Clearance Hole Calculator finds bolt or screw clearance drill size using with ISO or ASME fit class, radial margin, max hole size, and tap drill reference.
Clearance Holes Are Not Tap Holes — And the Mix-Up Costs More Than You’d Think
Grab the wrong number off a drill chart and you get a fastener that won’t pass through, or one that rattles loose the moment vibration enters the picture. Clearance holes — the unthreaded holes a bolt or screw passes through freely — follow a completely different size logic than tap drill holes, and on a busy shop floor or under deadline pressure, it’s easy to pull the wrong column. This calculator handles both the clearance hole recommendation and the tap drill reference in one pass, so the comparison is right in front of you.
How the Calculator Works
The core output is simple: given a fastener’s major diameter, what’s the minimum hole diameter that lets it slide through cleanly — with predictable, standardized clearance rather than a guessed offset? The answer isn’t one number. It depends on three decisions you make about the assembly.
The fit class is the most consequential choice. Close fit (ISO H12 / ASME Close) gives just enough room for the fastener to pass with minimal side play — used when you need precise alignment between two mated parts and your tolerances are already tight. Normal fit (ISO H13) is the everyday default for general-purpose assemblies, enclosures, and structural joints where a small amount of float is acceptable and even desirable. Loose fit (ISO H14 / ASME Loose) allows significant lateral movement, typically used when parts need to shift slightly during assembly or when thermal expansion is a concern.
The calculator then derives four things from that combination: the recommended drill diameter, the diametral clearance (total gap around the fastener), the machining tolerance band (the max hole you can accept before the fit class is violated), and the clearance area — the actual cross-sectional void around the shank, useful for load path and sealing calculations.
On the metric side, these values trace directly to ISO 273. On the US side, they follow ASME B18.2.8 recommendations. The alternate unit card converts the result mathematically — divide mm by 25.4 or multiply inches by 25.4 — so you can sanity-check a metric spec against an imperial drill index without switching the whole calculator.
The Thread Pitch Field Does Less Than You’d Expect Here
Here’s the part that trips people up: changing the thread pitch series from Coarse to Fine has no effect on the clearance hole size. Not a small effect — zero effect. This is correct. Clearance hole sizing is based entirely on the fastener’s major (nominal) diameter, not its thread pitch. An M8 Fine pitch bolt has the same 8.0mm major diameter as an M8 Coarse pitch bolt, so both need the same clearance hole.
The pitch field exists in this calculator for a different reason: the tap drill reference in Card 4 changes significantly between coarse and fine series. If you’re working with a plate that needs a clearance hole on one side and a threaded hole on the other — a common gusset arrangement — you can read both drill sizes from one tool without flipping between charts.
Watch the M8 Normal Fit Jump
Look at the M8 data and you’ll notice something that doesn’t follow the pattern of surrounding sizes. The close-fit drill for M8 is 8.4mm — a 0.4mm clearance over the 8.0mm major, which tracks with M6 (0.4mm) and M5 (0.3mm). But the normal-fit drill jumps to 9.0mm (0.9mm clearance) and the loose-fit to 10.0mm (2.0mm clearance). This isn’t a data entry error — it reflects the ISO 273 standard, which groups M8 into a higher clearance tier. The practical implication: if you’ve been mentally interpolating “about half a millimeter over nominal” for normal fit and apply that habit to M8, you’ll undersized the hole. The calculator catches this because it’s table-driven, not formula-driven.
Worked Example: Bracket-to-Frame Assembly
A mild steel mounting bracket needs to bolt to a tube frame using four M6 cap screws. The drawing calls out normal clearance holes in the bracket — the screws thread into the frame itself. Pull up the calculator: Metric system, M6, Coarse pitch, Normal fit. Result: 6.6mm drill diameter.
The diametral clearance comes out to 0.60mm, radial margin 0.30mm per side. The machining limit tells you the hole can open up to 6.82mm before it steps outside ISO H13 — so if your drill runs slightly oversize, you’ve got 0.22mm of tolerance before you need to reconsider the fit class. Card 4 shows the tap drill reference at 5.0mm for coarse pitch: if one of those holes accidentally gets tapped instead of cleared, you’d immediately see you’re running the wrong drill entirely.
If the bracket is a close-fit alignment bracket instead — one that locates the assembly — switch to Close fit. Drill drops to 6.4mm with only 0.40mm diametral clearance. The tolerance band tightens to 0.15mm (max 6.55mm), which means your drill selection matters more and you’ll want to spot-drill first to control the entry.
Frequently Asked Questions
Why does the calculator show a tap drill size if this is a clearance hole tool?
The tap drill reference is shown as a companion value — not because the calculator drills tapped holes, but because in real parts, clearance holes and tapped holes often appear on opposite ends of the same fastener stack. Having both numbers visible lets you verify you have the right drill loaded for each operation without switching between charts.
The clearance area output — what is that actually used for?
It’s the cross-sectional annular space between the hole wall and the fastener shank, in square millimeters (or square inches in US mode). This matters when you’re calculating sealant volume for weathertight assemblies, checking whether a grommet or bushing will fit the annular gap, or doing conservative load-path analysis where the shank area needs to be distinguished from the hole area.
Does switching to Fine pitch ever change the clearance hole result?
No. The clearance hole diameter is solely a function of the major diameter, not the thread geometry. Coarse and fine variants of the same nominal size are identical from a clearance hole standpoint. Fine pitch only changes the tap drill reference in Card 4.
My shop works in fractional inches. Are the US Customary results exact fraction equivalents?
No, and this is a real gotcha. The ASME clearance values are expressed as decimal inches, not neat fractions. For example, a 1/4-20 normal fit calls for a 0.281in clearance drill — that’s close to 9/32in (0.281in exactly), which happens to work out. But #10 normal fit is 0.221in, which sits between a #2 drill (0.221in) and needs to be verified against your drill index. The alternate unit card converts the decimal inch value to millimeters, which can help identify which metric drill you already have that will clear the hole.
What happens if I need a clearance hole for a size not in the list?
The calculator only covers the sizes hardcoded in the lookup tables: M3 through M12 for metric, #4 through 1/2in for US customary. Non-standard sizes, metric fine coarse variants like M7 or M9, or fractional sizes outside the list aren’t covered. For those, you’d apply the ISO 273 or ASME B18.2.8 clearance allowances manually to the actual major diameter.
Why do the tolerance values increase with fastener size?
This reflects the ISO tolerance grade system — the absolute size of the tolerance band scales with the nominal diameter. A 0.18mm band on an M3 hole represents a tighter relative tolerance than 0.43mm on an M12, but both correspond to equivalent functional grades (H12, H13, H14). Expect tighter process control on smaller fasteners if you’re holding close-fit tolerances.
Standards This Calculator References
- ISO 273:1979 — Fasteners: clearance holes for bolts and screws. Defines the close, normal, and loose clearance series for metric fasteners along with tolerance grades H12, H13, and H14.
- ASME B18.2.8 — Clearance holes for bolts, screws, and studs (US customary). Defines the close, normal, and loose designations used in the US mode.