What Size Fender Do I Need for My Trailer? (Full Measurement Guide)

Choosing the right trailer fender size is critical for safety, tire clearance, appearance, and overall trailer function. The wrong fender leads to rubbing, cracking, or insufficient coverage—while the right one fits perfectly, protects the trailer, and meets industry standards.

This guide shows you exactly how to measure your trailer and determine the correct fender size for single-axle, tandem-axle, utility, equipment, enclosed, and boat trailers.

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1. The Three Measurements That Decide Fender Size

Every correct fender fit comes down to:

1. Tire Diameter

Determines fender height.

2. Tire Width (Section Width)

Determines fender width.

3. Axle Spacing

Determines fender length—especially on tandem trailers.

If you measure these three correctly, the fender will fit perfectly every time.

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2. How to Measure Tire Diameter

Look at the tire size printed on the sidewall:

Example: ST225/75R16

Use this quick chart:

Tire Size	Approx Diameter
13” wheel	24”–25”
14” wheel	26”–27”
15” wheel	27”–28.5”
16” wheel	28”–31”
17.5” wheel	30”–32”

Fender Height Rule:

The fender must be 2–4 inches taller than the tire’s radius.

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3. How to Measure Tire Width

Tire width is the first number in the tire size:

Example: 225/75R16

→ The tire is 225 mm wide ≈ 8.85 inches.

Fender Width Rule:

You need at least 1 inch of clearance on each side.

So if your tire is 9 inches wide → choose a 10-inch-wide fender.

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4. How to Measure Axle Spacing (Tandem Trailers)

Measure from the center of the front axle to the center of the rear axle.

Standard U.S. spacing:

33–36 inches

Fender Length Rule:

• Standard spacing requires 72-inch fenders.

• Shorter spacing (rare) uses 69-inch radius fenders.

If your spacing is within normal range, 72×9×17 or 72×10×17 is correct.

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5. Fender Size Guide by Trailer Type

Single-Axle Trailers

Use this chart:

Wheel Size	Recommended Fender Size
13”	23”–25” long, 7”–8” wide
14”	26”–28” long, 8”–9” wide
15”	28”–32” long, 9” wide
16”	28”–32” long, 9”–10” wide

Best Profiles:

• Radius fenders

• Flat-top fenders

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Tandem-Axle Trailers

Industry Standard Sizes:

• 72×9×17 (15–16” wheels)

• 72×10×17 (16–17.5” wheels)

When to choose 72×10×17:

• Tire section width ≥ 8.5”

• Heavy-duty or equipment trailer

• Using 17.5” wheels

When to choose 72×9×17:

• Narrow 15–16” tires

• Light-duty tandem trailers

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Car Haulers & Equipment Trailers

Always use:

✔ 72×10×17 steel teardrop fenders

or

✔ 13-gauge versions for extreme-duty

These trailers require maximum strength and clearance.

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Enclosed Trailers

Choose:

• 28–32” single-axle

• 72” tandem-axle

• 9”–10” width depending on tire

Aluminum or smooth steel preferred for aesthetics.

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Boat Trailers

Common sizes:

• 23–25” (12–13” wheels)

• 28–32” (14–15” wheels)

• 60–72” radius (tandem)

Material: aluminum diamond plate for corrosion resistance.

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6. Clearance Requirements (Must Follow)

Vertical Clearance

2–4 inches above the tire.

Side Clearance

1 inch minimum per side.

Tandem Tire Spacing

3–4 inches between tires.

If any of these clearances are not met, the fender is the wrong size or mounted incorrectly.

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7. How to Tell If the Fender Is Too Small

• Tire sticks out beyond the fender

• Tire rub marks underneath

• Fender sits too close (less than 2” clearance)

• Fender flexes due to insufficient thickness

If any of these occur → choose a larger or wider fender.

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8. How to Tell If the Fender Is Too Big

• Excessive gap above tire (>4”)

• Looks oversized and unprofessional

• Debris spray increases

• Fenders sit higher than trailer frame

A fender that’s too tall or too wide reduces effectiveness and appearance.

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9. Quick Selection Cheat Sheet

If you run 17.5” wheels → 72×10×17 (10-inch width)

If you run 16” wheels → 72×9×17 or 72×10×17

If you run 15” wheels → 72×9×17

If you have a single axle → 28–32” length

If you’re unsure → choose 10” width for future tire upgrades

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Conclusion

The right fender size is determined by tire diameter, tire width, and axle spacing. Once you understand these measurements, choosing the correct fender becomes simple.

For most U.S. tandem trailers, the answer is straightforward: 72×9×17 or 72×10×17 steel fenders depending on tire width. Single-axle and boat trailers vary more widely, but the measurement rules stay the same.

Correct fender sizing ensures safety, proper clearance, and a professional finish for any trailer.