ISO Metric & Whitworth Thread & Caliper Measurement Calculator
Calculate thread pitch diameter, minor root diameter, tap drill sizes, and caliper 3-wire measurements.
Last reviewed by the myCalculator Editorial Team
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
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Formula & How ISO Metric & Whitworth Thread & Caliper Measurement Calculator Works
Thread formulas let you find the pitch diameter and minor diameter of a screw thread so you can check it with a calliper or choose a tap drill. For an ISO metric M12 × 1.75 thread, the pitch diameter is D − 0.6495 × P = 12 − 1.137 = 10.86 mm, the minor diameter is D − 1.0825 × P = 10.11 mm, and the tap drill size is approximately D − P = 10.25 mm, so a standard 10.2 mm drill is used. Whitworth threads use a 55° profile with their own constants and are specified in threads per inch. Always check against the standard tables and a thread gauge for critical parts, because tolerances and thread class change the limits.
- 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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Cite this Calculator
Are you using our calculation results for a university assignment, research paper, or academic report? Use the auto-generated citations below to accurately reference this tool in your bibliography.
MyCalculator.com.au. (2026). ISO Metric & Whitworth Thread & Caliper Measurement Calculator. Retrieved October 24, 2026, from https://www.mycalculator.com.au/calculators/engineering/iso-whitworth-caliper
MyCalculator.com.au. (2026) 'ISO Metric & Whitworth Thread & Caliper Measurement Calculator'. Available at: https://www.mycalculator.com.au/calculators/engineering/iso-whitworth-caliper (Accessed: 24 October 2026).
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.
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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.
Dedicated Australian home loans, borrowing capacity, mortgage repayments & refinance modeling.