Quick Answer

Choose a diamond core bit whose inner diameter is the pipe outside diameter (OD) plus 10-25 mm clearance — for a 110 mm PVC pipe, drill a 125 mm or 150 mm core hole; for a 4 inch (114 mm) steel pipe, use a 162 mm bit. Always size to the pipe's outside diameter, not the nominal size, and add clearance for insulation, fire-stop sleeve, or sleeve coupling. Use a wet diamond core drill bit for walls and floors over 100 mm thick, or a dry diamond core bit with a vacuum shroud for small penetrations in occupied buildings. Never drill a pipe-hole so tight that the pipe cannot flex or be fire-stopped.

Nominal Pipe Size vs Outside Diameter

This is the #1 sizing mistake. Nominal pipe size (NPS) is not the outside diameter. A 3 inch NPS steel pipe is actually 88.9 mm OD. A 110 mm PVC pipe is 110 mm OD. If you order a "3 inch core bit" thinking you will fit a 3 inch pipe, you will either drill too small (if the bit is 76 mm) or waste money on an oversized hole. Read the pipe spec, measure the OD with calipers, and then size the bit.

Pipe	OD	Recommended bit	Clearance
50 mm PVC	50 mm	63 mm	13 mm
110 mm PVC / soil	110 mm	125 or 150 mm	15-40 mm
3" NPS steel pipe	88.9 mm	100 or 117 mm	11-28 mm
4" NPS steel pipe	114.3 mm	132 or 152 mm	18-38 mm
150 mm drainage	160 mm	180 or 200 mm	20-40 mm
HVAC duct 300 mm	300 mm	350 mm	50 mm

Account for Insulation, Sleeves and Fire Stop

If the pipe is insulated (chilled water, hot water, refrigerant), the hole must clear the insulation, not just the pipe. Add 2× insulation thickness to the OD. If the spec calls for a fire-stop sleeve (required in most commercial walls and floors), the bit must clear the sleeve OD — usually a steel or PVC sleeve that is 1-2 sizes larger than the pipe. Ask the MEP engineer for the penetration detail before you order bits. Sizing on the fly at 7 am on site is how you end up with a 100 mm hole that needs to be re-cored at 150 mm.

Wet vs Dry Core Bits

Wet core bits use water to cool the segments and flush the core. They cut faster, last longer, and are required for holes over 100 mm or in reinforced concrete. Dry core bits (threaded 5/8-11 or M16, toothed pilot) are for small holes up to 150 mm in walls and floors where water is impractical. Our full diamond core drill range covers both. Always use water with wet bits — dry-running a wet-rated bit destroys the segments in minutes. Use a vacuum shroud and dust collector adapter when dry drilling indoors.

Drilling the Hole Cleanly

Mark the centre with a centre punch. Use a drill stand or rig for holes over 76 mm — free-handing a 150 mm core drill wanders. Start at slow speed until the pilot bit bites, then increase. For wet drilling, feed water through the bit centre at 2-4 L/min. Keep the drill level; do not pry on the barrel. When the bit breaks through the far side, slow the feed — that is where spalling happens. Catch the core inside the barrel; a falling core can crack the slab.

• Use a torque-limited drill or a core drill rig — they bind less.
• Rebar slows the cut; keep water flowing and feed steady.
• For through-holes in finished floors, drill from the finished side to prevent spall-out.
• Deburr the edge of the hole after the core comes out.

Standard Bit Sizes to Stock

For a plumbing or MEP service crew, stock these diameters on the truck: 27, 32, 52, 63, 76, 102, 117, 127, 152, 162, 200 mm. That covers 90% of plumbing, electrical and HVAC penetrations. Order larger sizes (250, 300, 350 mm) to job when needed — they are bulky to carry and rarely used.

Frequently Asked Questions

What size hole do I drill for a 110 mm soil pipe?

125 mm or 150 mm. A 125 mm bit gives 7.5 mm clearance each side, which is enough for a clean fit but tight for fire-stop mastic. Most plumbers spec 150 mm to leave room for the seal.

Can I use a dry core bit on reinforced concrete?

Not for deep or large holes. Dry bits are for 100-150 mm maximum and light reinforcement. For rebar-heavy slabs and walls, switch to a wet rig — dry bits overheat and lose segments when they hit rebar.

Do I need a larger hole for a pipe with insulation?

Yes. Add 2× insulation thickness to the pipe OD. A 114 mm steel pipe with 25 mm insulation needs a 170-180 mm hole, not a 152 mm one.

How do I stop the hole spalling on the back side?

Drill from the finished side whenever possible. When you near breakthrough, slow the feed. For wall holes in finished interiors, use a core rig with a vacuum base to keep the drill steady.

Conclusion

Sizing a core bit is simple once you measure the pipe OD, add clearance for insulation and sleeves, and choose wet or dry based on hole size. Stock the standard diameters on the truck, order specials to job, and never size by nominal pipe — measure, add clearance, and drill.