--- type: "thread" name: "1 1/2-11.5 NPT" designation: "1 1/2-11.5 NPT" series: "npt" series_name: "NPT Taper Pipe Thread" system: "pipe" size_label: "1 1/2" tpi: 11.5 pitch_in: 0.086957 pitch_mm: 2.2087 basic_major_dia_in: 1.9 basic_major_dia_mm: 48.26 basic_pitch_dia_in: 1.82234 basic_pitch_dia_mm: 46.2874 basic_minor_dia_in: 1.7528 basic_minor_dia_mm: 44.5211 tap_drill: "1" tap_drill_dia_in: 1 tap_drill_dia_mm: 25.4 standard: "ASME B1.20.1, ASME B1.20.3; public-domain primary FED-STD-H28/7A (1984)" verified: "August 1, 2026" url: https://shopmath.org/threads/npt/1-1-2 --- # 1 1/2-11.5 NPT Thread — Dimensions, Tap Drill & Clearance The tap drill for 1 1/2-11.5 NPT taper pipe thread is 1 — 1.0000 in (25.400 mm). Basic pitch diameter is 1.8223 in (46.287 mm) and basic minor diameter is 1.7528 in (44.521 mm). - **Series**: NPT Taper Pipe Thread - **Designation**: 1 1/2-11.5 NPT - **Threads per inch**: 12 - **Tap drill**: 1 - **Standard**: ASME B1.20.1, ASME B1.20.3; public-domain primary FED-STD-H28/7A (1984) ## Basic Thread Dimensions for 1 1/2-11.5 NPT | Property | Inch | Millimetre | | --- | --- | --- | | Basic major diameter | 1.9000 | 48.260 | | Threads per inch | 12 | — | | Pitch | 0.0870 | 2.209 | | Basic pitch diameter | 1.8223 | 46.287 | | Basic minor diameter | 1.7528 | 44.521 | | Sharp V thread height (H) | 0.0753 | 1.913 | _Basic dimensions are the theoretical values before any tolerance class is applied._ ## Taper Pipe Thread Geometry for 1 1/2-11.5 NPT 1 1/2-11.5 NPT is a taper thread: the diameter changes along the thread, so every diameter below is stated at a named plane. The taper is 0.7500 in per foot on diameter. | Property | Inch | Millimetre | | --- | --- | --- | | Pipe outside diameter | 1.9000 | 48.260 | | Taper on diameter, per foot | 0.7500 | 19.050 | | Pitch Ø at end of pipe (E₀) | 1.7961 | 45.621 | | Pitch Ø at hand-tight plane (E₁) | 1.8223 | 46.287 | | Pitch Ø at effective plane (E₂) | 1.8413 | 46.769 | | Hand-tight engagement (L₁) | 0.4200 | 10.668 | | Effective thread length (L₂) | 0.7235 | 18.377 | | Overall external thread length (L₄) | 1.0252 | 26.040 | | Thread height (h) | 0.0696 | 1.767 | | Sharp V height (H) | 0.0753 | 1.913 | | Minor Ø at hand-tight plane | 1.7528 | 44.521 | _ASME B1.20.1 publishes these dimensions in inches only; the millimetre column is an exact conversion (1 in ≡ 25.4 mm). L₁ is the hand-tight engagement length and is tabulated in the standard, not derived._ ## Other Thread Pitches in the Same Size as 1 1/2-11.5 NPT A 1 1/2 fastener is not always 1 1/2-11.5 NPT. These are the other standard pitches cut on the same nominal size. | Thread | Series | Pitch | Tap drill | | --- | --- | --- | --- | | [1 1/2-11.5 NPTF](https://shopmath.org/threads/nptf/1-1-2.md) | NPTF Dryseal Pipe Thread | 12 TPI | 1 | ## Sources and Method for 1 1/2-11.5 NPT ### Basic dimensions - **Source**: ASME B1.20.1 taper pipe thread formulas; E0 and L2 reproduced to 5 decimals in docs/research/data-sourcing.md §3.3. Public-domain primary: FED-STD-H28/7A (1984).; pipe OD / tpi / L1 / L4: ASME B1.20.1 pipe OD / tpi / L1 / L4 via Wikipedia National pipe thread (CC BY-SA 4.0); OD and tpi cross-checked against FreeCAD FeatureHole.cpp NPT block (LGPL-2.1+) - **Method**: computed+compiled - **Verified against**: Wikipedia National pipe thread standard-sizes table (CC BY-SA 4.0) — 98 checks passed to 1e-5 in ### Taper pipe geometry - **Source**: ASME B1.20.1 taper pipe thread formulas; E0 and L2 reproduced to 5 decimals in docs/research/data-sourcing.md §3.3. Public-domain primary: FED-STD-H28/7A (1984).; pipe OD / tpi / L1 / L4: ASME B1.20.1 pipe OD / tpi / L1 / L4 via Wikipedia National pipe thread (CC BY-SA 4.0); OD and tpi cross-checked against FreeCAD FeatureHole.cpp NPT block (LGPL-2.1+) - **Method**: computed+compiled - **Verified against**: Wikipedia National pipe thread standard-sizes table (CC BY-SA 4.0) — 98 checks passed to 1e-5 in ### Assumptions - NPT threads are not a seal on their own — the standard assumes a sealing compound or tape on the joint. - Dataset scope: L1 is tabulated, not derivable (data-sourcing.md §3.3). EXCLUDED: NPTF crest/root truncation limits (single-sourced), NPSM/NPSC/NPSF straight pipe threads, BSPT/BSPP. **Data last verified**: August 1, 2026 ## Common Questions About 1 1/2-11.5 NPT ### What is the tap drill size for 1 1/2-11.5 NPT? The tap drill for 1 1/2-11.5 NPT is 1 — 1.0000 in (25.400 mm). Taper pipe threads are tapped with a taper tap and sealed with compound or tape. ### What is the pitch diameter of 1 1/2-11.5 NPT? The basic pitch diameter of 1 1/2-11.5 NPT is 1.8223 in (46.287 mm). ### What is the minor diameter of 1 1/2-11.5 NPT? The basic minor diameter of 1 1/2-11.5 NPT is 1.7528 in (44.521 mm). That is the theoretical root of the internal thread, not the drilled hole size — see the tap drill table for the hole to drill. ### What is the thread pitch of 1 1/2-11.5 NPT? 1 1/2-11.5 NPT has 12 threads per inch, a pitch of 0.0870 in (2.209 mm). ### What is the hand-tight engagement length (L₁) of 1 1/2-11.5 NPT? The hand-tight engagement length L₁ for 1 1/2-11.5 NPT is 0.4200 in (10.668 mm), on a pipe outside diameter of 1.9000 in (48.260 mm). ## Related pages - Smaller in NPT Taper Pipe Thread: [1 1/4-11.5 NPT](https://shopmath.org/threads/npt/1-1-4.md) — tap drill 1 - Larger in NPT Taper Pipe Thread: [2-11.5 NPT](https://shopmath.org/threads/npt/2.md) — tap drill 1 - All NPT Taper Pipe Thread sizes: [NPT Taper Pipe Thread thread chart](https://shopmath.org/threads/npt.md) - Every thread series: [Screw thread reference](https://shopmath.org/threads.md) - Cross-series chart: [Tap drill chart](https://shopmath.org/charts/tap-drill.md) --- *Source: [ShopMath](https://shopmath.org/threads/npt/1-1-2) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*