What to measure
- Receiver height: from the ground to the top of the square receiver opening, with the truck parked on level ground.
- Coupler height: from the ground to the bottom of the trailer's coupler, with the trailer level. Adjust the tongue jack until a level on the frame reads flat.
- Squat (optional): how much the receiver drops when the loaded trailer's tongue weight goes on. It depends on your truck and load, so measure it; leave it empty if you do not know it.
Why this works: ball mount geometry
A ball mount's drop is measured from the top of its shank to the top of the platform the ball bolts to. The shank fills the receiver, so its top sits at the top of the receiver opening. When the trailer is coupled, the bottom of the coupler sits close to that platform. So the drop you need is the receiver height minus the coupler height. The small gap between the coupler and the platform depends on the ball and the coupler, so check the trailer with a level once it is hitched.
The formula
drop = receiver height − squat − coupler height
positive: drop needed, negative: rise needed
gooseneck clearance = overhang height + (hitch height − trailer coupler height) − bed rail heightWorked example
The receiver top is 20.5 in off the ground, and the bottom of the level coupler is 17 in. The drop needed is 20.5 − 17 = 3.5 in. Ball mounts are sold in whole-inch steps, so a 3 in mount leaves the tongue 0.5 in high and a 4 in mount leaves it 0.5 in low. If the truck squats 1 in under the tongue weight, the drop needed is 2.5 in.
For a gooseneck: bed rails at 56 in and the underside of the level trailer's overhang at 62 in leave 6 in of clearance. If the gooseneck ball is 57 in high and the trailer coupler sits at 56 in when level, hitching lifts the trailer front by 1 in, and the clearance becomes about 7 in.
Rise instead of drop
On a low car with a tall trailer, the coupler can be above the receiver. The result then shows a rise. Some drop mounts can be turned over to give a rise; check the maker's instructions.
Fifth wheel hitch height
For a fifth wheel, the trailer's pin plate should sit on the hitch plate with the trailer level. If the two heights differ, the trailer tilts, and the overhang moves up or down by about the difference at the front. Enter both heights to see the effect on the bed rail gap.
Questions
How do I calculate hitch drop?
Measure from the ground to the top of the receiver opening, and from the ground to the bottom of the coupler with the trailer level. Subtract the coupler height from the receiver height: 20.5 in − 17 in = 3.5 in of drop.
My drop is 3.5 inches. Should I get a 3 or 4 inch drop?
Either leaves the trailer 0.5 in off level: the 3 in mount leaves the tongue 0.5 in high, the 4 in mount 0.5 in low. If your truck squats when the trailer goes on, the smaller drop is usually the closer match, so measure the squat and enter it.
What if the coupler is higher than the receiver?
You need a rise instead of a drop. With the receiver at 17 in and the coupler at 19 in, you need 2 in of rise.
How much clearance do I need between the bed rails and a gooseneck trailer?
The calculator does not set one. Use your trailer or hitch maker's minimum. It shows the gap from your measurements: bed rails at 56 in and an overhang at 62 in leave 6 in.