Clearance Hole Calculator: Drill Size for Bolts and Screws
A clearance hole is the hole a bolt passes through, not the hole it threads into. It has to be big enough for the fastener to slide in without hammering, but tight enough to keep the joint from shifting. This clearance hole calculator takes the fastener size, fit type, and standard, then returns the recommended hole diameter, a min and max range, the closest standard drill bit, and the tap drill size for reference.
Both metric and imperial fasteners are covered. The metric table spans M1.6 to M36. The imperial table covers #0 through 1″. Fit type is set by the standard: close, normal, or loose. Custom tolerance is available if your drawing or your shop uses different rules.
How to Use the Clearance Hole Calculator
Pick your unit system. Metric uses millimeters, imperial uses inches.
Choose the fastener size from the dropdown. Thread pitch fills automatically.
Set the fit type. Normal is the default for most work.
Pick the standard: ISO 273 for metric, ANSI B18.2.8 for imperial, or Custom.
If you picked Custom, set the clearance value in the active unit.
Enter how many holes you’re drilling.
Add cost per hole and cost per drill bit if you want a total.
Read the recommended hole size and the closest standard drill bit.
The Summary tab leads with the recommended hole size. Detailed Breakdown lists the min, max, drill bit, and tap drill reference.
How the Clearance Hole Calculator Works
Every clearance hole comes from a standard table. This calculator looks up the value, then shows the min and max tolerance window around it.
Formula: Clearance hole = Table value (ISO 273 or ANSI B18.2.8)
Formula: Hole min = Table value × 0.98
Formula: Hole max = Table value × 1.02
Formula: Custom hole = Fastener diameter + Custom clearance
Formula: Total cost = (Holes × Cost per hole) + Cost per drill bit + Additional cost
For a standard lookup, the tolerance band is ±2 percent of the nominal hole size. That’s a reasonable approximation of the tolerance a machinist would see on a properly sized hole. For custom tolerance, the min and max are 80 percent and 120 percent of the specified clearance.
The recommended drill bit is the closest match from a standard drill index. Metric indexes step in 0.1 mm increments below 3 mm and coarser above that. Imperial indexes use both numbered bits, lettered bits, and fractional sizes. The tool picks the one that lands closest to the calculated hole size, so you can buy a bit that’s actually available.
The tap drill reference is the drill you’d use to tap the hole for the fastener. That’s a different hole than the clearance hole. Tap drill sizes come from standard tapping charts. They appear on the summary so you can check both numbers when you’re setting up a job.
Worked Example: A 1/2″ Bolt, Normal Fit
Say you’re drilling a clearance hole for a 1/2″ UNC bolt in a steel plate. Normal fit, ANSI standard, four holes. Cost per hole is $0.50, drill bit is $12, no additional cost.
Fastener: 1/2″ (0.500″)
Nominal diameter: 0.5000 in
Fit: Normal
ANSI B18.2.8 normal clearance for 1/2″: 0.531 in
Hole min: 0.531 × 0.98 = 0.5204 in
Hole max: 0.531 × 1.02 = 0.5416 in
Recommended drill: 17/32″ (0.5313 in)
Tap drill reference: 27/64″ (0.4219 in)
Number of holes: 4
Cost for holes: 4 × $0.50 = $2.00
Total cost: $2.00 + $12.00 = $14.00
The ANSI normal clearance for a 1/2″ bolt is 0.531″. That’s 17/32″. The tap drill is 27/64″, which is what you’d use if you were threading the plate rather than bolting through it. Those two numbers are commonly confused, and the tool shows them side by side so you can tell them apart before you drill.
