Last updated: 15 October 2026
Buying the wrong padlock is almost always a shackle problem. The body size gets the attention, but it is the shackle — the U-shaped bar — that must fit the thing you are locking. Measure it properly once and you will never buy a lock that does not close again.
The three dimensions that matter
1. Shackle diameter (thickness)
This is the thickness of the steel bar itself, usually quoted in millimetres or inches. A thicker shackle resists cutting and prying better and also decides whether the lock will pass through a hole: a 10mm shackle will not fit a hasp drilled for 6mm. If your hasp is tight, measure the hole, not the old lock.
2. Vertical clearance (the "shackle height")
The most common source of a lock that will not close. Vertical clearance is the usable height inside the shackle — the gap the hasp must fit into. If you are locking two overlapping plates, the clearance must exceed their combined thickness with a little room to spare. A standard shackle suits a single plate; a long-shackle padlock suits a gate with a recessed hasp or two plates stacked together.
3. Horizontal width (the "shackle span")
How wide the opening is between the two legs of the shackle. It must be at least as wide as the hasp you are threading. Too narrow and the lock will not go on at all.
How to measure your hasp in one minute
- Close the hasp as if locked.
- Measure the vertical gap from the top of the staple (the loop) to the plate beneath it — that is your required vertical clearance.
- Measure the width of the staple — that is your required horizontal span.
- Note the diameter of the hole — that caps your shackle thickness.
Write those three numbers down before you shop. They answer every sizing question a product page will ask.
Quick sizing table
| Where | Typical shackle | Body |
|---|---|---|
| Gym locker / suitcase | Standard, 25–30mm clearance | 30–40mm |
| Shed door / toolbox | Standard, 25–40mm | 40–50mm |
| Garden gate (recessed hasp) | Long, 50–60mm | 40–50mm |
| Double-plate / chained fittings | Extra-long, 60mm+ | 50–60mm |
Standard vs long shackle: which to pick
Choose a standard shackle when the lock sits in a hasp and is protected by it — a shorter exposed shackle is harder to cut. Choose a long or extra-long shackle only when the hasp geometry demands it: a recessed gate fitting, a thick hasp, or locking two things together. A long shackle exposes more steel, so if you need one outdoors, insist on a hardened shackle. See the full range in our heavy duty padlock collection.
Why hardened steel changes the maths
A long shackle is only as strong as its steel. A soft shackle is the first thing a thief attacks with bolt cutters, and length makes it easier. Hardened boron steel resists cutting and sawing, so a long hardened shackle is safer than a soft short one. Our long-shackle padlocks all use hardened steel for exactly that reason.
Frequently asked
How do I measure a padlock shackle? Measure the vertical gap inside the shackle (clearance), the width between the legs (span), and the thickness of the bar (diameter). Compare all three to your hasp.
What size padlock fits a shed door? Usually a 40–50mm body with a standard or slightly long shackle; measure the hasp clearance to be sure.
Can I use a long-shackle padlock on a normal hasp? Yes, but it exposes more of the shackle. Use a hardened long-shackle lock if it is outdoors.
Does a bigger body mean a stronger lock? Not on its own — shackle hardness and the lock's internal mechanism matter more than body size.