BEGINNER’S GUIDE · PLAIN LANGUAGE
How to measure a thread and find the right fitting
Standing by a tap with a ruler, wondering whether you need ½ or ¾ inch? Start with where you measure. The outside width of a nut and the diameter of its threaded opening are different measurements.
This guide and calculator cover G pipe threads. A measurement helps you find a likely size; it does not confirm that any two parts will fit together.
First, look at the part
A projecting end with grooves around the outside has a male thread. A hole with grooves inside has a female thread, such as the nut on a flexible tap connector. Choose the matching picture in the calculator.
Do not dismantle plumbing just to guess a size. Measure an exposed part. If you need to disconnect a water pipe, turn off its water supply first. If you are unsure how to do that safely, show a photo to a supplier or ask for help.
A caliper makes the job easier
Caliper — the first choice
The large jaws measure around the outside of a part; the small jaws measure an opening. A digital display makes reading easier. Clean the measuring faces, close the jaws and check zero before measuring. Do not squeeze the part hard.
Ruler — for a first estimate
Hold the millimetre scale close to the edge and look straight at it. A tape measure can help, but its moving hook and awkward access make small diameters harder to read. Check an uncertain result with a caliper.
Good lighting and a photo of the whole connection help. Write the measurement in millimetres. Do not add decimal places that your measuring tool cannot show.
Sources: 2
Outside thread: measure across the raised edges
- Remove loose dirt so you measure the part rather than a layer of deposits.
- Place the large caliper jaws across the thread. Touch the opposite raised edges, not the grooves between them.
- Keep the tool square to the length of the part. Do not measure at an angle.
- Measure again after rotating the tool slightly. Choose “Outside thread” in the calculator and enter the reading.
Do not measure the smooth pipe behind the thread or the width across the flats of a nut. If the diameter changes noticeably along the thread, do not assume it is G.
Inside a nut: measure the opening, not the body
- Find the actual thread inside. A washer, smooth entrance or sloping edge is not the measuring surface.
- Touch opposite inward-facing thread crests with the small caliper jaws. Do not push them into the deep grooves.
- Measure through the centre of the opening. A short line near the edge of the circle gives too small a reading.
- Measure again and choose “Inside a nut” in the calculator. For the same nominal thread size, the inside reading is smaller than the outside reading.
Do not force a washer out of position. If it covers the thread or you cannot reach the measuring surface, take the safely disconnected part to a supplier instead.
Why is ½ inch not 12.7 mm here?
For a G pipe thread, the inch fraction is a nominal size name. It is not the diameter you read on a ruler. Do not divide your measurement by 25.4: the calculator compares it with a thread table instead.
ONE SIZE · TWO MEASUREMENTS
About 21 mm outside or about 18.6 mm across the threaded opening may both point to the same size:
G ½″These readings describe the two mating thread sides, not two different inch sizes.
The calculator uses identification reference values. Its ±1 mm search window helps with ruler measurements; it is not an allowed thread clearance or a guarantee that parts fit.
Four easy mistakes to avoid
- Measuring the spanner size: the outside width of a nut does not tell you its thread size.
- Measuring a smooth pipe: this calculator identifies threads, not arbitrary pipe diameters.
- Choosing the wrong picture: entering an inside reading as an outside reading may suggest a different size.
- Rounding straight to an inch size: enter the actual millimetre reading. If there is no match, check where you measured.
Sources: 4
Take three pieces of information to the shop
Note the suggested size, which thread side you measured and what the part is for. If possible, take the old, safely disconnected part or a clear photo together with the measurement.
- The size, such as G ½″, and whether the new part needs an outside or inside thread. An outside thread connects to a suitable inside thread, not another outside thread.
- Both ends of a hose or adapter. Do not assume they have the same size.
- The sealing method and intended use. A matching size does not confirm the right washer, thread length or suitability for the application.
If the new part will not start turning easily by hand, do not force it with a spanner. Check the size and thread type. This guide helps identify a part; it does not replace the manufacturer’s installation instructions.
Two common questions
I measured about 21 mm. Do I definitely need ½ inch?
That is a possible reading for a G ½″ outside thread. First check that you measured an outside thread, rather than an opening or an aerator. Confirm the thread type and connection design before buying.
The calculator found no size. What should I do?
Check mm/cm, the selected picture and the measuring surface. Measure again with a caliper. The part may use another thread type or a size outside our table. Do not force a match by rounding.