Start with the ball size stamped on the coupler, then identify the coupler maker, model, latch and lip shape. Finally, check the dimensions requested for the particular lock. A pin through the latch needs different measurements from a lock that encloses the whole coupler.
Common US bumper-pull sizes are 1-7/8, 2 and 2-5/16 inches. A lock marketed as universal may cover all three, but its seating and clearance requirements still apply. The printable card below gives you somewhere to record the details at the trailer before ordering.
Step one: ball size
These are the three common US bumper-pull ball sizes. Other systems exist, so use the coupler specification rather than choosing from trailer weight.
| Stamped as | Ball diameter in decimal inches |
|---|---|
| 1-7/8 in | 1.875 in |
| 2 in | 2.000 in |
| 2-5/16 in | 2.3125 in |
Read the coupler and tow-ball markings and make sure they agree. Ball size is separate from the ball’s shank size and load rating; check those too when choosing towing hardware. CURT’s ball-sizing guide explains the distinction. A conventional fifth-wheel hitch connects to a kingpin, not a ball socket.
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
Pivoting yoke over the head
A U-shaped yoke pivots down over the ball and pins in place.
Sleeve / collar
Collar slides along the neck
A collar slides along the neck to lock the socket shut.
Identifying yours
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Inspect with the trailer secured
Park on firm, level ground and secure the trailer against movement. Follow its support and unhitching instructions before opening the coupler; do not release a latch while it is carrying an unsupported trailer.
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Find the moving part
Use the drawings as visual clues: a trigger, a flip-up lever, a pivoting yoke or a sliding collar. They are simplified examples, not a model-identification chart. A brand can sell more than one mechanism, so compare your exact model with the lock maker's fit guide.
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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: measure for the lock type
For a latch-pin lock, check the hole diameter and the width the pin must span between the lock’s retaining ends. Use the manufacturer’s usable span, not its overall product length. For example, CURT 23503 specifies a 1/4 in pin and an adjustable 1–3 in latch span. Those dimensions describe that lock, not every latch with the same ball size.
For a socket-blocking or whole-coupler lock, match the exact coupler and lip profile first. If its fit guide asks for width, depth or lip clearance, measure between the points shown there. There is no single span-and-depth table that makes every enclosing lock fit.
Universal or precision: the honest trade-off
A universal lock offers broader compatibility across its listed sizes and styles. Fastway describes the Fortress as fitting virtually all common 1-7/8, 2 and 2-5/16 in couplers. That is not a promise about every shape: check how it seats and closes against your coupler using the manufacturer’s instructions.
A precision-fit lock uses a body or insert matched to particular coupler shapes. It can reduce clearance around the head, but you must choose the right variant. A wrong order may not close, and a close fit alone does not prove how well the lock resists attack.
Which of these locks needs which measurement
| Lock | Ball size | Coupler details | Fit check |
|---|---|---|---|
| Fastway Fortress | 1-7/8, 2 or 2-5/16 in | Broad compatibility, not every shape guaranteed | Seats and closes as instructed |
| Altor ICON | 2 in or 2-5/16 in, selected by variant | Match the OEM coupler style | Exact size and style variant |
| Proven Model 2516 | 2-5/16 in | Most trigger and thumb-latch couplers | Match the index; send photos if uncertain |
| AMPLock BRP-2516 | 2-5/16 in only | Flat lip | BRP-2 is the separate 2 in variant |
Fitment checked against the Fastway Fortress, Altor ICON, Proven 2516 and AMPLock flat-lip range on October 4, 2026. AMPLock’s range offers both sizes, but each is a separate SKU; the family description does not mean one BRP-2516 fits both.
Download the coupler and ball-size fitment card
Take the card to the trailer, record the markings and measurements, and keep it with your coupler photos when confirming a lock. Download a one-page PDF in US Letter or A4, or print it in your browser.
Coupler and ball-size fitment card
For identifying common bumper-pull ball couplers and choosing a matching lock. Gooseneck, fifth-wheel kingpin and lunette-ring locks need their own fitment guides.
Match the coupler's specified ball size exactly. A smaller ball is not a substitute. Ball diameter alone does not establish towing capacity or lock compatibility.
| Coupler marking | Matching tow ball | Decimal inches | Millimeters |
|---|---|---|---|
| 1-7/8 in | 1-7/8 in | 1.875 | 47.625 |
| 2 in | 2 in | 2.000 | 50.800 |
| 2-5/16 in | 2-5/16 in | 2.3125 | 58.7375 |
Conversions are references, not a measuring template. A 50 mm ball is not a 2 in ball (50.8 mm). If markings are missing or disagree, confirm the coupler model with its manufacturer before towing.
Record at the trailer
Trailer / date: __________________________________________________________________
Coupler maker / model: ___________________________________________________________
Coupler ball-size marking: __________________ Tow-ball marking: _____________________
Latch mechanism / lip profile (flat, rolled, other): _____________________________________
Measure for the lock you are buying
| Lock type | What to confirm / record |
|---|---|
| Socket-blocking / whole-coupler lock | Match ball size, exact coupler model, latch and lip shape to the maker's fit guide. Measure body / lip clearances only where its diagram asks. |
| Latch-pin lock | Match pin diameter to the latch hole and usable span to the latch width. Check room for the lock head. These are separate from ball size. |
| Universal lock | Check the listed sizes and exclusions; universal is not a guarantee for every coupler. Confirm it seats and locks as its instructions show. |
Requested dimensions (label each; circle units: in / mm): ______________________________
Before ordering
- Photograph the top, side, latch, lip and all markings. Match the coupler itself, not just the trailer brand.
- Confirm the exact lock SKU / variant and required clearances with the manufacturer. Send photos if your coupler is not listed; do not guess from a similar picture.
- Check nearby brackets and chain hooks for interference. Follow the lock instructions when fitting it; a socket-blocking storage lock must be removed before hitching.
Lock SKU / maker confirmation / date: ______________________________________________
Fitment sources checked October 4, 2026. Inch-to-mm conversions use 25.4 mm per inch. This worksheet records identification; it does not certify fit or towing capacity.
- CURT: trailer-ball sizing and matching
- CURT 23503: latch-pin diameter and span example
- Proven Model 2516: fitment index and photo confirmation
- Fastway Fortress: stated compatibility and installation
rvtrove.com/towing/what-size-coupler-lock-do-i-need/
What to buy once you know
Broad compatibility
Fastway Fortress
Covers virtually all three common ball sizes and many latch designs. Confirm it seats and locks on your coupler.
Best precision fit
Altor ICON
Choose the variant matched to your ball size and OEM coupler design.
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. Check both couplers against the exact lock’s compatibility list. Shared ball size alone is insufficient; an adjustable lock may fit both, and a fitted lock can too if both couplers match its listed applications.
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?
Start with the coupler's stamped ball size, maker and model. Record the latch and lip shape, then measure only the clearances requested by the lock maker's fit guide. A latch-pin lock also needs the hole diameter and usable span. If the ball-size stamp is unreadable, confirm the model with the coupler manufacturer rather than selecting a towing ball from a rough socket measurement.
What are the common ball-coupler sizes?
The common US bumper-pull sizes are 1-7/8 inch, 2 inch and 2-5/16 inch. Read the coupler marking rather than inferring its size from trailer type or weight. A conventional fifth-wheel hitch uses a kingpin, and a 50 mm ball is a different size from a 2 inch ball.
Will a 2-5/16 coupler lock fit a 2 inch coupler?
Only if that exact lock is listed for both sizes and your coupler design. Some universal locks cover all three common sizes; size-specific models do not. For example, AMPLock's BRP-2516 is the 2-5/16 inch flat-lip variant, while BRP-2 is the separate 2 inch version.
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 fitted locks use different variants for different coupler designs. Altor asks for ball size and coupler style; Proven asks you to match its fitment index. Ordering by ball size alone can result in a lock that will not close.
Do I need to measure span and depth as well?
Measure the dimensions specified for the lock type. A latch-pin lock needs a suitable pin diameter and enough usable span across the latch. A lock enclosing the coupler may need body, lip or depth clearances measured at particular points shown in its fit diagram. There is no universal span-and-depth pair that proves all coupler locks will fit.
Is a universal coupler lock good enough?
It can be a convenient choice across several listed coupler sizes, but universal does not mean every coupler. Check the maker's compatibility notes, seating and locking instructions. A fitted lock needs a more specific model match; neither label alone proves security performance.