Measuring a trailer wheel correctly requires finding three numbers — diameter, width, and bolt pattern — each measured from specific surfaces on the rim, not the outer edges.
Guessing the size usually means buying the wrong tire or a rim that won’t bolt up. The most common mistake is measuring from the outside rim edge instead of the bead seat. Here is how to get each one right using tools you already have.
Measuring Wheel Diameter Correctly
Wheel diameter is the distance from one bead seat to the opposite bead seat — the surface the tire seals against. Measuring from the outer lip gives a number roughly half an inch to an inch too large. Lay the wheel flat, place a tape measure across the rim, and measure from the inside of the upper bead seat to the inside of the lower. On most trailer wheels this falls between 12 and 16 inches. If the wheel is mounted, remove the tire and measure across the rim’s inner surface while bolted on — the mounting surface gives access to the bead seat.
How to Determine Wheel Width
Measure inside the flanges: place the tape between the inner bead seat on one side and the inner bead seat on the other, keeping it parallel to the rim faces. Trailer wheel widths are typically whole or half inches — common sizes include 5, 5.5, 6, and 7 inches. A marking like 4.5J13 indicates internal width (4.5 inches) and diameter (13 inches). You cannot get this number accurately without removing the tire first.
Finding the Bolt Pattern
The bolt pattern is the number of lugs followed by the bolt circle diameter — for example, 5 on 4.5″ or 6 on 5.5″. For 4-, 6-, and 8-lug wheels: measure center-to-center directly across between two opposite holes. For 5-lug wheels: measure from the center of one hole to the outer edge of the hole straight across (the back of the lug stud to the center of the opposite hole) to approximate the bolt circle diameter. Alternatively, measure center-to-center between two neighboring bolts and multiply by 1.701.
Review the best trailer wheels for your setup after you confirm the bolt pattern.
Don’t Skip Center Bore and Backspacing
Even a wheel with the right bolt pattern can be a bad fit. Pilot bore / center hole: measure the diameter of the center hole with a tape or caliper. If the trailer uses lug bolts instead of nuts, the center hole is more critical because the hub carries the load. On hub-centric trailers, the wheel centers on the bore. Backspacing / offset: lay a straightedge flat across the back of the wheel (across the rim flanges), then measure from the straightedge down to the wheel’s mounting surface. Too much backspace rubs the suspension; too little sticks out past the fender. Offset expresses the mounting surface’s position relative to the wheel’s centerline — it matters most when swapping wheels from a different axle.
Common trailer tire sizes tell you the allowed rim width. For example, an ST225/75R15 tire fits rims 6 to 7 inches wide. Mismatch the rim width and the tire either bulges oddly or does not seat properly.
| Measurement | Where to Measure From | Common Mistake |
|---|---|---|
| Diameter | Bead seat to bead seat (inside to inside) | Measuring outer rim edge instead |
| Width | Inner flange to inner flange (bead seat to bead seat) | Measuring the overall outside width |
| Bolt pattern | Center-to-center across (even lugs) or center-to-outer edge (5 lugs) | Using the same method for odd and even lug counts |
| Pilot bore | Center hole diameter | Assuming all wheels center the same way |
| Backspacing | Straightedge across rim to mounting surface | Ignoring it if the bolt pattern fits |
If replacing wheels on a trailer with original rims, replicate the original specs. Check the wheel well for clearance — inset and offset need room on both sides. A wheel can bolt on and still rub if backspacing is off. Load rating matters too: always confirm the wheel’s rated capacity matches or exceeds the axle requirement.
FAQs
Can I measure wheel size while the tire is still on?
You can get a rough diameter measurement from the bead seat if you access the rim from behind, but width and backspacing require removing the tire. The bolt pattern and pilot bore are accessible with the wheel mounted.
What is the difference between wheel diameter and tire diameter?
Wheel diameter is the rim’s bead-seat measurement. Tire diameter is the overall height of the mounted tire and rim together. A 15-inch wheel means the rim opening is 15 inches, not the tire height.
How do I write down a trailer wheel size for ordering?
Use the format diameter × width plus bolt pattern — e.g., 16 x 6, 6 lug on 5.5. Include pilot bore and backspacing when the supplier asks. If you have the original wheel, include those numbers.
References & Sources
- etrailer. How to Measure Trailer Wheels Covers diameter, width, bolt pattern, center bore, and backspacing measurement methods.
- etrailer. Determining Tire and Wheel Diameter Explains bead seat measurement and common mistakes.
- etrailer. Wheel Bolt Pattern Guide Details even-lug and odd-lug measurement techniques.