Before you begin. This procedure is general reference information, not professional advice. Equipment varies by manufacturer, model, and year. Always follow the documentation supplied with your equipment — where it differs from this page, follow the manufacturer. Verify that you have the tools, the working conditions, and the ability to do this task safely. If anything is unclear, if the equipment is damaged, or if the task exceeds your experience, stop and use a qualified professional. You are responsible for your own safety and for the outcome. This site accepts no liability for any loss, damage, or injury arising from the use of this information.
Specifications marked unverified could not be traced to a manufacturer source. Do not rely on them — obtain the value from your equipment documentation.
Summary
A pipe or a hose is sold and labeled by a nominal size, and a nominal size is a name for a product family, not a measurement of the object in your hand. A pipe marked half inch can measure five eighths of an inch across, a garden hose fitting marked three quarter inch is sized by an old thread standard rather than by a ruler across the opening, and the gap between the label and the measurement is different for every material. Buying a fitting from the label alone is one of the most common ways a repair trip to the store turns into a second trip. This page shows you how to read the measurements a fitting seller actually needs: the outside diameter, whether a thread is straight or tapered, and the thread pitch.
This page teaches measuring and identifying, and it stops there. It does not print a table that converts a nominal size into a measured one, because that conversion belongs to the pipe or the fitting manufacturer and differs by material and by standard. It does not cover cutting a pipe, installing a fitting, or sealing a joint. It does not cover gas pipework at all; that work goes to a licensed plumber.
Necessary equipment
| Item | Note |
|---|---|
| Caliper | A caliper measures the outside diameter and the thread diameter. |
| Tape measure optional | A tape measure finds the diameter when a caliper does not fit around the pipe. |
| Thread pitch gauge optional | This tool has blades of different tooth width. One blade fits the thread and shows the pitch. |
| Pen and paper | Record every measurement before you go to the store. |
Identify the pipe or the hose
Estimated 2 min
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
Examine the cut end of the pipe or the hose before you handle it.
A sharp edge can scrape or nick your skin.
- Look at the pipe or the hose. Note the material.<1mReport a problem with step 1, Identify the pipe or the hose
- Common materials are copper, PEX, CPVC, PVC, steel, and reinforced rubber hose.
- Look at the end of the pipe or the hose. Determine if it has a plain end or a screw thread.<1mReport a problem with step 2, Identify the pipe or the hose
- Look for a printed size on the pipe or the fitting. Record any size you find.<1mReport a problem with step 3, Identify the pipe or the hose
Measure the diameter and the thread
Estimated 5 min
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
Do not force a caliper or a gauge onto the pipe, the hose, or the thread.
Force can damage the tool or give a false reading.
- Close the caliper jaws around the widest point of the pipe or the hose. Read the outside diameter from the display or the scale.1mReport a problem with step 1, Measure the diameter and the thread
- If a caliper is not available, wrap a tape measure around the pipe.
- Divide the circumference by 3.14 to estimate the diameter.
- Determine if the connection has a screw thread.<1mReport a problem with step 2, Measure the diameter and the thread
- If the connection has a thread, measure the thread crest-to-crest diameter with a caliper. Measure at the first, the fourth, and the last full thread.2mReport a problem with step 3, Measure the diameter and the thread
- A crest is the top ridge of one thread turn.
- Compare the three measurements. Equal numbers show a straight thread. Numbers that increase or decrease show a tapered thread.<1mReport a problem with step 4, Measure the diameter and the thread
- Match a thread pitch gauge to the thread. If no gauge is available, count the thread peaks across one inch of thread.1mReport a problem with step 5, Measure the diameter and the thread
- Record the material, the outside diameter, the thread shape, and the thread pitch.<1mReport a problem with step 6, Measure the diameter and the thread
Find the correct fitting
Estimated under a minute (partly estimated)
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
- Do not choose a fitting from the nominal size alone. A nominal size is a label. It is not always the measured size.<1mReport a problem with step 1, Find the correct fitting
- Take the pipe, the hose end, or the old fitting to a store or a supplier.Report a problem with step 2, Find the correct fitting
- Make sure the fitting matches your measurements before you buy it. Ask store staff, or look at the fitting manufacturer's own chart.Report a problem with step 3, Find the correct fitting
Terminology
This page uses one term for each item and holds it throughout, as ASD-STE100 requires. Equipment sellers and other guides use the names below for the same parts.
- Nominal size
- Also called: labeled size, trade size, the size printed on the box, half inch pipe, pipe size
- Outside diameter
- Also called: OD, pipe diameter, how wide the pipe is, outer measurement
- Thread pitch
- Also called: threads per inch, TPI, thread spacing, how fine the thread is
- Tapered thread
- Also called: pipe thread, NPT thread, cone thread, thread that gets tighter
- Straight thread
- Also called: parallel thread, hose thread, garden hose thread, straight-cut thread
- Fitting
- Also called: coupling, adapter, connector, the part that joins two pipes
When to use a professional
Never use this method for a gas pipe or a gas fitting. Call a licensed plumber for gas pipework instead.
This page stops at the correct measurement. It does not give steps to cut a pipe, join a fitting, or seal a thread.
Take the pipe, the hose, or the old fitting to a store if you are not sure of the material.
Call a licensed plumber for a pipe or a fitting you cannot safely reach or identify.
Common questions
Why isn't a half inch pipe actually half an inch?
A nominal size is a label a manufacturer gives to a product family. It does not have to equal the outside measurement you get with a caliper or a tape measure, and the gap between the label and the measurement is different for every material. Copper, PEX, CPVC, PVC, steel, and hose all use different labeling systems. Measure the item in your hand instead of trusting the number printed on the old part or the packaging, then check that measurement against the specific manufacturer's own chart for that material.
What is the difference between a straight thread and a tapered thread?
A tapered thread narrows along its length, so the crest-to-crest diameter changes from the first thread to the last thread. A straight thread stays the same diameter along its whole length. Many household water supply pipe fittings use a tapered thread, while many hose and appliance connections use a straight thread, but the only sure way to know is to measure your own fitting.
Do I need a caliper, or can I use a tape measure?
A caliper gives a more precise reading, and it is the tool a fitting manufacturer's own identification guide recommends. A cloth or metal tape measure works too: wrap it around the pipe, read the circumference, and divide by 3.14 to estimate the diameter. Either tool is enough to bring an accurate measurement to the store.
The store has a size chart on the wall. Can I just use that?
A chart on the wall may cover only the materials that particular store stocks, and a chart for one manufacturer's product is not guaranteed to match a different brand's fitting. Bring your own measurement and the old part or a cut sample if you have one. Store staff can then match your numbers against their current stock instead of you guessing from a general chart.
Does this method work on a gas connection?
No. Never use this method on a gas pipe or a gas fitting. Call a licensed plumber for gas pipework instead. A mistake in a gas system can leak a dangerous, invisible gas. Measure only water pipe, drain pipe, or a hose with this method.
What do 'GHT' and 'NPT' mean on a fitting box?
GHT is short for garden hose thread, the straight thread on most outdoor hose connections. NPT is short for National Pipe Taper (also written National Pipe Tapered), a tapered thread standard used on many water and gas fittings. The two do not interchange without an adapter, and the straight-versus-tapered check on this page is how you tell them apart on the part in your hand.
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Sources
Every specification on this page is traced to one of these sources. Tier 1 is a manufacturer document, a published standard, or a government safety authority. See Standards for the full tiering rule.
- Tier 1SharkBite Installation Guide
- Tier 1Thread and End Connection Identification Guide (MS-13-77, RevH)
Revision history
Current version 1 · updated 2026-08-04 · next review due 2027-08-04
- v · 2026-08-04Initial draft from tier-1 manufacturer sources: SharkBite (Reliance Worldwide Corporation) installation guide and Swagelok thread and end connection identification guide. Deliberately does not publish a nominal-to-measured size table; every such value routes to the manufacturer.
Corrections to this page are listed in the public corrections log. Reports that are credible and not yet verified are listed in the public errata register, and an open one renders above the step it concerns.
Comments and corrections
Found an error? A correction with a manufacturer source is applied and logged publicly. You can also write to corrections@fixchecklist.com.
Every step above carries its own report control, which tags the report with that step so a correction reaches the right place. For anything about the procedure as a whole — the order of the sections, a step that is missing, something that did not match your equipment — use this: report something about this whole procedure.