Thread - ISO Metric Thread Sizing & Limits

A thread calculator turns a metric thread callout into basic and limit dimensions for an ISO metric screw thread. Type the pitch and the major diameter, pick a tolerance grade and a position, and the tool returns the minor diameter d1, the pitch diameter d2, the height of the fundamental triangle H, and the max and min limits for each diameter.

• • Sizing a metric bolt for a drawing: Designers turn an M-style callout into d2 and d1 without re-deriving the ISO 965 equations.
• • Choosing a tap drill: Shop-floor users read d1 to size a tap drill, or read d2 to verify a thread plug or ring gauge.
• • Confirming tolerance class on a print: Quality teams check whether an M10 x 1.5 thread is supposed to meet 6g, 6h, or 4h6h limits.
• • Re-checking a thread after plating: Engineers pick a position such as g or f to see how much room is left for zinc or chrome.

The ISO metric thread callout shows up on every drawing, every bolt bin, and every inspection report. The number after the x is the pitch in mm, the number before the x is the basic major diameter, and the suffix such as 6g tells the inspector how much the actual thread can deviate.

Once the thread dimensions are fixed, the same major diameter and pitch feed straight into the bolt torque calculator to size the tightening torque from the nut factor and the bolt grade.

The geometry starts from the height of the fundamental triangle H = P * sqrt(3) / 2 and uses the 60 degree V-thread profile from ISO 965-1 to derive d1 and d2 from the major diameter. The chosen tolerance position then sets the fundamental deviation es, and the chosen grades set Td and Td2.

• P and d: P is the pitch in mm; d is the major diameter in mm. The two numbers come from the metric callout.
• H, d1, d2: H = P * sqrt(3) / 2, d1 = d - 2 * (5/8) * H, d2 = d - 2 * (3/8) * H, in mm.
• es and grade tolerances: es is the external fundamental deviation: 0 at h, -(15 + 11 * P) / 1000 at g, -(30 + 11 * P) / 1000 at f, -(50 + 11 * P) / 1000 at e.

After the basic diameters are known, the upper limit is set by adding es to the basic diameter, then the grade tolerance is subtracted to land on the lower limit. The min and max minor diameter add a small adjustment y or z that accounts for the curved thread root.

The grade table in ISO 965-1 ties a k multiplier to each grade number: k = 0.5 at grade 3, k = 1.0 at grade 6, k = 2.0 at grade 9 for pitch diameter.

According to ISO 965-1:2013, the basic minor diameter d1 = d - 2 * (5/8) * H, the basic pitch diameter d2 = d - 2 * (3/8) * H, and H = P * sqrt(3) / 2 for a 60 degree metric thread.

Four short definitions keep the result panel honest for any M-style callout.

ISO 965-1 also defines an internal companion to es called EI, which is 0 at H and +(15 + 11 * P) / 1000 at G. The same k table applies, so a 6g external thread and a 6H internal thread fit together by design.

Readers who need only the basic pitch diameter d2 without the tolerance class can cross-check the value with the pitch diameter calculator, which solves the same d2 formula in a standalone form.

Type the pitch and the major diameter, pick the tolerance class the callout uses, and read the basic and limit dimensions from the result panel. The defaults match a general M10 x 1.5 - 6g thread so the thread calculator shows a typical case on first load.

1. 1 Read the metric callout: M10 x 1.5 means d = 10 mm and P = 1.5 mm.
2. 2 Enter the pitch P: Type the pitch into the Thread Pitch box, in mm.
3. 3 Enter the major diameter d: Type the major diameter into the Basic Major Diameter box, in mm.
4. 4 Pick the tolerance grades: Choose the ISO 965-1 grade for the pitch diameter and the grade for the major diameter.
5. 5 Pick the tolerance position: Position g is the default for bolts, h sets es to 0, and e or f leave more room for plating.
6. 6 Read the result panel: Read the basic diameters, the height of the fundamental triangle, and the max and min limits.

A reader verifies an M8 x 1.25 - 6g bolt. They enter P = 1.25 and d = 8, leave the grades on 6, and leave the position on g. The result panel reports d2 = 7.188 mm, d1 = 6.647 mm, dmax = 7.971 mm, dmin = 7.757 mm.

Before tapping the hole, the reader can pre-check the through-hole size in the clearance hole calculator so the bolt head and the mating part sit on the right Close, Medium, or Free fit.

The thread calculator turns a callout into numbers a shop or a designer can act on, in a single read instead of by hand.

• • One read for basic and limit diameters: The result panel returns d, d1, d2, H, and the max and min limits at the same time.
• • ISO 965-1 equations in plain sight: The formula box and the variables list walk through the geometry and the tolerance math.
• • Tolerance position and grade are user-controlled: The position and grade selectors let the same tool answer a 6g general bolt, an h-position precision thread, and a coated part in the f or e position.
• • Worked example ties to a real M-size: A worked example with M10 x 1.5 - 6g shows the equations applied to a common metric thread.

The thread calculator is a useful sanity check when a vendor calls out a non-standard pitch or tolerance class. Setting the pitch and the position exposes the limit diameters without re-reading the ISO tables.

For multi-bolt assemblies, the same major diameter and pitch feed a pattern of holes through the bolt circle calculator to lay out a flange, a pump base, or a wheel hub on a bolt circle.

Five short factors describe which inputs change the thread calculator's result.

• • The tool follows the ISO 965-1 equations for general-purpose metric threads. It does not cover MJ, aerospace, or proprietary thread forms.
• • The result is a dimension, not a fit decision. A go/no-go plug or ring gauge is still the right way to confirm a thread on the bench.

Before ISO 965-1, the same tolerances were published in BS 3643 and DIN 13, and the result panel lines up with the same M-style callouts a reader would see on a fastener purchased in the United Kingdom, Germany, or the rest of the European Union.

According to Omni Calculator (citing BS 3643-1), an M10 x 1.5 - 6g thread has a basic pitch diameter d2 of about 9.026 mm and a basic minor diameter d1 of about 8.376 mm, matching the ISO 965-1 equations.

Readers who want to confirm the bar stock the threaded hole lives in can move the major diameter d into the metal weight calculator and read the kilograms per metre for the same stock.