Trailer Ball Size and Drop Hitch Calculator

US & Canada

Two measurements give you the drop. The coupler tells you the ball. Here is how to get both right, and what the numbers stamped on each part mean.

Your hitch and trailer

1. Measure both, parked on level ground

in

Ground to the top inside edge of the receiver tube.

in

Ground to the bottom of the coupler, with the trailer level. Renting from U-Haul? U-Haul says to use 18 in.

in

How far the back of the vehicle sinks once hitched. Leave 0 to measure the way CURT and U-Haul do.

Drop when the receiver is higher than the coupler, rise when it is lower. Many mounts flip over to give rise, but not every one is built to, so check yours.

2. The ball

lb

Everything aboard. The ball's rating has to be at least this.

Result

0

What to buy

The fixed mount is rounded to the nearest whole inch; the row under it shows what that rounding leaves. Capacity ranges are CURT's published line, not a law, and the rating stamped on the ball you buy is the one that counts.

How the drop is worked out

  • Receiver height: ground to the top inside edge of the receiver tube.
  • Coupler height: ground to the bottom of the coupler, with the trailer level.
  • Drop or rise = receiver height − coupler height. Positive is drop, negative is rise.

That is CURT's method and U-Haul's. CURT, The Ultimate Guide to Drop Hitch Towing: "calculate your drop or rise number by subtracting the coupler height from the receiver height." U-Haul says the same thing in its own words: "Subtract the height of the trailer coupler from the height of the receiver. This difference is the amount of drop you need in your ball mount."

CURT's worked example runs through the calculator above unchanged: a receiver at 21.75 in (552 mm) and a coupler at 15.5 in (394 mm) need 6.25 in (159 mm) of drop. The nearest whole-inch mount is 6 in, which leaves the coupler 0.25 in high. CURT rounds it the same way, to a drop of approximately 6 inches.

The other way round: a car with its receiver at 15 in towing a U-Haul trailer at the 18 in coupler height U-Haul suggests needs 3 in of rise.

The squat allowance is optional. Once the tongue is on the ball the back of the vehicle sinks, which lowers the receiver. If you know by how much (hitch up and measure again), enter it and the drop shrinks by the same amount. We print no rule-of-thumb figure for it, because it depends on your vehicle and your tongue weight.

What is a drop hitch?

A ball mount with its ball platform set below the receiver, so a tall truck can tow a low trailer level. CURT, How to Measure Hitch Drop & Select a Ball Mount defines drop as the distance the ball platform sits below the top of the shank, and rise as the distance it sits above it. Many mounts can be flipped to turn drop into rise, but CURT warns that not all models can safely be used in the rise position.

  • Fixed mount: one drop, one rise. Right for towing the same trailer every time.
  • Adjustable mount: the ball head slides up and down a channel. Right for several trailers. CURT says to choose one with at least 1 in more drop or rise than you need, and that adjustable mounts tend to cost more.
  • Starter kit: a mount sold with a ball already fitted. CURT notes a loose ball takes a lot of torque to install, which is the case for buying them together.

Why level matters

CURT's guide puts it plainly: when towing, the vehicle and trailer must be level. There is no standard hitch height to aim for instead, because, as CURT puts it, every vehicle rides at a different height. Your measurement is the only right number.

What size trailer ball do I need?

  1. Diameter: the size stamped on the trailer's coupler. Not the size on your neighbour's trailer, and not the most common size.
  2. Shank: the threaded stem must fit the hole in your ball mount or bumper.
  3. Rating: at least the loaded weight of the trailer. A ball's capacity is a gross trailer weight figure.

CURT, Standard Trailer Hitch Ball Sizes | How to Choose: "In short, your trailer ball diameter must match your coupler size, the shank diameter must fit the ball hole and the weight capacity must be enough to safely tow your trailer."

The standard trailer ball sizes

CURT lists four standard diameters, and publishes the spread of shanks and ratings it makes in each. These are one maker's ranges, not a regulation: CURT notes that capacities vary by part number.

Standard trailer ball sizes, from CURT's chart
Ball diameter Shank diameter Shank length Rated for (GTW)
1-7/8 in (47.6 mm) 3/4 to 1 in 1-5/8 to 3 in 2,000 lb to 3,500 lb
2 in (50.8 mm) 3/4 to 1-1/4 in 1-3/4 to 3 in 3,500 lb to 12,000 lb
2-5/16 in (58.7 mm) 1 to 2 in 2-1/8 to 3-1/2 in 6,000 lb to 30,000 lb
3 in (76.2 mm) 2 in 3-1/4 in 30,000 lb
  • Most common: 2 inch. CURT calls it the standard size, because most trailers have a 2 inch coupler.
  • Smallest: 1-7/8 inch, for light trailers.
  • Largest in CURT's chart: 3 inch, which CURT lists for gooseneck towing.
  • Metric: CURT also mentions a 50 mm ball, more common on European trailers. 2 inches is 50.8 mm, so it is not the same ball.
  • Anything else, such as a 2-5/8 inch search: none of the four standard sizes. Read the coupler.

Ball size by trailer type

A general guide, from CURT's sizing page and product pages. The coupler on your trailer overrides all of it.

  • Small utility trailers, small boats, teardrops: CURT puts 1-7/8 inch balls on light-duty trailers towed by cars, minivans and crossovers.
  • Boat and utility trailers: CURT names boat trailers, utility trailers and campers as the popular trailers on a 2 inch ball.
  • Travel trailers and campers: 2 or 2-5/16 inch, CURT says. Most weight distribution hitches come with a 2-5/16 inch ball.
  • Gooseneck trailers: CURT's sizing page lists 3 inch for gooseneck towing, and its Double Lock gooseneck ball (#60616) is a 2-5/16 inch ball it says "couples to most gooseneck trailers". Both exist, which is exactly why you read the coupler.
  • U-Haul rentals: see below.

Ball size for a specific trailer brand

People search for the ball size of a Jayco, a Karavan, a Kendon, an Airstream or a particular boat trailer. We do not publish a list by brand, because the same brand fits different couplers to different models and years, and a list would be a guess. The coupler is where the answer is: CURT says most manufacturers stamp or label the ball size on it. If the stamp is unreadable, measure the inside opening of the coupler, or ask the trailer maker for your exact model and year.

What size ball U-Haul uses

  • Trailers and towing equipment: a 1-7/8 or 2 inch ball, rated for the total weight towed, per U-Haul, Trailer Rentals FAQ.
  • Cargo trailers: U-Haul, 6x12 Cargo Trailer Rental asks for a 1-7/8 or 2 inch ball rated at least 2,000 lb (907 kg).
  • Car trailer (Auto Transport): a 2 inch ball, per U-Haul, Auto Transport Rental.
  • Ball height: U-Haul gives 18-1/2 to 18-3/4 inches to the middle of the ball, before hitching, and a maximum of 25 inches.
  • For the calculator: U-Haul's towing expert suggests using 18 inches as the coupler height, the standard for U-Haul trailers.

U-Haul's FAQ: "U-Haul couplers take either a 2" or 1-7/8th" ball. The ball must also be rated for the total weight that will be towed."

Real balls and their ratings

The rating is set by the ball you buy, not its diameter. Here are ten balls from CURT's catalogue, each read off its own product page. Same diameter, very different numbers.

Ten CURT hitch balls: diameter, shank, rating and nut torque
CURT part Ball Shank Capacity Nut torque
#40001 1-7/8 in 3/4 x 1-5/8 in 2,000 lb 160 ft-lb
#40002 1-7/8 in 1 x 2-1/8 in 3,500 lb 250 ft-lb
#40003 2 in 3/4 x 1-3/4 in 3,500 lb 160 ft-lb
#40004 2 in 1 x 2-1/8 in 6,000 lb 250 ft-lb
#40037 2 in 1 x 2-1/8 in 7,500 lb 250 ft-lb
#40090 2 in 1-1/4 x 2-5/8 in 10,000 lb 450 ft-lb
#40005 2-5/16 in 1 x 2-1/8 in 7,500 lb 250 ft-lb
#40007 2-5/16 in 1 x 2-1/4 in 10,000 lb 250 ft-lb
#40006 2-5/16 in 1-1/4 x 2-5/8 in 10,000 lb 450 ft-lb
#40085 2-5/16 in 1-1/4 x 2-5/8 in 30,000 lb 450 ft-lb

Two things stand out. The higher ratings sit on the thicker shanks, and a 2-5/16 inch ball can be rated anywhere from 7,500 lb to 30,000 lb. Buy by the rating stamped on the ball, which U-Haul says you will find on the top of the ball or around the base of the shank.

Where the size and rating are stamped

  • The ball: diameter on top. COMAR 11.13.08.07 (Maryland), Identification makes that law in Maryland: "Each hitch ball sold for use in a primary connecting system shall be permanently and legibly marked to show the spherical diameter of the ball; e.g., 1-7/8“, 2”, etc."
  • The coupler: the ball size it takes. CURT says most manufacturers stamp or label it on the coupler. Maryland also requires each coupling to be marked with its class, its maximum gross trailer weight and its maximum tongue weight.
  • The ball mount: its rating, usually on a label on the platform or along the shank, per U-Haul.
  • The hitch: its maximum tow weight, usually on a label on the hitch itself.

WAC 204-70-050 (Washington), Coupling classification and attachment goes further for trailers of 10,000 lb or less: "Couplings must be classified, tested, attached and labeled by the manufacturer in accordance with SAE J684 revised July 2005." SAE J684, Trailer Couplings, Hitches, and Safety Chains, Automotive Type (J684_201405, reaffirmed May 2014) is the industry standard those labels come from. It is paywalled, so we cite what it covers rather than quoting it. This page cites two US state rules because they are the ones we could read in full; we did not find a Canadian rule on ball marking to cite.

Shank diameter, ball hole, nut and torque

  • Shank diameter is the diameter of the threaded stem. It is also what people mean by hitch ball "thread size" or "bolt size": 3/4, 1 and 1-1/4 inch are the shanks in the table above, and CURT's chart runs up to 2 inch.
  • Ball hole in the mount or step bumper must match the shank. CURT's product pages say the hole must be no more than 1/16 inch larger than the shank, and the mounting platform at least 3/8 inch thick.
  • Shank too small for the hole? CURT says a reducer bushing can make up the difference.
  • Nut torque, as CURT's product pages publish it: 160 ft-lb on a 3/4 inch shank, 250 ft-lb on a 1 inch shank, 450 ft-lb on a 1-1/4 inch shank. The ball's own packaging wins if it says otherwise.
  • Nut and wrench size: CURT's ball wrench has 1-1/8 inch (28.6 mm) and 1-1/2 inch (38.1 mm) ends for balls with 3/4 and 1 inch shanks.
  • Lock washer: Washington requires every hitch ball to be installed with a lock washer or equivalent.

Ball mount shank and receiver size

The square bar that slides into the receiver is the mount's shank, a different thing from the ball's shank. CURT, How to Measure Hitch Drop & Select a Ball Mount says these are standardized to four sizes, 1-1/4, 2, 2-1/2 and 3 inch, and the mount's must match your receiver tube or you need a receiver tube adapter. Which receiver size and class you have is covered on the hitch class finder.

Ball mount length and extenders

CURT, How to Measure Hitch Drop & Select a Ball Mount: "Length is the distance from the pin hole center to the ball hole center." A longer mount or a receiver extender gives clearance for a bumper or a spare tire, but CURT warns it adds leverage on the receiver hitch.

  • Extenders cut capacity. CURT, The Complete Guide to Hitch Extenders: "Extenders under 12” long reduce load capacity by 25%, and extenders over 12” reduce load capacity by 50%." Extender length is measured pin hole to pin hole.
  • A longer mount is the alternative. CURT points to two of its mounts with the same ratings and drop, 8-1/4 and 13 inches long.
  • Rattling in the receiver? CURT suggests an anti-rattle hitch pin or clamp to tighten the joint.

Your rating is the lowest-rated part

U-Haul puts it simply: your hitch system rating is the lowest rating among the hitch, the hitch ball and the ball mount. Add the coupler on the trailer and any extender to that list. A 10,000 lb ball on a 3,500 lb mount is a 3,500 lb setup, and the vehicle's own tow rating and payload still come first.

Everything on this page from CURT and U-Haul is manufacturer and rental-company guidance, not law. The Maryland and Washington rules are law in those two states only.

Sources

The drop figures in the worked examples are calculated by the same code as the calculator.

Common questions

What size trailer ball do I need?
The size stamped on your trailer's coupler. The ball diameter has to match the coupler exactly, the ball's shank has to fit the hole in your ball mount, and the ball's rating has to be at least the loaded weight of the trailer. CURT, Standard Trailer Hitch Ball Sizes | How to Choose puts it in one line: "In short, your trailer ball diameter must match your coupler size, the shank diameter must fit the ball hole and the weight capacity must be enough to safely tow your trailer."
What is the standard trailer ball size?
2 inch. CURT calls it the standard size because most trailers have a 2 inch coupler, and the most common size for the same reason. The other standard sizes are 1-7/8, 2-5/16 and 3 inch. Standard does not mean yours: the coupler decides.
What size trailer ball does U-Haul use?
1-7/8 or 2 inch. U-Haul's own FAQ says "U-Haul couplers take either a 2" or 1-7/8th" ball. The ball must also be rated for the total weight that will be towed." The car trailer (Auto Transport) is the exception: it needs a 2 inch ball. U-Haul's 6x12 cargo trailer page asks for a ball rated at least 2,000 lb.
What hitch ball height does a U-Haul trailer need?
U-Haul gives 18-1/2 to 18-3/4 inches from the ground to the middle of the ball, measured before you hitch up, and a maximum ball height of 25 inches. For the calculator, U-Haul suggests using 18 inches as the coupler height.
Does hitch ball size matter?
Yes, twice over. The diameter must match the coupler: CURT says a ball and coupler of different sizes can lead to the trailer disconnecting. And a ball's rating is set by the part, not the diameter: CURT sells 2-5/16 inch balls rated 7,500 lb, 10,000 lb and 30,000 lb.
How do I know what size my trailer ball is?
Read the top of the ball. COMAR 11.13.08.07 (Maryland), Identification requires every hitch ball sold there to be "permanently and legibly marked to show the spherical diameter of the ball". If the stamp is worn off, measure across the widest point. For the coupler, look for the size stamped or labelled on it, or measure the inside of the socket.
What is a drop hitch?
A ball mount whose ball platform sits below the receiver, so the trailer tows level behind a vehicle that sits higher than the trailer's coupler. Turn many of them over and they give rise instead, for a trailer that sits higher than the vehicle.
How do I measure for a drop hitch?
Park on level ground. Measure from the ground to the top inside edge of the receiver tube, then level the trailer and measure from the ground to the bottom of the coupler. Subtract the coupler height from the receiver height. With CURT's own numbers, 21.75 in minus 15.5 in is 6.25 in of drop.
How much is a drop hitch?
It depends on the type more than the drop. A fixed mount is the cheaper kind; CURT says adjustable mounts "tend to be more expensive" but let you level more than one trailer. Prices move too often for us to print one; the drop you need, which is free to work out above, is what to shop with.
Is the ball rating the same as the tongue weight rating?
No. CURT notes that a ball's capacity is a gross trailer weight limit, not a tongue weight. Hitches and ball mounts carry both figures, and your setup is limited by whichever part is rated lowest.
Is a 50 mm ball the same as a 2 inch ball?
No. 2 inches is 50.8 mm, and CURT lists 50 mm as a separate size, more common on European trailers. The coupler must match the ball exactly, so use the size the coupler is stamped with.
Can I put a 2 inch ball in a 2-5/16 inch coupler?
No. CURT's rule is that the ball diameter should be exactly the same as the inside opening of the coupler, and it warns that a mismatch can lead to the trailer disconnecting. Use a 2-5/16 inch ball.