How To Measure NPT Fittings And Pipe Sizes

What's the deal with measuring pipe threads? Do you order the wrong size and wonder why? This very easy rule will save you headaches for the rest of your pipe measuring life. 3/4" NPT pipe refers to the nominal INSIDE diameter of the pipe, not the outside diameter.

There are several types of pipe thread fittings, each designed for specific applications and purposes.

Here are some common types: 

1. Tapered Threads: These threads are used for creating a tight seal and are often found in pipe fittings (NPT threads are always tapered.)

2. Square Threads: These threads have a square cross-section and are used in heavy-duty applications where high strength is required.

3. Acme Threads: Similar to square threads but with a trapezoidal shape, these threads are used in lead screws and other applications requiring high load capacity.

4. Metric Threads: These are standardized threads based on the International Organization for Standardization (ISO) and are used globally.

5. British Standard Threads (BSPT): These are similar to metric threads but follow British standards.

6. National Pipe Thread (NPT): These are used for sealing pipe threads in North America (these threads are tapered.) Most air compressor pipe fittings will be NPT.

7. Unified Thread Standard (UTS): This includes both Unified Coarse (UNC) and Unified Fine (UNF) threads, commonly used in the United States

Measuring Threads:

To measure threads, you can use several tools and methods:

1. Tape measure: Use a tape measure to measure the Outside Diameter (OD) of a male thread to deduce the thread size of an NPT thread (then subtract 1/4" to find your nominal thread size.)

2. Vernier Caliper: Measures the major and minor diameters of threads.

3. Pitch Gauge: Determines the thread pitch by matching the gauge with the threads.

4. Micrometer: Used for precise measurements of the pitch diameter.

5. Three-Wire Method: A more accurate method for measuring the pitch diameter using three wires of known diameter placed in the thread grooves.

6. Thread Gauges: These are specialized tools that check if a thread is within specified tolerances