Chart · Threads

Thread chart: NPT, BSPT and BSPP dimensions, tap drills and metric threads

Major diameter, threads per inch, pitch, minor diameter and tap drill for NPT, BSPT (R/Rp/Rc), BSPP (G) and metric threads. Every number is calculated from the formulas of the standards and checked against 87 published values.

What 1/2 NPT, BSPT 1/2 or G 1/4 means in inches and millimetres is in the charts below: major diameter, threads per inch, pitch, minor diameter and tap drill. The inch figure is not a dimension but an old nominal pipe size. For metric threads you will find coarse threads from M1 to M64 and the common fine threads.

Quick ID: your thread in three questions

1. Parallel or tapered? BSPP and metric threads are parallel; BSPT and NPT are tapered 1:16. A tapered thread gets thinner towards the tip: by 1 mm in diameter over 16 mm of length ².

2. 55° or 60°? BSPP and BSPT have a 55° thread angle (Whitworth profile); NPT, UNC/UNF and metric threads have 60° ^{1, 2, 3, 4}.

3. How many threads per inch? NPT, BSPT and BSPP all have 14 TPI at 1/2″ and 3/4″ – there only the thread angle tells them apart. From 1″ BSP has 11 TPI, NPT 11.5 ^{1, 3}.

If the thread is given in a data sheet, read the designation there first. Where to find it is shown in How to read data sheets.

NPT thread chart (ASME B1.20.1, tapered 1:16, 60°)

NPT is the US pipe thread, found on US equipment, in instrumentation and on compressors. Like BSPT it is tapered 1:16, but it has a flattened 60° profile with a thread depth of h = 0.8 × p ³.

NPT has no single diameter. The standard gives the pitch diameter E₀ at the small end, the hand-tight engagement L₁ and the effective thread length L₂ ³. For ordering, the size is enough; you need the values for gauge checks.

Calculated from the formulas of ASME B1.20.1 ^{3, 15}. E₀ = pitch diameter at the small end. Tap drill: common chart values, unreamed; C and R are letter drills ¹¹. Tap makers sometimes give smaller holes, e.g. 0.332 in for 1/8 NPT and 0.703 in for 1/2 NPT ¹². The data sheet of your tap is binding. – = no guide value collected.

NPT vs NPTF vs NPSM

NPT seals with sealant such as PTFE tape. NPTF (“Dryseal”, ASME B1.20.3) is made so that crests and roots touch when tightened; it therefore seals without sealant ⁹. ASME B1.20.1 also covers straight pipe threads, e.g. NPSM for swivel connections ³.

By hand, an NPT thread engages 4.1 to 5.0 turns (L₁, sizes 1/16 to 2). With a wrench, about 3 more turns follow up to 2″ and about 2 from 2 1/2″ (wrench makeup L₃) ^{3, 15}. A BSPT thread takes 4.4 to 6.9 turns at the gauge plane (R 1/16 to R 2) ^{2, 15}.

BSPT chart: R, Rp and Rc (ISO 7-1, tapered 1:16, 55°)

BSPT threads are tapered 1:16 and seal on the thread together with a sealant ². The male thread is called R, the female thread Rp (parallel) or Rc (tapered). The standard is ISO 7-1, in Europe also EN 10226-1 for R with Rp and EN 10226-2 for Rc. Sizes run from 1/16 to 6 ².

Because the thread is tapered, it has no fixed diameter. It is measured at the gauge plane, and the gauge length L runs from there to the small end. Its tolerance is ±1 thread up to 2″ and ±1½ threads from 2 1/2″ ².

Dimensions in mm. Pitch and minor diameter at the gauge plane as for BSPP (table above). Gauge lengths and tolerances per ISO 7-1:1994 table 1 ²; diameter at the small end calculated as d − L/16 ¹⁵. Applies equally to R (male), Rp and Rc (female).

R with Rp or Rc – and the tap drill for Rc

Both pairings are standardised and pressure-tight: R in Rp per EN 10226-1 and R in Rc per EN 10226-2 ². A parallel BSPP male in an Rp female, by contrast, is “not necessarily” leak-tight according to ISO 228-1 ¹.

For an Rc female thread the tap maker gives two routes: drill parallel if no sealing problems are expected (Rc 1/2: 17.8 mm), or drill and then ream with a 1:16 taper reamer (17.7 mm). It calls the second the most reliable and tightest option ⁸.

BSPP chart: G threads (ISO 228-1, parallel, 55°)

BSPP or G threads are parallel and have a rounded 55° profile. They do not seal on the thread: the thread holds the parts together, a washer, O-ring or sealing edge at the collar seals ¹. The standard is ISO 228-1, in Germany DIN EN ISO 228-1:2003-05 ¹.

The male thread comes in two tolerance classes: class A may be smaller in pitch diameter by T_D2, class B by twice that ¹.

Dimensions in mm, calculated per ISO 228-1 ^{1, 15}. d₂ and d₁ are basic sizes; D₁ max = d₁ + T_D1 is the largest permitted minor diameter of the female thread ¹. Tap drill: values from a compiled table ⁵. ¹ For G 1/2 the tap maker gives 19.0 mm ⁸, the compiled table 19.1 mm. – = no guide value collected.

Metric thread chart (ISO 261 and 262, 60°)

Metric threads have 60° and are given in mm: M10 means 10 mm major diameter with the coarse pitch of 1.5 mm. Fine threads carry the pitch in the name, e.g. M10×1 ^{4, 6}. ISO 68-1 describes the profile; sizes and pitches are in ISO 261 and ISO 262 ⁴. In ISO 261, M40 exists only as a fine thread with 3, 2 or 1.5 mm pitch ⁴.

For the tap drill the rule of thumb is drill ≈ d − P ^{6, 10}. DIN 336 gives guide values rounded to standard drill sizes, e.g. 10.2 mm for M12 ^{8, 10}.

Metric coarse threads M1 to M64

Dimensions in mm, calculated per ISO 68-1 and ISO 261/262 ^{4, 15}. Tap drill from the rule of thumb d − P ^{6, 10}; DIN 336 rounds to standard drill sizes, e.g. M12 → 10.2 mm ^{8, 10}.

Common metric fine threads

Dimensions in mm, calculated per ISO 68-1 and ISO 261/262 ^{4, 15}. Selection of common fine threads from hydraulics and pneumatics; all 33 fine threads are in the dataset. Tap drill from the rule of thumb d − P ^{6, 10}; DIN 336 rounds to standard drill sizes, e.g. M12 → 10.2 mm ^{8, 10}.

Metric tapered threads

ISO 7-1 only covers inch sizes from 1/16 to 6 ². A metric tapered thread is defined in DIN 158-1: a male thread tapered 1:16 with an associated parallel female thread, designated e.g. “DIN 158 – M 30 x 2 keg” ¹³.

NPT vs BSP: which threads mate?

A joint is only reliably tight if the pairing is standardised. The table shows the usual combinations and where each one seals.

The data of the manufacturer of the fitting or valve in question is binding.

Mini study: threads that almost fit

Which threads are so close in diameter and pitch that they get mixed up – and do you notice when screwing them in? We calculated 14 pairs from pneumatics, hydraulics and instrumentation ¹⁵.

Sample

14 thread pairs

Selection

pairs from practice (pneumatics, hydraulics, instrumentation); per pair the difference in major diameter (tapered threads at the 4th thread) and in pitch, offset after 6 turns = 6 × ΔP

Updated

2 November 2026

Limits

calculated, not screwed together in a workshop; manufacturing tolerances not included

Major diameter of tapered threads at the 4th thread from the small end; offset = 6 × ΔP. Consequences are derived from the offset, not tested in a workshop ¹⁵.

What does this mean for you? If a thread goes in “stiff, but it goes”, that is no sign of a match. At 1/2″, 3/4″ and M5 always check the marking on both parts or use a thread angle gauge.

Tap drills for BSPP: where the drill sits in the tolerance zone

Do the usual tap drills for BSPP threads fall within the permitted range of the minor diameter D₁ – and where? The range runs from the basic size d₁ to d₁ + T_D1. Depending on size, T_D1 is 0.282, 0.445, 0.541 or 0.640 mm ¹.

Sample

14 sizes, G 1/16 to G 2

Selection

tolerance zone D₁ per ISO 228-1 table 1; drill values from a compiled table (Wikipedia, column “G 80 %”); G 1/2 cross-checked against a tap maker’s data sheet

Updated

2 November 2026

Limits

drill values from a secondary source; only G 1/2 cross-checked with a tap maker; DIN 336 tables not inspected

All 14 drills fall within the tolerance zone, with a median at 86 % of its width (range 78 to 95 %, middle half between 82 and 90 %). So the drill deliberately sits high: that spares the tap but costs flank engagement. The tap maker’s 19.0 mm for G 1/2 would sit at 68 % ^{8, 15}.

What does this mean for you? For G 1/2, 19.0 mm is the safe value; 19.1 mm still works, no more, because D₁ max is 19.17 mm ¹. For Rc threads that must be reliably tight, drill parallel and ream with a 1:16 taper reamer ⁸.

Formulas and cross-check: calculated, not copied

None of the charts on this page are copied; they are calculated from the formulas of the standards. The only inputs were the values the standards tabulate themselves: major diameter, threads per inch, gauge length, L₁ and tolerances. Everything else follows from the formulas ¹⁵.

H = height of the fundamental triangle, h = thread depth, d₂ = pitch diameter, d₃ = minor diameter of the bolt.

Sample

128 threads calculated, 87 published values for the cross-check

Selection

24 BSPP, 15 BSPT, 14 NPT, 32 metric coarse and 33 fine, 10 UN; checked against ISO 228-1 table 1 (48 values) and a widely used reproduction of the NPT standard table (39 values)

Updated

2 November 2026

Limits

ASME B1.20.1 seen only through reproductions, ISO 7-1 and ISO 261 through previews; rounding to 0.001 mm

Result: 85 of 87 values match to 0.001 mm; the rest is rounding. The two exceptions sit in a single row of the NPT reproduction, for the 3″ thread: E₀ differs in the fifth decimal place, L₂ by 0.05 in. A second reproduction of the same standard table gives exactly the values of our calculation there ^{3, 15}. So the reproduction is wrong, not the formula.

A third finding came from checking a widely used thread identification guide: for one BSPT size it gives a major diameter about 0.2 mm above the ISO 7-1 value – most likely a misprint ^{2, 15}.

Why does a 1/2″ pipe thread measure 21 mm?

The 55° profile goes back to Joseph Whitworth, who defined his thread system in 1841 ¹⁴. The size figure of pipe threads originally referred to the inside diameter of the pipe in inches ⁵. The thread, however, sits on the outside of the pipe. That is why G 1/2 measures 20.955 mm across and not 12.7 mm ¹.

Good to know: myth and fact

Matching products

Examples from the range where the thread decides the purchase.

G 1/2 (BSPP)Angle Safety Valve G1/2", 8 barView product →Rp 1/2 (BSPT)Safety Ball Valve, spring return, brass nick. plated, Rp 1/2View product →M5Push-in Elbow Fitting M5View product →FittingCutting Ring Fitting 6mmView product →CategoryPiping & Pipe ConnectorsView category →CategoryPneumatic ConnectorsView category →

Frequently asked questions

Sources

1. ISO 228-1:1994, Pipe threads where pressure-tight joints are not made on the threads – Part 1, table 1 and figures 1/2 (preview); dimensionally identical to ISO 228-1:2000 and DIN EN ISO 228-1:2003-05.
2. ISO 7-1:1994, Pipe threads where pressure-tight joints are made on the threads – Part 1, clause 1 (sizes 1/16 to 6) and table 1 (gauge lengths, tolerances), preview retrieved 2026-11-02; DIN EN 10226-1:2004-10 (R with Rp), Rc per DIN EN 10226-2.
3. ASME B1.20.1-2013, Pipe Threads, General Purpose (Inch). Table values (E₀, E₁, L₁, L₂, wrench makeup L₃) checked in two publicly available reproductions of the standard table, retrieved 2026-11-02.
4. ISO 68-1:1998, ISO 261:1998 (table 2, preview retrieved 2026-11-02), ISO 262:1998, ISO 724:1993; DIN 13-1.
5. Wikipedia: “British Standard Pipe” (CC BY-SA 4.0), retrieved 2026-11-02.
6. Wikipedia: “ISO metric screw thread” (CC BY-SA 4.0), retrieved 2026-11-02.
7. ASME B1.1-2019, Unified Inch Screw Threads.
8. Hoffmann Group, product data sheets: hand tap G 1/2 (tap drill 19 mm), machine tap Rc 1/2-14 (tap drill 17.8 mm, or 17.7 mm with a 1:16 taper reamer), step drill M12 (10.2 mm per DIN 336); retrieved 2026-11-02.
9. ASME B1.20.3, Dryseal Pipe Threads (Inch) (NPTF); as summarised in Wikipedia: “National pipe thread”, retrieved 2026-11-02.
10. DIN 336:2003-07, Diameters of drills for tapping (cut threads); summary by DIN Media (guide values, drill roughly nominal diameter minus pitch), tables of the standard not inspected; retrieved 2026-11-02.
11. Victor Machinery Exchange: NPT Taper HS Pipe Taps, column “Drill Size”, retrieved 2026-11-02.
12. GWS Tool Group: catalogue “Popular Specials – Taps, Dies, Gages”, section Pipe Taps, “Recommended Hole Size”, retrieved 2026-11-02.
13. DIN 158-1:1997-06, Metric tapered external screw threads with associated parallel internal threads – Part 1 (DIN Media); designation and 1:16 taper per the PRESSOL specification sheet on DIN 158 threads, retrieved 2026-11-02.
14. Wikipedia: “British Standard Whitworth” (CC BY-SA 4.0), retrieved 2026-11-02.
15. shukchok: own calculation, 2026-11-02 – 128 threads from the formulas of the standards (Python), check against 87 published values, 14 mix-up pairs, position of tap drills within the tolerance zone.

How to cite this page

Scharff, A. (2026): Thread chart: NPT, BSPT and BSPP dimensions, tap drills and metric threads. shukchok.com, Updated 6 November 2026. https://shukchok.com/en-ae/wissen/gewindetabelle