Three measurements decide whether a coupler lock fits, and every other guide to this question gives you one of them.
Ball size — 1-7/8, 2 or 2-5/16 inches — is the one everybody publishes, and it is enough if you are buying a universal lock. If you are buying anything better than that, you also need your latch style, because the good coupler locks are sold as a separate variant for each design. And for locks that seat around the latch rather than filling the socket, you need span and depth too.
Step one: ball size
The coupler socket is machined for one ball diameter, and there are only three in general use.
| Stamped as | Actual diameter | Typically found on |
|---|---|---|
| 1-7/8 in | 1.875 in | Older and very light utility trailers |
| 2 in | 2.000 in | Small utility and boat trailers |
| 2-5/16 in | 2.3125 in | Travel trailers, fifth-wheel pin boxes, anything much over 5,000 lb |
Read it rather than measure it wherever you can. The size is normally stamped into the top or the side of the coupler casting, and the hitch ball on your tow vehicle carries its own stamp on the top of the ball. Two independent stampings that agree is better evidence than one tape measure reading.
Step two: latch style — the part nothing else tells you
Every guide ranking for this question either measures a diameter and stops, or lists latch types and assumes you already know which one you own. Working it out is the step that is missing, and it takes about ten seconds once you know what to look at.
Look at how the coupler clamps onto the ball.
Trigger latch
Squeeze handle under the head
You pull a trigger upward beneath the coupler to release it.
Thumb / lever latch
Flip-up lever on top
A flat lever lies along the top and lifts from the front.
Yoke latch
Atwood, Pro Series
A U-shaped yoke pivots down over the ball and pins in place.
Sleeve / collar
Bulldog, Ram, Demco cast
A collar slides along the neck to lock the socket shut.
Identifying yours
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Unhitch, or look at the coupler with the trailer parked
The mechanism is easiest to read when it is open. Lift whatever moves and watch which way it travels.
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Find the moving part
If you squeeze something upward from underneath, it is a trigger latch. If a flat lever lies along the top and lifts from the front, it is a thumb or lever latch. If a U-shaped piece pivots down over the ball, it is a yoke latch — common on Atwood and Pro Series. If a collar slides along the neck instead of pivoting, it is a sleeve or collar coupler, which Bulldog, Ram and Demco cast couplers use.
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Check the lip profile while you are there
Look at the edge of the socket from the side. A straight lip sits flat and square; a rolled lip curves under. Some locks fit only one of the two — AMPLock's BRP is straight-lip only — so this single detail can decide the purchase.
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Or skip all of it and read the label
Couplers usually carry a manufacturer name — Demco, Fulton, Bulldog, Dexter, Atwood, Ram, Husky, Curt, GEN-Y. Lock makers publish their variants against those names, so matching the brand and model is faster and less error-prone than describing the mechanism yourself.
Step three: span and depth, if you are buying a precision lock
Locks that clamp around the latch rather than filling the socket need two more numbers. Span is the width the lock has to bridge across the coupler. Depth is how far down it has to reach to sit properly.
Manufacturers selling latch-specific locks publish these in ranges — spans of 3/4, 1-5/8, 2-1/8, 2-3/8, 2-5/8, 2-15/16 and 3-1/8 inches, with depths from 3/4 up to about 2-5/8 inches — and list which range suits which coupler model. In practice you will match your coupler’s make and model against their list rather than measuring, which is both quicker and harder to get wrong.
If you are buying a lock that fills the socket or covers the whole coupler head, ignore this step. It does not apply.
Universal or precision: the honest trade-off
A universal lock is built to span all three ball sizes. It needs one measurement, it fits whatever you own, and it is the right purchase for most people — including anyone who has read this far and still is not certain what latch they have.
A precision-fit lock is moulded or machined to one coupler shape. It seats close enough that there is no useful gap to insert a pry bar into, which is a genuine security difference rather than a marketing one. The cost is that you have to get the identification right, and a wrong order does not fit at all.
Which of these locks needs which measurement
| Lock | Ball size | Latch style | Span and depth |
|---|---|---|---|
| Fastway Fortress | Any of the three — universal | Not needed | Not needed |
| Altor ICON | 2 in or 2-5/16 in | Required — nine variants | Matched by variant |
| Proven Model 2516 | 2-5/16 in | Required — four models | Matched by model |
| AMPLock BRP-2516 | 2 in or 2-5/16 in | Straight lip only | Not needed |
Two of the four cannot be ordered correctly from ball size alone, and both of those are the ones worth buying if the trailer is valuable. That is the whole reason this page exists.
What to buy once you know
Fits without measuring the latch
Fastway Fortress
Universal across all three ball sizes, so ball size is the only measurement you need to get right.
Best precision fit
Altor ICON
Nine variants matched to OEM latch designs — the closest fit available once you know what you have.
Straight-lip couplers
AMPLock BRP-2516
Moulded to a straight-lip coupler so there is no gap to work a tool into. Check your lip profile first.
Full reviews of these and the hitch pin locks, with cylinder grades and warranty terms, are in our guide to the best trailer hitch locks.
If you also tow with weight distribution
The A-frame is crowded hardware. A coupler lock has to clear whatever else is bolted there, and a weight distribution hitch is usually the biggest thing competing for the space — Equal-i-zer’s rigid L-brackets in particular, which mount to the frame rails and sit close to propane bottles and battery boxes. Blue Ox’s rotating clamp-on latches are designed around that clearance problem, and the two approaches differ in more than mounting: Equal-i-zer vs Blue Ox SwayPro works through it.
Measure once for both while you are under there — and if the weight distribution hitch is still to be bought, size it on your tongue weight rather than your trailer weight. The towing payload calculator works that number out and shows what it costs your tow vehicle’s payload.
Edge cases worth knowing about
Adjustable and gooseneck couplers. An adjustable-height coupler often carries a different latch to a fixed one on the same trailer brand, so check the coupler itself rather than assuming from the model. Gooseneck couplers are a separate product category and take a different lock entirely.
Two trailers, one lock. This works only with a universal lock, and only if the ball sizes are among the three it covers. A precision lock bought for one trailer will not move to another unless both couplers are the same make and model.
Safety chain hooks. Some coupler locks will also capture 5/16 in safety chain hooks, which closes a small extra gap. Proven says its 2516 does this in most cases but does not guarantee every hook size, and manufacturing variation between chain suppliers is why. Treat it as a bonus rather than a specification you can rely on.
Couplers with no stamp and no label. Photograph the coupler from the side and from directly above and send it to the lock manufacturer. Proven identifies fitment from emailed photographs, and it is a faster route to a right answer than guessing from a diagram — including this one.
Questions worth answering
How do I measure my trailer coupler?
Measure the inside diameter of the ball socket at its widest point. It will come out at 1.875, 2.000 or 2.3125 inches, which are the 1-7/8, 2 and 2-5/16 inch sizes. Most couplers are also stamped with the size on the top or side of the casting, and the hitch ball you tow with is stamped too — check both before reaching for a tape, because the two larger sizes are only 5/16 of an inch apart and easy to misread.
What are the three trailer coupler sizes?
1-7/8 inch, 2 inch and 2-5/16 inch. Travel trailers, fifth-wheel pin boxes and anything much above 5,000 lb are almost always 2-5/16. Small utility and boat trailers are usually 2 inch, and 1-7/8 is mostly older or very light trailers.
Will a 2-5/16 coupler lock fit a 2 inch coupler?
Not if it is a precision-fit lock — those are moulded to one size and will either not seat or will leave a gap wide enough to defeat the point of fitting one. Universal locks are the exception and are built to span all three sizes deliberately.
What is a coupler latch style and why does it matter?
It is the mechanism that clamps your coupler onto the ball — a trigger you squeeze, a lever on top, a pivoting yoke, or a sliding collar. It matters because the better coupler locks are sold as a separate variant for each design. Altor lists nine, and Proven splits one model number four ways. Ordering by ball size alone is how people end up with a lock that will not close.
Do I need to measure span and depth as well?
Only for precision-fit locks that seat around the latch rather than filling the socket. Span is the width the lock has to bridge and depth is how far down it has to reach. Manufacturers selling latch-specific locks publish these in ranges — spans from 3/4 up to about 3-1/8 inches — and list them per coupler model, so matching your trailer's make and model is usually faster than measuring.
Is a universal coupler lock good enough?
It is the right answer for most people, and it is the only answer if you are not going to identify your latch. The trade-off is real though: a universal lock covers the socket rather than closing to the coupler's own shape, which leaves more room to work a tool in. If the trailer is high value and lives outside, that gap is worth closing with a precision fit.