How to Measure a Trailer Fender (DIY Step-by-Step Guide)

Measuring a trailer fender correctly is the difference between a perfect bolt-on/weld-on fit and a frustrating replacement that rubs the tire, sits too high, or doesn’t align with your axle spacing.

This step-by-step guide walks you through exactly how to measure ANY trailer fender—single axle, tandem axle, steel, aluminum, radius, or teardrop—so you always order the correct size.

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1. Tools You Need

• Tape measure (minimum 10 ft)

• Straight edge or level

• Notepad or phone for notes

• Tire pressure gauge (optional, but recommended)

Tip:

Always measure while the trailer is on level ground with properly inflated tires.

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2. The Three Critical Measurements

Every trailer fender is sized based on these three numbers:

1. Fender Length

Affects axle spacing fitment.

2. Fender Width

Determined by tire section width.

3. Fender Height

Determined by wheel diameter + clearance.

If you record these accurately, you’ll never order the wrong fender again.

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3. How to Measure Fender Length (Tandem & Single Axle)

Single-Axle Fender Length

Measure from:

• Front tip of the fender

to

• Rear tip of the fender

Typical lengths:

• 23–25” for 13” wheels

• 26–28” for 14” wheels

• 28–32” for 15”–16” wheels

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Tandem-Axle Fender Length

Measure from:

• Front of the fender

to

• Rear of the fender

Standard lengths:

• 72” (most common)

• 69” (some OEM radius setups)

Matching Axle Spacing

Most U.S. tandem trailers use 33–36 inches spacing, which matches 72-inch fenders perfectly.

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4. How to Measure Fender Width

Place your tape across the top of the fender from outer edge to inner edge.

Standard widths:

• 9-inch → Light-duty tandem trailers, 15–16” wheels

• 10-inch → Heavy-duty tandem trailers, 16–17.5” wheels

• 7–8” → Small single-axle trailers

Rule of Thumb

Your fender should always be 1 inch wider on each side of the tire.

Example:

A tire measuring 8.5” wide → choose a 10-inch fender.

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5. How to Measure Fender Height

Measure from:

• Bottom of the fender (mounting surface)

to

• Highest point on the fender arch

Typical heights:

• 12–15” for single-axle

• 17” for nearly all tandem-axle steel fenders

Height correlates with tire diameter:

Wheel Size	Approx Fender Height
13” wheel	12”–13”
14” wheel	13”–14”
15” wheel	14”–16”
16” wheel	16”–18”
17.5” wheel	17”–19”

 

6. How to Measure Tire Diameter (For Correct Fender Height)

Look at the tire sidewall:

Example: ST225/75R16

Diameter is roughly:

• 225 mm → 8.85” width

• 75% aspect ratio → 6.64” sidewall

• Multiply by 2 → 13.28”

• Add rim diameter (16”)

= 29.28 inches

Fender height must be at least half the tire diameter + 2–4 inches for clearance.

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7. How to Measure Axle Spacing (Tandem Only)

Measure center-to-center from front axle hub to rear axle hub.

Standard spacing:

• 33–36 inches → requires 72-inch fenders

• <33 inches → may require 69-inch radius fenders

If your spacing is within normal range, pick the standard 72-inch fender.

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8. How to Check Fender Clearance

Correct clearance prevents tire rubbing.

Vertical Clearance:

2–4 inches above tire

Side Clearance:

1 inch per side

Tandem Tire Spacing:

3–4 inches

Measure clearance after you install one side before securing both.

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9. How to Confirm Fender Profile (Teardrop vs Radius)

Teardrop Fenders

• Have a center “dip” between tandem wheels

• Used on almost all equipment trailers

• More rigid

Radius Fenders

• Smooth curve across entire length

• Common on aluminum and utility trailers

Match the fender profile to what is already on your trailer unless you are doing a full upgrade.

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10. DIY Measuring Cheat Sheet

If your tire is 15”–16”, your tandem fender is:

→ 72×9×17 or 72×10×17

If your tire is 17.5”:

→ 72×10×17 mandatory

If your single axle fender is under 30”:

→ You likely have 13–14” wheels

If your single axle fender is 28–32”:

→ You have 15–16” wheels

If your axle spacing is 33–36”:

→ Choose 72” teardrop or radius fenders

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Conclusion

Measuring a trailer fender is simple once you know the three essential dimensions: length, width, and height. Combine those with tire diameter, section width, and axle spacing, and you can instantly determine the perfect fender size for any trailer.

Accurate measurement ensures:

• No rubbing

• Perfect fit

• Correct protection

• Long-lasting performance

This DIY guide guarantees you order the right fender every time.