class (6H, 6g, etc.) shift real dimensions. Standard tables also round crests and roots, especially on Whitworth threads.
Can this identify UNF versus UNC?
Only when pitch differs. If two standards share the same nominal size and pitch, the calculator cannot distinguish them from geometry alone.
How do I design the mating thread?
Use the mating thread diameters in the result panel. For a mating tapped hole, start with the recommended tap drill size and cut threads with the matching tap or single-point tool.
- Basic profile only — no tolerance class (6H, 6g) or allowance applied
- Worn, damaged, or plated threads skew measurements
- Whitworth matching limited to catalogued BSW sizes
- Not a substitute for go/no-go gauges or certified inspection
After identifying a thread, use the Thread Engagement calculator to check minimum engagement and strip force.