You found the perfect wheel. The listing says "19x9.5 ET35" and your brain says "sure, sounds right." That is exactly how people end up with a caliper grinding a spoke or a tire slicing a fender liner. Learning how to measure wheel offset takes ten minutes, a straightedge and a tape measure, and it turns fitment from a gamble into arithmetic. This guide gives you the formula, a worked Audi example, the poke and inner clearance math, and the brake checks a spec sheet can't do for you. Rather skip the homework? Our made-to-order VF-08 forged Audi wheels are built to the width and offset your car actually needs.
What Wheel Offset Actually Means
Offset (the "ET" number, from the German Einpresstiefe) is the distance in millimeters from the wheel's mounting face, the flat pad that bolts to the hub, to the centerline of the wheel's overall width.
- ✅ Positive offset: the mounting face sits outboard of the centerline, toward the street side. Almost every modern Audi runs positive offset.
- ✅ Zero offset: the mounting face sits exactly on the centerline.
- ✅ Negative offset: the mounting face sits inboard, so the wheel sticks out. Common on trucks, rare on Audis without fender work.
Higher ET pulls the wheel into the car. Lower ET pushes it out. A B9 S4 wears its factory 18x8 and 19x8.5 wheels at ET40, so an ET35 wheel of the same width sits 5 mm further out, and an ET45 sits 5 mm further in.
Offset vs Backspacing
Backspacing is the distance from the mounting face to the inner edge of the wheel, usually quoted in inches. Offset and backspacing describe the same position two different ways, and you can convert between them once you know the wheel's overall width. Here is the catch that trips up most people: a wheel sold as "8.5 inches wide" is measured at the bead seats. The lips add roughly half an inch per side, so the overall width is about the nominal width plus 1 inch.
| Nominal size | Approx. overall width | Offset | Backspacing |
|---|---|---|---|
| 19x8.5 | 9.5 in (241.3 mm) | ET40 | 160.7 mm (6.32 in) |
| 19x9 | 10 in (254.0 mm) | ET35 | 162.0 mm (6.38 in) |
| 19x9.5 | 10.5 in (266.7 mm) | ET35 | 168.4 mm (6.63 in) |
| 20x9 | 10 in (254.0 mm) | ET30 | 157.0 mm (6.18 in) |
Formula: Backspacing = (overall width / 2) + offset, or flipped, Offset = backspacing - (overall width / 2). Keep everything in millimeters (1 inch = 25.4 mm) and the math stays clean.
How to Measure Wheel Offset Step by Step
Tools: a flat floor or bench, a rigid straightedge longer than the wheel is wide, a tape measure or ruler in millimeters, and a notepad. A bare wheel is easiest, but a mounted tire works if the straightedge can reach the lip.
- Lay the wheel face-down on a flat surface with a towel under it to protect the finish.
- Bridge the back lip with the straightedge so it sits flat across the inner lip on both sides without rocking.
- Measure overall width: from the floor up to the underside of the straightedge. That is lip-to-lip width. Write it down.
- Measure backspacing: from the mounting pad up to the straightedge. Write it down.
- Find the centerline: divide the overall width by 2.
- Calculate offset: backspacing minus centerline. A positive result is positive offset; a negative result is negative offset.
Worked example: a B9 S4 factory 19
You measure an overall width of 241 mm and backspacing of 161 mm. Centerline = 241 / 2 = 120.5 mm. Offset = 161 - 120.5 = 40.5 mm, which rounds to ET40, right on the published factory spec for a B9 S4 19x8.5 wheel. Measure twice: a straightedge that rocks or a tape read at an angle can throw you off by several millimeters, and on an Audi caliper that matters.
Before you measure, flip the wheel over and look. Most wheels have the size and offset cast or stamped on the back of a spoke or inside the barrel, such as "8.5Jx19 ET40." Measuring confirms the stamp; it doesn't replace it.
Predict Poke and Inner Clearance Before You Buy
Measuring one wheel is step one. The real question is where a new wheel sits compared with the one on your car now. Two quick formulas, in millimeters:
- ✅ Outer change (poke) = (width change / 2) - offset change
- ✅ Inner change (toward the strut) = (width change / 2) + offset change
Example: going from a B9 S4 19x8.5 ET40 to a 19x9.5 ET35. Width change is 1 inch, or 25.4 mm, so half is 12.7 mm. Offset change is -5. The outer face moves out 12.7 + 5 = 17.7 mm, and the inner barrel moves toward the strut 12.7 - 5 = 7.7 mm. Now you know exactly how much fender and strut room you need, instead of guessing from forum photos.
| Change | Outer face | Inner barrel |
|---|---|---|
| +1 in width, same ET | 12.7 mm out | 12.7 mm in |
| Same width, ET 5 lower | 5 mm out | 5 mm more room |
| +1 in width, ET 5 lower | 17.7 mm out | 7.7 mm in |
| Add a 5 mm hub-centric spacer | 5 mm out | 5 mm more room |
A spacer simply lowers effective offset by its thickness. It can fix inner clearance, but it can't fix a spoke that hits the caliper face, and it needs longer bolts for proper thread engagement.
Measure the car, not just the wheel
- ✅ Hang a plumb line or level from the top of the fender lip and measure in to the current tire sidewall and wheel face to find your outer room
- ✅ Measure from the inner barrel of the current wheel to the strut and control arm at the tightest point
- ✅ Check at ride height, not with the suspension hanging on a lift, since clearance shrinks as the car settles
- ✅ Turn the wheels to full lock both ways and check the liner and tie rod
Audi Brake Clearance: What the Offset Number Won't Tell You
Offset tells you where the wheel sits. It says nothing about the shape of the spokes. Concave rotor designs taper toward the hub, and the spoke back can hit the front caliper face even when the barrel clears the caliper body with room to spare. Bigger brakes shrink the margin further: the C8 RS6 and RS7 need a genuine 21-inch minimum to clear the factory brakes, and an aftermarket big brake kit changes the picture again. Planning an Anker Street big brake kit for a B8, B8.5 or C7? Choose wheels after the brakes, not before.
How to check it:
- ✅ Test-fit one bare wheel on the front hub with two or three bolts snugged by hand
- ✅ Spin the wheel slowly and look at the gap between the spoke backs and the caliper all the way around
- ✅ Check the gap between the inner barrel and the caliper body as well
- ✅ Leave margin: components flex under hard cornering and braking, so "just clears" on the stand is not good enough
- ✅ Ask the seller for a brake clearance template or confirmation for your exact caliper
Our Audi wheel fitment guide covers bolt pattern, center bore, hub rings and lug seats, and the Audi forged wheels buying guide breaks it down by B8, B9, C7 and C8.
Skip the Guesswork With Made-to-Order Forged Wheels
Catalog wheels come in fixed widths and offsets, so you measure, calculate and hope the closest option works. Our forged sets flip that. You order your style, size and finish, then our Wheel Survey collects your car, brake package and suspension, and we confirm width, offset, bore and brake clearance before anything is machined.
| Vorsprung made-to-order | Typical catalog wheel | Dealer/OEM | |
|---|---|---|---|
| Offset | Specified for your car | Pick the closest listed ET | Factory ET only |
| Brake clearance | Confirmed in the Wheel Survey | Buyer test-fits | Designed for that model |
| Center bore | Machined for a hub-centric fit | Often oversized, needs rings | Exact |
| Construction | Fully forged 6061-T6 | Often cast or flow-formed | Usually cast |
| Set of four | $1,504.99-$2,794.99 | Varies | Dealer pricing |
That is the real shortcut on how to measure wheel offset: let the people building the wheel do the math for your exact Audi. Start with the VF-08 forged rotor-style wheels, or browse every design in our forged wheels collection.
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✅ Fully forged 6061-T6, made to order ✅ Offset and brake clearance confirmed for your exact car before we build ✅ Help from real Audi owners
Lead time is 45 to 60 days. While you wait, sort the hardware: Audi wheels use ball-seat bolts, and our titanium ball seat wheel lug kit ($259.99-$314.99) matches the factory seat. If you add spacers, you need longer bolts anyway. Curious about construction? Read forged wheels vs cast wheels, monoblock forged wheels and why we build custom forged wheels.
How to Measure Wheel Offset: FAQ
How do you measure wheel offset?
Lay the wheel face-down, put a straightedge across the back lip, and measure overall width (floor to straightedge) and backspacing (mounting pad to straightedge). Offset = backspacing minus half the overall width, in millimeters.
What is the formula for wheel offset?
Offset = backspacing - (overall width / 2). Overall width is roughly the nominal width plus 1 inch because of the lips. Convert inches to millimeters by multiplying by 25.4.
What is the difference between offset and backspacing?
Offset is measured from the wheel centerline to the mounting face in millimeters. Backspacing is measured from the inner lip to the mounting face, usually in inches. Both describe the same wheel position.
Does higher offset push the wheel in or out?
Higher offset pulls the wheel into the car. Lower offset pushes it out toward the fender.
What offset do Audi B9 S4 wheels use?
Published factory specs list the B9 S4 18x8 and 19x8.5 wheels at ET40, with a 5x112 bolt pattern and 66.5 mm center bore.
Will the right offset guarantee brake clearance?
No. Offset sets wheel position, but spoke shape decides whether the spokes clear the caliper. Test-fit a bare wheel or get clearance confirmed for your exact brakes.
Do wheel spacers change offset?
Yes. A spacer lowers effective offset by its thickness, so a 5 mm spacer turns ET40 into an effective ET35. Use hub-centric spacers and longer bolts.
Can I order wheels without knowing how to measure wheel offset?
With our made-to-order forged wheels, yes. The post-order Wheel Survey confirms your car, brakes and suspension, and we specify width and offset before machining.
Bottom Line on How to Measure Wheel Offset
Knowing how to measure wheel offset comes down to two numbers and one subtraction: backspacing minus half the overall width. Add the poke and inner clearance formulas and a bare-wheel brake check, and you will never buy a wheel on hope again. Or hand the math to us and order the VF-08 forged Audi wheels, built to the offset your car needs.
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Drive the difference. Drive Vorsprung.
Prices checked September 25, 2026 and subject to change.
