NPT Tap Drill Size
Pick a pipe thread size from 1/16 to 3 inch and get the tap drill for NPT — with its decimal and millimetre size and the NPS straight-thread drill — from a tap maker's published chart.
The answer updates as you choose. Nothing is sent anywhere.
1/8″-27 NPT tap drill
R drill — 0.3390″ (8.61 mm)
27 threads per inch. For a straight (NPS) pipe thread of the same size the chart gives S — 0.3480″ (8.84 mm).
What this page cannot tell you
These are the drill sizes one tap maker publishes. Charts from other makers sometimes list a slightly different drill for the same thread. Material, tap condition and the required thread engagement (checked with a gauge or the mating fitting) decide the final result; follow your tap manufacturer’s figure where it differs.
Full NPT / NPS tap drill chart
| Size | TPI | NPT drill | Decimal (in) | mm | NPS drill |
|---|---|---|---|---|---|
| 1/16″ | 27 | D | 0.2460 | 6.25 | — |
| 1/8″ | 27 | R | 0.3390 | 8.61 | S (0.3480) |
| 1/4″ | 18 | 7/16" | 0.4375 | 11.11 | 29/64" (0.4531) |
| 3/8″ | 18 | 37/64" | 0.5781 | 14.68 | 19/32" (0.5938) |
| 1/2″ | 14 | 23/32" | 0.7188 | 18.26 | 47/64" (0.7344) |
| 3/4″ | 14 | 59/64" | 0.9219 | 23.42 | 15/16" (0.9375) |
| 1″ | 11-1/2 | 1-5/32" | 1.1563 | 29.37 | 1-3/16" (1.1875) |
| 1-1/4″ | 11-1/2 | 1-1/2" | 1.5000 | 38.10 | 1-33/64" (1.5156) |
| 1-1/2″ | 11-1/2 | 1-47/64" | 1.7344 | 44.05 | 1-3/4" (1.7500) |
| 2″ | 11-1/2 | 2-7/32" | 2.2188 | 56.36 | 2-7/32" (2.2188) |
| 2-1/2″ | 8 | 2-5/8" | 2.6250 | 66.67 | — |
| 3″ | 8 | 3-1/4" | 3.2500 | 82.55 | — |
How to find the tap drill for a pipe thread
- Pick the nominal pipe size. Pipe threads are named by nominal pipe size, not the hole diameter — a 1/8" NPT hole is about a third of an inch across.
- Read the NPT drill. The result gives the drill as the chart prints it (a letter or a fraction), plus its decimal size in inches and millimetres.
- Check the threads per inch. The TPI is shown alongside, so you can confirm you have the right tap — 27, 18, 14, 11-1/2 or 8 TPI depending on size.
- Use the NPS figure for straight pipe threads. For a straight (NPS) pipe tap of the same size the chart lists a different, slightly larger drill. It is shown where the chart gives one.
- Confirm against your tap maker. The values are one manufacturer's published chart. If your tap's maker lists a different drill, theirs applies to their tap.
10 situations where the NPT drill size is needed
Adding a 1/8" NPT port for a gauge or sensor
When a gauge, sender or fitting has a 1/8" NPT thread, the chart's R drill (0.339") is the hole to drill before tapping.
Tapping an air compressor manifold
1/4" NPT fittings need a 7/16" drill by this chart — a standard fractional bit most shops have.
Machining a hydraulic or fuel block
Fabricators tapping 3/8" or 1/2" NPT ports need 37/64" and 23/32" drills, sizes that are easy to confuse without a chart.
Converting a letter drill to decimal
Letter drills such as D and R are not obvious; the tool shows them as 0.246" and 0.339" and in millimetres.
Working with metric drill sets
The millimetre equivalent helps pick the nearest metric drill when a letter or fractional size is not available — then check it against the tap maker.
Telling NPT from NPS drill sizes
Straight pipe threads use a slightly larger drill than tapered ones of the same nominal size. Seeing both side by side avoids drilling the wrong one.
Buying the right drills before a job
A plumbing or fabrication job with several pipe sizes needs several drills. The full chart lists them all at once.
Repairing a stripped pipe thread port
Moving up a pipe size means a new drill and tap. The chart shows what the next size needs.
Teaching machining or plumbing students
The chart shows the relationship between nominal pipe size, threads per inch and drill size in one table.
Checking a drill size quoted on a forum
Forum answers sometimes give different drills for the same NPT size. A manufacturer's chart is a fixed reference to compare against.
Where the drill sizes come from
The sizes are taken from Sowa Tool's published 'Tap & Drill Charts', from its sections for taper pipe taps (NPT) and straight pipe taps (NPS). For NPT it lists: 1/16-27 D, 1/8-27 R, 1/4-18 7/16", 3/8-18 37/64", 1/2-14 23/32", 3/4-14 59/64", 1-11-1/2 1-5/32", 1-1/4-11-1/2 1-1/2", 1-1/2-11-1/2 1-47/64", 2-11-1/2 2-7/32", 2-1/2-8 2-5/8" and 3-8 3-1/4". Letter-drill decimals (D 0.2460", R 0.3390", S 0.3480") come from the same document's decimal-equivalents table.
Fractional sizes are converted to decimals exactly, and millimetres are inches × 25.4. Nothing is fetched while you use the page.
The limits: this is one tap maker's chart. Other manufacturers' charts sometimes give a slightly different drill for the same pipe thread, and when that happens the maker of the tap you are using is the authority. The hole size is a starting point — material, tap wear and how far the tap is run in all affect the finished thread, which is checked with a thread gauge or the mating fitting.
This page lists published sizes; it is not a machining procedure, and it does not cover NPTF dryseal threads, BSPT or metric pipe threads.
Sources
- Sowa Tool — Tap & Drill Charts (PDF) — Taper Pipe Taps (NPT), Straight Pipe Taps (NPS) and decimal equivalents
Frequently asked questions
What size drill for 1/8 NPT?
An R drill, 0.339" (8.61 mm), according to Sowa Tool's chart. 1/8" NPT has 27 threads per inch.
What size drill for 1/4 NPT?
7/16" (0.4375", 11.11 mm). 1/4" NPT has 18 threads per inch.
What size drill for 3/8 NPT and 1/2 NPT?
3/8"-18 NPT takes a 37/64" drill (0.5781") and 1/2"-14 NPT a 23/32" drill (0.7188"), by the same chart.
Why is the hole so much bigger than the pipe size?
Pipe thread sizes are nominal names, not hole diameters. The chart's drill for a 1/8" NPT thread is 0.339" and for a 1/2" thread 0.7188" — always look the drill up rather than reading it off the size.
Is the NPS drill the same as NPT?
No. In the chart, straight pipe taps (NPS) use a slightly larger drill than taper pipe taps (NPT) of the same size — for 1/8" it is S (0.348") instead of R (0.339").
Another chart gives a different drill. Which is right?
Tap makers publish their own recommendations, and they do not always agree exactly. Use the figure from the maker of the tap in your hand.
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