How to Choose the Right Brass Padlock Size: Body Width, Shackle Diameter and Clearance

Size is the most underrated decision in padlock buying. Buyers usually check the material and the keying first — and they should. But a lock made from the right material in the wrong size still fails at the hasp: it either won't pass through the hole, or it hangs loose with the shackle exposed and easy to cut.

Getting the size right is mechanical, not cosmetic. Once you understand the three numbers that define a padlock, you can specify the correct lock from a catalogue without ever holding it in your hand.

The Three Numbers That Define a Padlock's Size

Every padlock size is described by three measurements. Learn to read them and you'll avoid most sizing mistakes.

1. Body width (lock body)

Body width is the horizontal span of the lock body, measured in millimetres. It's the number most catalogues use to name the lock — "40 mm brass padlock" means a 40 mm wide body.

Common body widths run 20, 25, 30, 32, 38, 40, 45, 50, 60 and 70 mm. The body width matters for two reasons:

• It sets the internal space available for the mechanism — a bigger body fits a larger brass cylinder or combination dial, which generally means more key or code combinations and better resistance to prying and picking.
• It's what the customer sees and compares, so it's the first number buyers quote.

2. Shackle diameter

Shackle diameter is the thickness of the U-shaped loop that does the actual locking. It scales roughly with body width, but not exactly, and this is the number to watch closely:

• 20 mm body → roughly 3–4 mm shackle
• 25 mm body → roughly 4 mm shackle
• 30 mm body → roughly 5 mm shackle
• 40 mm body → roughly 6–7 mm shackle
• 45 mm body → roughly 7 mm shackle
• 50 mm body → roughly 8–9 mm shackle
• 60 mm body → roughly 9–10 mm shackle
• 70 mm body → roughly 10–11 mm shackle

Treat the shackle as the real security number. A thick body with a thin shackle is still a weak lock, because a thief attacks the thinnest part. In practice, most bolt cutters beat a shackle of 8 mm or less fairly easily, so anything securing real assets should start from the shackle diameter and work backwards.

3. Shackle clearance (vertical and horizontal)

Clearance describes how much room the shackle leaves for the thing you're locking:

• Vertical clearance is the gap between the inside top of the shackle and the body — how thick an object the lock can wrap around.
• Horizontal clearance is the inside width of the loop — how far apart the two legs sit.

Standard (short) shackles usually give around 15–25 mm of vertical clearance. Long-shackle versions extend that to 25–50 mm or more, for fitting through double hasps, thick panels or multiple hardware parts stacked together.

Matching Size to the Application

Size is driven by the hasp, the enclosure and the threat — not by personal preference. A useful starting map:

• Luggage, briefcase, small personal lockers — 20–25 mm body, short shackle, light duty.
• Gym and school lockers — 20–30 mm body, fits standard locker hasps.
• Toolboxes, small cabinets — 30–40 mm body, medium hardened-steel shackle.
• Utility and electrical enclosures — 40–50 mm body, often needs a long shackle.
• Industrial safety (LOTO), gates, high-value enclosures — 50–70 mm body, thick shackle, tamper-resistant.

For lockout/tagout in particular, the rule is one lock per worker per isolation point, and the lock needs a shackle thick enough that it can't be casually cut, plus enough clearance for multi-person hasps. That pushes the spec toward 50 mm and up.

Five Things to Check Before You Order

1. Measure the hasp hole first. The shackle must pass through the hole with a little room to spare. If the hole is 6 mm and your shackle is 8 mm, the lock is useless regardless of how good it is.
2. Confirm vertical clearance wraps the object. It needs to fit over the hasp, the plate, and anything stacked in between, with room left to close and lock.
3. Match security to the threat, not to looks. A larger body does not automatically mean a stronger lock. Check the shackle diameter and material — that's where a cutting attack lands.
4. Consider a long shackle for enclosures. Panel-mounted hasps and electrical cabinets routinely need more clearance than a standard shackle provides. Specify long-shackle up front rather than forcing a short lock.
5. Ask about custom shackle length. If your hasp geometry is non-standard, a factory that does OEM can often set the shackle to your length, diameter and finish — this is cheaper than redesigning your hardware around an off-the-shelf lock.

Common Sizing Mistakes (and How to Avoid Them)

• Buying a body that's too wide for the hasp. The lock looks serious but won't fit, or it forces the hasp out of position. Measure the hasp before choosing the body.
• Choosing a thick body with a thin shackle. The customer pays for a big lock and gets the security of a small one. Verify the shackle diameter, not just the body width.
• Forgetting clearance. A lock that fits the hole but can't close over the hardware is a common, expensive return. State the required vertical clearance in the spec.
• Oversizing as a substitute for quality. A 70 mm body with a weak internal mechanism is still a weak lock. Size sets the fit; material and mechanism set the security.

Why the Supplier Matters for Sizing

Most sizing problems are solved before the order is placed, which is exactly when a good supplier earns their keep. A factory that manufactures the lock — rather than reselling a stock SKU — can:

• Set the shackle length, diameter and finish to your hasp geometry.
• Match body width to the exact locker or enclosure you're outfitting.
• Supply samples so you can confirm the fit against your own hardware before committing to volume.

If a supplier can't tell you the shackle diameter and clearance off the top of their head, keep looking. That detail is the difference between a lock that fits and a lock that ships back.