Bolt Hole Size Calculator

Metric rows reflect common ISO-style series. Imperial rows show typical clearance add-ons, not fixed drill sizes.

• Round hole diameter: D = d + c + t
• Slot width: W = D
• Slot length: L = d + e + t
• Round hole area: A = π·D²/4
• Slot area (obround): A = W·(L − W) + π·W²/4

d is bolt diameter, c is clearance, t is drill tolerance add-on, and e is slot extra length. Outputs can be rounded to preferred increments.

1. Select the unit system used on your drawings.
2. Enter the bolt diameter, or choose a quick size.
3. Pick a fit/series that matches erection conditions.
4. Select round, short slot, or long slot hole type.
5. Use custom clearance when specifications require it.
6. Add drill tolerance to reflect fabrication variability.
7. Choose a rounding step to match available drill bits.
8. Press Calculate, then export CSV/PDF for documentation.

1) Why bolt hole sizing matters

Correct hole sizing protects alignment, erection speed, and final fit-up. Undersized holes cause binding and damaged coatings, while oversized holes reduce bearing area. A controlled clearance supports bolt insertion without forcing, especially when plates arrive with mill tolerances and site handling variations.

2) Clearance categories in practice

Most projects use three clearance categories: close, normal, and loose. For many metric sizes, fine holes are only about 0.5–1.0 mm over nominal, normal holes roughly 1.0–2.0 mm over, and loose holes commonly 2.0–3.0 mm over. Always confirm the governing standard.

3) Metric series overview

In typical ISO-style tables, M10 normal clearance often targets 11.0 mm, M16 normal targets 17.5 mm, and M24 normal targets 26.0 mm. These values balance assembly tolerance with reasonable bearing area. If your nominal size is nonstandard, apply a conservative fallback and document the assumption.

4) Imperial field fit guidance

Imperial jobs frequently specify clearance as an add-on to nominal bolt diameter. A common approach is +1/32 in for close work, +1/16 in for standard erection, and +1/8 in for loose field adjustment on smaller bolts. Larger diameters may shift up one step to avoid fit-up delays.

5) Slotted holes and movement

Slots accommodate construction tolerances, thermal movement, and staged alignment. The calculator uses a width equal to the recommended hole diameter and a length of d + e + t. Typical slot extras are about 6 mm (short) and 12 mm (long), or 1/4 in and 1/2 in, depending on detailing.

6) Drill tolerance and coatings

Drill tolerance captures the real-world variation between nominal drill size, tool wear, and setup. Adding 0.2 mm to 0.5 mm in metric work, or about 1/64 in in imperial work, can better represent shop practice. For galvanized parts, consider extra allowance for coating thickness and handling.

7) Checking bearing and edge distance

Hole size is not a stand‑alone decision. Larger holes can reduce net section and bearing resistance, and may increase the required edge distance. When you select a loose clearance or slot, re-check connection detailing, washers, plate thickness, and the load path so the joint remains compliant and durable.

8) Documentation and QA

Track hole types, clearances, and drilling assumptions for every connection package. This tool exports CSV for spreadsheets and PDF for submittals, allowing quick verification during inspections. Record the unit system, fit category, and any custom clearance so future teams can reproduce the decision reliably.

1) What clearance should I use for typical steel erection?

Use the normal/medium setting unless the drawings specify otherwise. It usually provides enough tolerance for plate alignment while maintaining reasonable bearing area and predictable fit-up.

2) When should I choose a slotted hole?

Choose slots when you need adjustment along one axis, staged alignment, or movement accommodation. Confirm washer requirements and detailing rules for slotted holes on your project.

3) Why add drill tolerance if clearance already exists?

Clearance is a design choice. Drill tolerance reflects fabrication reality, including tool wear and setup variation. Adding it helps the output match typical shop results and improves documentation accuracy.

4) Can I enter fractions like 7/8 or 1 1/8?

Yes. The imperial input accepts decimals, simple fractions, and mixed numbers. You can also use the quick size picker for common bolt diameters.

5) What does the “total removed area” mean?

It is the sum of hole areas for the specified quantity. It can support reporting and rough comparisons, but it does not replace full connection checks like net section and bearing.

6) Should I always round to a drill increment?

Rounding is helpful when you must match available drill bits or reamers. If your project requires exact nominal holes, leave rounding off and rely on the specified sizes.

7) Which option should I use if a standard table is mandated?

Use custom clearance and enter the required allowance, then keep the exported PDF with your submittal package. This preserves traceability when contract documents govern the hole sizing.