ISO Metric & Whitworth Thread & Caliper Measurement Calculator
Calculate thread pitch diameter, minor root diameter, tap drill sizes, and caliper 3-wire measurements.
Input Parameters
e.g. 12mm for M12, 12.7mm for 1/2" BSW, 6mm for M6
For ISO Metric enter Pitch in mm (e.g. 1.75). For Whitworth enter TPI (e.g. 12 TPI).
Calculation Results
Formula & How ISO Metric & Whitworth Thread & Caliper Measurement Calculator Works
Screw thread geometry determines mechanical fastener strength and machining tolerances. ISO Metric threads feature a 60° symmetrical triangular flank profile, whereas British Standard Whitworth (BSW) threads feature a 55° flank angle with radiused crests and roots ($r = 0.1373 \times p$). Caliper and micrometer thread pitch measurements use the 3-wire method over the pitch diameter.
- Enter Thread Standard System: Input your target parameter value.
- Enter Nominal Major Diameter (mm) (mm): Input your target parameter value.
- Enter Thread Pitch (mm) OR Whitworth TPI: Input your target parameter value.
- Apply Formula: Calculate using
ISO: d₂ = D - 0.6495P, d₁ = D - 1.0825P, Tap Drill ≈ D - P | Whitworth: d₂ = D - 0.6403p, d₁ = D - 1.2807p, Depth = 0.6403p. - Review Results: View exact computed answers, metrics, and worked mathematical proofs.
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Frequently Asked Questions (FAQs)
What is the difference between ISO Metric and Whitworth threads?
ISO Metric threads have a 60-degree included thread flank angle with flat crests and rounded roots, specified in millimeters. Whitworth (BSW/BSF) threads have a 55-degree flank angle with rounded crests and roots, specified by diameter in inches and pitch in Threads Per Inch (TPI).
How do you calculate the correct tap drill size for metric threads?
For standard 75% thread engagement in ISO metric tapping, Tap Drill Size (mm) = Nominal Major Diameter (mm) - Thread Pitch (mm). For example, an M10 × 1.5 thread requires an 8.5 mm drill bit (10 - 1.5 = 8.5 mm).
What is the 3-wire method for measuring pitch diameter with calipers or micrometers?
The three-wire method places three precision cylinders of known diameter in the thread grooves to measure distance across the wires with a micrometer or caliper, calculating effective pitch diameter free of thread crest errors.
Authoritative Standards & Verified Reference Citations
Calculations and mathematical logic on this page are grounded in published statutory rules, regulatory standards, and official government data published by the following bodies:
Australian Standards including AS/NZS 3000 (Wiring Rules), AS 3600 (Concrete Structures), and AS 4100 (Steel Structures).
Accreditation and technical guidelines for civil, structural, mechanical, and electrical engineering.
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