Wheel Offset Calculator: Backspacing, Poke & Will It Fit?

Quick answer: backspacing (inches) = rim width ÷ 2 + 0.5 + offset (mm) ÷ 25.4. A 9-inch-wide wheel at ET35 has about 6.4 in of backspacing; the same wheel at ET0 has 5.0 in. Higher positive offset means more backspacing and a wheel that sits further inboard. Use the calculators below to convert either way and to check poke and inner clearance for your car.

Not sure if a set of wheels will fit your car? The Will It Fit? checker below compares any wheel size and offset against your vehicle’s factory wheels, bolt pattern, center bore and the most aggressive setup we recommend for it, using data on 10,000+ vehicle variants from our open wheel fitment dataset. Further down the page you will find the offset-to-backspacing and tire diameter calculators.

Getting offset and backspacing right is the difference between a wheel that clears your suspension and one that rubs the fender liner on the first turn. This calculator uses the same formula ANKUSI's engineering team applies on every fitment page in our fitment guide library, so the numbers here match what you will see quoted for your specific vehicle.

This tool gives you a calculated reference number, not a clearance guarantee. Suspension geometry, brake calipers, and fender lips vary by trim and modification — always confirm final clearance with a physical test fit before production.

Will It Fit? Wheel Fitment Checker

Choose your make, model and year, then enter the diameter, width and offset (ET) of the wheel you are considering. Add the wheel’s bolt pattern and center bore if you know them. The checker tells you whether the wheel should fit, fits with caution, or is likely to rub, and shows how many millimeters the outer and inner lips move compared with the factory wheel.

How the fitment check works

Every wheel has two edges that matter for clearance. The outer lip sits (width ÷ 2) − offset millimeters outside the hub face and decides whether the wheel pokes past the fender. The inner lip sits (width ÷ 2) + offset millimeters inside the hub face and decides whether it touches the strut, spring or control arm. The checker calculates both edges for your wheel and compares them with the factory wheel and with the flush and maximum setups we recommend for that vehicle. If the new wheel stays inside that envelope it should fit. Up to 6 mm beyond it usually means fender rolling, extra camber or a small spacer; more than that usually means rubbing.

It also checks the bolt pattern, which must match exactly; the center bore, which must be equal to or larger than the hub (use a hub-centric ring to fill any gap); and the diameter, because going smaller than the factory wheel can foul the brake calipers.

Wheel Offset and Backspacing Calculator

It works in both directions: as a wheel offset calculator when you know the backspacing, and as a backspace-to-offset converter when all you have is the ET figure stamped on the wheel.

Enter any two known values and select the corresponding calculation to solve for the third. Wheel width is the nominal stamped width in inches. Offset is measured in millimeters from the wheel centerline to the hub-mounting surface. Positive offset places the mounting surface toward the wheel’s street-side face and positions the wheel farther inboard.

Formulas Used

Given nominal wheel width W (in) and offset ET (mm):

Building a drag car? Our drag wheel backspacing guide walks through measuring backspacing and typical values for 15x10 rears and 15x3.5 front runners.

Backspacing(in) = W ÷ 2 + ET(mm) ÷ 25.4 + 0.5

Given backspacing BS (in) and nominal width W (in):

ET(mm) = [BS − W ÷ 2 − 0.5] × 25.4

The 0.5 in constant is a standard single-flange allowance used industry-wide when working from nominal (stamped) width rather than a caliper-measured total outer width.

Tire Diameter Calculator

Compare two tire sizes to check how much the overall rolling diameter changes when you go up or down in wheel diameter — useful for keeping speedometer accuracy and fender clearance in check. Calculated using the standard ETRTO/TRA sidewall geometry formula.

Tire Diameter Formula

Overall Diameter(in) = Rim Diameter(in) + 2 × [Width(mm) × Aspect Ratio(%) ÷ 100] ÷ 25.4

This is the standard ETRTO/TRA sidewall-height calculation used across the tire industry. Actual measured diameter can vary roughly ±1–2% by manufacturer and tread design, so treat this as a close reference rather than an exact spec.

Frequently Asked Questions

How do I know if a wheel will fit my car?

Check four things: the bolt pattern must match exactly, the wheel’s center bore must be equal to or larger than the hub, the diameter must clear the brakes, and the width and offset must keep the outer lip inside the fender and the inner lip clear of the suspension. The Will It Fit? checker at the top of this page runs all four checks for your vehicle using its factory wheel size and the setups recommended in our fitment data.

What is the difference between offset and backspacing?

Offset is the distance in millimeters from the wheel's true centerline to its mounting (hub) surface, and can be positive, negative, or zero. Backspacing is the distance in inches from the mounting surface to the back (inner) edge of the wheel, and is always a positive number. They describe the same physical geometry from two different reference points, which is why one can be calculated from the other once wheel width is known.

How do I calculate backspacing from an offset figure?

Backspacing in inches = (wheel width ÷ 2) + (offset in mm ÷ 25.4) + 0.5. The 0.5 accounts for the two half-inch flanges that the stamped “nominal” width leaves out, so a 9.0J wheel actually measures about 10.0 inches across the rim edges. Working the other way, offset in mm = (backspacing − wheel width ÷ 2 − 0.5) × 25.4. The calculator above does both conversions, so you can enter whichever two numbers you already have.

Does a higher positive offset increase or decrease backspacing?

At the same nominal wheel width, a higher positive offset increases backspacing. The hub-mounting surface moves toward the wheel’s street-side face, so the wheel sits farther inboard: the inner edge moves closer to the suspension while the outer edge becomes more tucked. A lower or negative offset decreases backspacing and moves the wheel outward. These measurements describe lateral wheel position only and do not guarantee spoke-to-caliper, suspension, or fender clearance.

Does this tool guarantee my wheel will clear my brakes or fenders?

No. The fitment checker and calculators work from geometry and published fitment data. Actual clearance also depends on caliper size, suspension travel, camber, tire size and fender lip shape, which these tools cannot measure. A physical test fit is recommended before final mounting, especially on aggressive or lowered setups.

Popular Fitment Guides

Already have your width and offset numbers? Check them against the factory-verified bolt pattern, offset and center bore data for these vehicles:

Looking for a different make? Browse the full fitment guide library by brand, year and model.

About This Calculator

Written by: ANKUSI Engineering Team
Published: August 22, 2026  •  Last updated: October 12, 2026

Formulas cross-checked against ANKUSI's own fitment-page generation process and standard ETRTO/TRA tire sidewall geometry conventions.

Use this calculator on your own site. Shops, clubs and forums can embed it for free, with no ads or sign-up. Get the embed code, along with a bolt pattern and center bore lookup for about 10,000 vehicles.

Written by Emma, ANKUSI Forged Wheels. ANKUSI is a forged wheel manufacturer in Guangzhou, China, certified to IATF 16949 and ISO 9001. Bolt pattern and center bore figures are verified against at least two independent sources before publication. Found a mistake? Email info@ankusi.com.

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Source: Wheel Offset, Backspacing and Tire Diameter Calculator (ankusiwheels.com)

ANKUSI Wheels. (2026). Wheel Offset, Backspacing and Tire Diameter Calculator. Retrieved October 20, 2026, from https://ankusiwheels.com/offset-backspacing-tire-diameter-calculator/

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  title = {Wheel Offset, Backspacing and Tire Diameter Calculator},
  year = {2026},
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<a href="https://ankusiwheels.com/offset-backspacing-tire-diameter-calculator/">Wheel Offset, Backspacing and Tire Diameter Calculator</a> (ANKUSI Wheels)