1/8 Inch NPT Tap Drill Size | Q Bit Or R Bit?

For a 1/8-27 NPT pipe thread, most tap-drill charts specify a letter Q bit at 0.3320 in (8.43 mm), though some charts list letter R at 0.3390 in.

Two different drill bits get printed for the same hole, and most tap-drill charts never explain why. Current charts put the 1/8 NPT tap drill size at a letter Q — 0.3320 in, or 8.43 mm — for a 1/8-27 tapered pipe thread. Other charts list letter R at 0.3390 in. Drill too small and the tap fights the metal; drill too large and the taper has less material to seal against.

Below is the bit the charts name, the point where they split, and the order of operations that keeps a pipe joint tight.

What Drill Bit Does 1/8 NPT Need?

Most current tap-drill charts call for a letter Q bit — 0.3320 in, or 8.43 mm — for a 1/8-27 NPT internal pipe thread. Letter R, 0.3390 in, appears on other charts for the same tap.

The size label is where people go wrong first. 1/8 NPT is a nominal pipe size, not a hole measurement, so the drilled hole runs far wider than 1/8 in. A published NPT reference puts the finished major diameter at 0.405 in, and those dimensions trace back to ANSI B1.20.1. The same chart logic applies across U.S. pipe-thread work using NPT or NPTF conventions.

The table below gathers the numbers — the two drill sizes first, then the thread specs that explain why the hole is bigger than the name suggests.

1/8 NPT Tap Drill Size: Where The Charts Disagree

The two charts are not really contradicting each other. Q, 0.3320 in, is the direct-tap figure on several current charts; R, 0.3390 in, shows up where a chart accounts for reaming or a different tap geometry. Charts also differ on whether the listed number assumes tapping without reaming, so read the column header before you trust the row.

That gap is 0.007 in — small on paper, meaningful at the joint. For anything under pressure, air, water, or gas, the tap manufacturer’s own chart beats a generic one, and its number is the one you drill. NPT threads are tapered and seal through engagement, so an oversized hole removes metal the taper needed. One source warns that the larger R size may raise leakage risk compared with Q.

Most entries on a published pipe tap drill chart land on Q for 1/8 NPT, which is why that letter ends up circled on so many shop walls. If you are still choosing the cutter itself, our tested 1/8 inch NPT taps roundup lines up the options by thread quality and price.

Tapping The Hole Without Wrecking The Thread

Drill the size your chart or tap maker gives, start the tap square, and test the fit by hand before forcing anything. That sequence is what keeps a pipe thread sealing.

1. Read the size as nominal. A 1/8 NPT fitting is labeled for the pipe it threads onto, not for the hole you drill.
2. Pull the drill size off an NPT pipe-thread chart. Machine-screw tap charts are not interchangeable with pipe-thread charts.
3. Drill the hole. Letter Q, 0.3320 in, covers the common direct-tap listing. Letter R, 0.3390 in, is the number on charts that call for it.
4. Tap with a 1/8-27 NPT tap, backing it out every turn or two to clear chips and keep the flutes from packing.
5. Check the fit. A mating fitting should thread in by hand for several turns and only then start to bind — that is the taper seating the joint.

Four mistakes account for most ruined pipe threads:

• Using the nominal size as the drill size. 1/8 NPT is a pipe label; the hole is much larger.
• Mixing NPT with BSP or BSPT. The two systems are not interchangeable, even at matching nominal sizes.
• Assuming one drill size is universal. Q and R both appear in print for 1/8 NPT.
• Drilling oversized in sealing service. An oversized hole can leave the taper short of full engagement.

For a 1/8-27 NPT internal thread, plan on letter Q at 0.3320 in (8.43 mm) unless your chart or tap manufacturer specifies R at 0.3390 in. On pressure-bearing work, the manufacturer’s number wins. Drill that size, tap square and slow, and the joint seals on the taper rather than on tape or paste.