Convert offset to backspacing, calculate poke or flush fitment, find the effect of wheel spacers on effective offset, and check the safe offset range for your vehicle type. The most complete wheel fitment calculator online — covering ET offset, backspacing, stance, and OEM reference data.
✓Verified: SAE J328 Wheel Standards & OEM Offset Reference Data
📋 Enter wheel width and either offset OR backspacing
in
e.g. 8 for an 8J wheel
Enter wheel width.
mm
e.g. ET35, ET+45, ET-15
in
Distance from inner lip to hub mounting surface
Enter either offset or backspacing (not both).
📋 Calculate wheel stance relative to fender
in
Your wheel width
Enter wheel width.
mm
e.g. ET35
Enter your offset.
in
Original stock wheel width
Enter OEM width.
mm
Original stock offset
Enter OEM offset.
📋 Calculate effective offset after adding a spacer
mm
Your wheel's current ET
Enter current offset.
mm
e.g. 15mm spacer
Enter spacer thickness.
in
For backspacing calculation
Enter wheel width.
📋 Compare current vs new offset — track width & clearance change
in
Enter current width.
mm
Enter current offset.
in
Enter new width.
mm
Enter new offset.
Result
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⚠️ Disclaimer: Wheel offset calculations are based on standard formulas. Actual fitment depends on brake caliper clearance, suspension geometry, fender flares, and spacers. Always verify fitment by physically checking clearance before final installation. Consult a professional installer for lifted or modified vehicles.
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Sources & Methodology
✓All offset and backspacing formulas use the SAE J328 standard definition. OEM offset ranges from published vehicle specifications and fitment industry data. All external sources cited with nofollow links per site policy.
SAE International J328 defines wheel offset (ET) as the distance from the hub mounting surface to the wheel centerline, measured in millimeters. This is the authoritative US standard used for all offset and backspacing formulas implemented in this calculator.
Tire Rack's technical guide on wheel offset and backspacing provides OEM fitment reference data, safe offset deviation ranges, and vehicle-type specific guidelines used to validate the safe offset range table and bearing load warning thresholds in this calculator.
Methodology:Backspacing (in) = (Wheel width in / 2) + (Offset mm / 25.4) + 0.5Offset (mm) = (Backspacing in - 0.5 - wheel width in / 2) x 25.4Outer position change (mm) = New offset - Current offset (negative = more poke)Inner clearance change (mm) = Current offset - New offset (positive = more inner clearance)Track width change per side (mm) = Current offset - New offset (positive = wider track)Effective offset with spacer = Original offset - Spacer thickness (mm)
The 0.5-inch constant in backspacing formula accounts for the inner barrel lip thickness per SAE J328 convention. Track width change shown is per side; multiply by 2 for total track change.
Last reviewed: April 2026
Wheel Offset Explained — The Complete Guide to ET, Backspacing & Fitment
Wheel offset is one of the most important — and most misunderstood — specifications when buying aftermarket wheels. Getting offset wrong means your wheels rub on suspension components, sit too deep inside the fender, or cause accelerated bearing wear. Getting it right means flush fitment, correct steering geometry, and years of trouble-free driving. This guide covers everything you need to know.
Positive, Negative, and Zero Offset — What Each Means
Offset determines whether the wheel sits inward or outward from the hub centerline. Positive offset means the mounting surface is toward the wheel's outer face — the wheel sits closer to the vehicle centerline (tucked). This is the most common on modern FWD passenger cars. Negative offset means the mounting surface is toward the wheel's inner face — the wheel sits further from the vehicle centerline (poked out). Zero offset means the mounting surface is exactly at the wheel's centerline.
Offset Type
ET Value
Wheel Position
Typical Application
High positive
ET+40 to ET+60
Tucked in, inner-biased
Compact FWD cars, stock OEM
Mild positive
ET+20 to ET+40
Near-flush or slight tuck
SUVs, sedans, many aftermarket
Zero
ET0
Exactly flush at centerline
Some trucks, custom builds
Negative
ET-5 to ET-40
Poked out, wide stance
Off-road trucks, aggressive fitment
OEM Offset Reference by Vehicle Type
Vehicle Type
Typical OEM Offset
Safe Aftermarket Range
Notes
Compact FWD cars
ET+35 to ET+50
ET+25 to ET+55
High positive needed for caliper/strut clearance
Midsize sedans
ET+35 to ET+48
ET+25 to ET+55
Varies by model; check caliper clearance
Crossovers / SUVs
ET+35 to ET+55
ET+25 to ET+60
AWD variants may need tighter tolerance
Pickup trucks
ET+10 to ET+30
ET+0 to ET+40
Wider range due to truck hub geometry
Sports / RWD
ET+20 to ET+45
ET+10 to ET+55
Flush fitment common goal
Off-road / lifted
ET+0 to ET+25
ET-30 to ET+30
Lift and fender flares affect safe range significantly
Poke, Flush, and Tucked — Stance Terminology Explained
These terms describe how the tire's outer edge sits relative to the vehicle's fender lip when viewed from the front or rear of the vehicle. Flush means the tire sits perfectly level with the fender lip — the most popular street fitment that looks intentional and clean. Poke means the tire extends beyond the fender lip — the wheel is visible sticking out. Small poke (2–5mm) is common for aggressive street builds; large poke requires fender rolling or flares. Tucked means the tire sits completely inside the fender lip — typical of stock vehicles and lifted trucks with narrow tires.
The amount of poke or tuck depends on the difference between your wheel's offset and the fender geometry of your specific vehicle. A wheel with lower offset (less positive) moves the tire outward, increasing poke. A wheel with higher offset (more positive) moves the tire inward, increasing tuck. Wider wheels also affect this — a wider wheel with the same offset will push both inner and outer edges outward proportionally.
Spacer Effect on Offset — What Does a 15mm Spacer Actually Do?
A wheel spacer physically moves the wheel outward by its thickness in millimeters, which effectively reduces the offset by the same amount. A 15mm spacer on a wheel with ET+45 creates an effective offset of ET+30. The track width increases by 30mm total (15mm per side × 2 sides). This is why spacers are popular for achieving flush fitment with wheels that have too much positive offset, without buying new wheels.
⚠️ Spacer safety rule: For slip-on spacers (no extended studs), maximum 5–10mm. For hub-centric bolt-on spacers with extended studs, up to 25–30mm is acceptable for street use. Always use hub-centric spacers with the correct bore and bolt pattern. A spacer exceeding 15mm with standard slip-on design reduces stud engagement below safe minimums and is a safety hazard.
How Offset Affects Wheel Bearings — The Safety Concern
The hub wheel bearing supports the wheel's weight and absorbs lateral forces during cornering. When you change offset, you change the moment arm — how far the wheel's weight acts from the bearing center. A wheel with significantly lower offset (more poke) increases the moment arm, multiplying the side load the bearing experiences. This is why going more than 15–25mm from OEM offset accelerates bearing wear, and going beyond 30mm can cause premature bearing failure on vehicles driven hard or at highway speeds.
Frequently Asked Questions
Wheel offset (ET) is the distance in mm from the wheel's hub mounting surface to its centerline. Positive ET = mounting surface closer to the outer face (wheel sits in). Negative ET = mounting surface closer to the inner face (wheel sticks out). ET comes from the German word Einpresstiefe meaning "press-in depth." Use Mode 1 above to convert any offset to backspacing instantly.
Offset (ET) = distance from wheel centerline to mounting surface, in mm, can be positive/negative/zero. Backspacing = distance from inner lip to mounting surface, in inches, always positive. Formula: Backspacing = (Width/2) + (Offset/25.4) + 0.5. Example: 8" wheel ET+35 = 5.88" backspacing. Use Mode 1 to convert in either direction.
Flush = tire outer edge level with fender lip. Poke = tire extends beyond fender. Tucked = tire sits inside fender. Lower offset (less positive) = more poke. Higher offset (more positive) = more tuck. Wider wheel with same offset = more poke on outer edge. Use Mode 2 (Poke/Flush/Tuck) above to calculate exactly how much poke or tuck your setup produces vs stock.
A spacer pushes the wheel outward, reducing effective offset by the spacer's thickness in mm. ET+40 with 15mm spacer = effective ET+25. This increases poke and widens track by 30mm total. Use Mode 3 (Spacer Effect) to calculate the new effective offset. Maximum safe spacer without extended studs: 5-10mm. Bolt-on spacers with extended studs: up to 25-30mm acceptable.
Safe ranges by type: Compact FWD cars: OEM typically ET+35 to +50, safe range ET+25 to +55. SUVs/crossovers: ET+35 to +55 OEM, safe ET+25 to +60. Pickup trucks: ET+10 to +30 OEM, safe ET0 to +40. Sports/RWD: ET+20 to +45 OEM. Going more than 15mm from OEM increases bearing load. More than 25mm accelerates bearing wear significantly. Off-road with lift: larger range, consult installer.
Yes. Lower offset (more negative/less positive) increases the lever arm on the hub bearing, multiplying side load. Up to 15mm offset change: generally safe for normal driving. 15-25mm change: noticeable bearing wear increase over time. More than 25mm: significant accelerated bearing wear; not recommended for highway/frequent use. The bearing load warning in Mode 4 (Compare) flags changes exceeding safe thresholds.
Remove wheel. Lay face-down on flat surface. Measure total width (inner lip to outer lip) in inches. Measure from floor (outer face) to hub mounting surface. Offset in mm = (width/2 - face-to-mount measurement) x 25.4. If mounting surface is farther from floor than centerline = positive offset. Closer = negative. Most OEM wheels also have the offset stamped on the inside of the wheel between spokes.
Track width changes by 2mm per 1mm of offset change (both sides). Example: Going from ET+45 to ET+35 (10mm less positive) = 20mm wider track total. Wider track improves cornering but may cause fender rub. Mode 4 (Compare) calculates the exact track width change, outer position shift, and inner clearance change for any two offset combinations.
Hub-centric: wheel center bore matches hub exactly — hub centers the wheel (best, eliminates vibration). Lug-centric: oversized bore, lug nuts center the wheel (can cause vibration at speed). If using aftermarket wheels with larger bore, use hub-centric rings to fill the gap. This is a simple, inexpensive fix that prevents high-speed vibration. Always verify center bore when ordering aftermarket wheels.
No. Fitment requires matching: bolt pattern (lug count + PCD), center bore (or use hub rings), offset within safe range, overall width/diameter clearing brakes and fenders. Even a correctly sized wheel with wrong offset can rub on struts, control arms, or fenders at full suspension travel. Always physically check clearance with full suspension compression and full steering lock before final installation.
Scrub radius is the distance between the steering axis and tire contact patch center at ground level. OEM engineers design specific scrub radius for stability and steering feel. Changing offset changes scrub radius: more positive offset on FWD cars increases scrub radius, worsening torque steer. Changing offset on RWD changes steering feel and return-to-center. Always try to stay close to OEM offset to preserve intended steering geometry.
ET comes from German Einpresstiefe (press-in depth). Always expressed as ET followed by the mm value: ET35 = positive 35mm offset. ET0 = zero offset. ET-15 = negative 15mm offset. The ET number tells you how far the mounting surface is from the wheel centerline: positive ET = mounting surface is toward the outer face (inside of vehicle when mounted). You will find the ET value stamped on the back of most OEM and aftermarket wheels.
A wider wheel with the same offset shifts both the inner and outer edges outward by half the width difference. Example: 7J ET+45 vs 9J ET+45 — the 9J wheel's outer face is 25.4mm (1") further out AND the inner face is 25.4mm further in. This means the wider wheel creates more poke AND needs more inner clearance. When going wider, you often need more positive offset to compensate for the outer edge moving out. Mode 4 (Compare) accounts for both width and offset changes.