Backspace | Offset Calculator

How to Use the Backspace & Offset Calculator

Three steps. Takes less than 30 seconds.

Choose Your Conversion Direction

The left calculator converts Backspace → Offset. The right calculator converts Offset → Backspace. Pick the one that matches the measurement you already have. Only one calculator can be used at a time — entering values in one automatically locks the other to prevent errors.

Enter Wheel Width and Your Known Value

Type in your wheel width in inches (e.g. 8, 10, 10.5) and then either the backspace in inches or the ET offset in millimeters. The result appears instantly — no button to press. The cross-section diagram below updates live to show exactly where the hub face sits relative to the wheel centerline.

Read Your Result and Reference the Diagram

Your converted value is displayed in large text — offset in mm or backspace in inches depending on which direction you chose. Review the wheel cross-section diagram to visually confirm the hub face position, mounting surface, backspace measurement, and offset relative to the wheel centerline. Hit Clear to start a new calculation.

Pro tip: Not sure which measurement you have? Check your wheel spec sheet or the sticker on the back of your wheel. Backspace is always listed in inches and measured from the inboard lip to the hub mounting face. ET offset is always in millimeters and is stamped or printed on most aftermarket wheels. If you only have one, use this calculator to find the other, then call our fitment team at 1-(833)-287-6088 to confirm your setup is correct for your vehicle before you buy.

Understanding the relationship between wheel backspace and ET offset is one of the most important and most misunderstood parts of building a drag racing or performance street car. Both measurements describe the same thing from different reference points: where the wheel's hub mounting face sits relative to the wheel itself. Get it wrong and your tire rubs a control arm, contacts a brake caliper, or sits tucked so far inside the fender it looks like a stock grocery-getter. Get it right and your fitment is flush, functional, and race-ready.

Backspace is the American standard measurement used by most domestic wheel manufacturers including Weld Racing, Billet Specialties, and Bogart Racing Wheels. It measures in inches from the inboard edge of the wheel (the back lip) to the hub mounting face. A deeper backspace pushes the wheel inboard toward the brake and suspension components. A shallower backspace pushes the wheel outboard, towards the fender and the street. For most rear drag racing applications on a full-size muscle car, you'll see backspace values between 4.5" and 6.5" depending on wheel width and the target stance.

ET offset (Einpresstiefe) is the European standard used by most import wheel brands and increasingly adopted by domestic manufacturers. It measures in millimeters from the hub mounting face to the wheel's geometric centerline. Positive ET means the hub face is outboard of center, the wheel sits more inboard. Negative ET means the hub face is inboard of center, the wheel sits more outboard, creating more poke and an aggressive stance. Zero ET puts the hub face exactly at the wheel centerline.

Backspace & Offset Calculator | DragRacingWheels.com

Backspace & Offset Calculator

Convert between wheel backspace (inches) and offset (mm) — use one calculator at a time

Backspace → Offset Calculate ET (mm)

Wheel Width

Backspace

Result

Offset (ET) —

Offset → Backspace Calculate Backspace (in)

Wheel Width

Offset (ET)

Result

Backspace —

Wheel Cross-Section — Backspace & Offset Reference

Formulas used:
Backspace → Offset (mm): Offset = (Backspace − (Width ÷ 2) − 0.5) × 25.4
Offset → Backspace (in): Backspace = (Offset ÷ 25.4) + (Width ÷ 2) + 0.5

The 0.5 in accounts for the wheel mounting flange thickness (industry standard). Positive offset = hub face is outboard of the wheel centerline. Negative offset = hub face is inboard of center.

DragRacingWheels.com · Need help? Call 1-(833)-287-6088 · Results are estimates. Always verify fitment before purchase.
The Wheel & Tire Fitment Calculator is provided for informational purposes only. The calculations and results generated by this tool are based on the input parameters provided by the user and are not guaranteed to be accurate or suitable for every application. By using this calculator, users acknowledge and agree to the terms of this disclaimer and understand that it is their responsibility to independently verify and validate the results obtained from the tool. Users are solely responsible for their decisions and actions based on the calculated results, and they should consider seeking professional advice when in doubt.

Wheel Backspace & Offset — Frequently Asked Questions

Common questions about converting between backspace and ET offset for drag racing and performance wheel fitment.

What is the difference between backspace and ET offset?

Both measurements describe where a wheel's hub mounting face sits — they're just measured from different reference points. Backspace is an American measurement in inches, taken from the inboard edge of the wheel (the back lip) to the hub mounting face. ET offset is a European measurement in millimeters, taken from the hub mounting face to the wheel's geometric centerline. A positive ET means the hub face is outboard of center; a negative ET means it's inboard. The two are directly convertible using the formula our calculator applies: Offset (mm) = (Backspace − (Width ÷ 2) − 0.5) × 25.4.

Why is there a 0.5 inch constant in the backspace-to-offset formula?

The 0.5 inch (approximately 12.7mm) constant accounts for the thickness of the wheel's mounting flange — the raised hat section that the lug nuts clamp against when the wheel is bolted to the hub. This flange sits between the inboard edge of the wheel and the actual hub contact surface. Because backspace is measured to the outer face of this flange and offset is measured from the hub contact face, the 0.5" flange thickness must be subtracted during conversion. This is an industry-standard value used by all major domestic wheel manufacturers including Weld Racing, American Racing, and Bogart.

How do I find my wheel's backspace or offset if I don't know it?

For aftermarket wheels, ET offset is typically stamped or printed on the back of the wheel near the center bore, and backspace is listed on the wheel's spec sheet or product page. For factory wheels, both measurements are usually listed in your vehicle's owner's manual, on the factory tire placard, or in a service manual. You can also measure backspace physically: lay the wheel face-down on a flat surface, place a straightedge across the back of the rim, and measure from the straightedge down to the hub mounting face. That measurement in inches is your backspace.

What is a typical backspace for a rear drag racing wheel?

Rear drag wheel backspace varies by vehicle platform and wheel width. On a full-size American muscle car running a 15×8" or 15×10" wheel, typical backspace ranges from 3.5" to 4.5" to push the tire outboard for a wide stance and tire clearance over the axle tubes. On a wider 10.5" or 12" wheel, you may see 4.5" to 5.5" depending on whether the car is a narrowbody or widebody. Front drag skinnies on a 3.5" to 4.5" wide wheel typically run 1.75" to 2.5" of backspace. Always verify against your specific vehicle's inner wheel well clearance and suspension geometry before purchasing.

What ET offset is equivalent to a 4.5" backspace on an 8" wide wheel?

Using the standard formula: Offset = (4.5 − (8 ÷ 2) − 0.5) × 25.4 = (4.5 − 4.0 − 0.5) × 25.4 = 0 × 25.4 = 0mm. So a 4.5" backspace on an 8" wide wheel equals ET0 — the hub face sits exactly at the wheel centerline. This is a very common rear drag racing setup for 8"-wide wheels on muscle cars. You can verify any backspace-and-width combination instantly using the calculator above.

Does a higher backspace push the wheel in or out?

A higher (deeper) backspace pushes the wheel inboard, towards the brakes, suspension, and vehicle centerline. This tucks the wheel further under the fender and reduces poke. A lower (shallower) backspace pushes the wheel outboard, towards the fender lip, creating more poke and a more aggressive stance. For drag racing, shallower backspace is often desirable at the rear to maximize tire clearance over the frame rails and axle housing, but it must be balanced against fender clearance at full suspension compression and steering lock at the front.

Can I use this calculator for Weld Racing, Billet Specialties, and other drag-specific wheels?

Yes, the calculator works for any wheel brand or style regardless of manufacturer. Weld Racing, Billet Specialties, Bogart Racing Wheels, and most domestic drag wheel brands spec their wheels in backspace (inches), while European and many modern aftermarket brands use ET offset (mm). This calculator converts in both directions so you can cross-reference specs between brands, verify that a replacement wheel matches your existing setup, or confirm that a new wheel will clear your brakes and suspension before you place an order. For Weld Racing fitment-specific guidance, call our team at 1-(833)-287-6088.

Does wheel width affect the backspace-to-offset conversion?

Yes, wheel width is a required input in the conversion formula because ET offset is measured from the centerline of the wheel, and the centerline moves as width changes. Two wheels with identical backspace but different widths will have different ET offsets. For example, a 4.5" backspace on an 8" wide wheel = ET0, but a 4.5" backspace on a 10" wide wheel = (4.5 − 5.0 − 0.5) × 25.4 = −25.4mm, which is approximately ET−25. This is why you must always enter the correct wheel width, not just the backspace or offset alone.

How accurate is the backspace-to-offset conversion?

The conversion formula is mathematically exact, there is no approximation or rounding in the underlying calculation. Our calculator returns results to two decimal places for backspace (inches) and one decimal place for offset (mm), which exceeds the measurement precision of any tape measure or wheel gauge you'd use in a shop. The only variable is the 0.5" flange thickness constant, which is the universal industry standard. In rare cases, specialty wheels with non-standard flanges may vary slightly, when in doubt, always verify with a physical measurement or contact our fitment team at 1-(833)-287-6088.

Is my wheel data saved or tracked when I use this calculator?

No. All calculations run entirely within your browser, nothing is sent to a server, stored in a database, or linked to your account or session. Your wheel width, backspace, and offset values are never transmitted anywhere. The calculator uses no cookies, no analytics on your inputs, and makes no external data requests of any kind. Your build specifications remain completely private from the moment you type them to the moment you close the tab.

Still unsure which backspace or offset is right for your vehicle?

Call Our Fitment Experts — 1-(833)-287-6088