Wheel offset and backspacing calculator
Compare your current wheel and tire with a new setup. See how far the tire moves toward the suspension and out toward the fender, drawn to scale.
- Current
- New
| Measured from the mounting face | Current | New | Change |
|---|---|---|---|
| Tire inner sidewall, inboard of the face | 6.95 in | 5.61 in | −1.34 in away from the suspension |
| Tire outer sidewall, outboard of the face | 3.48 in | 5.61 in | +2.13 in further out |
| Backspacing (rim edge) | 5.73 in | 4.50 in | −1.23 in |
| Front spacing (rim edge) | 2.27 in | 4.50 in | +2.23 in |
Inner sidewall: 1.34 in further from the suspension. Outer sidewall: 2.13 in further out toward the fender.
Measure today's gaps with the vehicle on level ground and the wheels straight, then enter them above to see the space left with the new setup.
How offset and backspacing work
Offset is the distance from the wheel's mounting face to the centre line of the rim. It can be positive, zero or negative (UN Regulation No. 124, para 2.6, where it is called inset). A positive offset puts the mounting face toward the outside of the wheel, so the wheel sits further in.
Backspacing is the distance from the mounting face to the inner edge of the rim: half the rim width plus the offset. This calculator measures to the rim width you enter. A wheel maker's backspacing figure is measured to the outer lip, which adds the thickness of the rim flange, so check the maker's figure before you order.
The tire's position assumes its nominal section width is centred on the rim. A wider rim spreads the sidewalls a little, and a new tire can be wider than nominal: for passenger tires, up to 7 percent under FMVSS No. 139. Leave some margin.
Example
From a 265/70R17 on an 8 in wheel at +44 mm to a 285/70R17 on a 9 in wheel at 0 mm: the offset change alone moves everything 44 mm (1.73 in) outward. The wider tire and wheel add half their extra width on each side. The tire's inner sidewall ends up 1.34 in further from the suspension, and its outer sidewall 2.13 in further out toward the fender.
How the clearance check works
Enter the gaps you measure on the current setup. The space left is today's gap minus the movement of the tire: the inner sidewall movement for the suspension gap, the outer sidewall movement for the fender lip, and half of the diameter change for the gap above the tire, because the body rises with the axle when the tire grows. A negative result means contact.
These are static gaps. When the suspension compresses or the wheels turn, the tire comes much closer to the body, so leave a margin.
This calculator does not tell you whether a wheel fits your vehicle. Brake caliper clearance, steering lock, suspension travel and fender liners decide that. See changing tire size.
Printed from https://tiresizecalc.net/wheel-offset-calculator/. Figures as published on 2026-11-01. The full working and the official sources are on that page.
Sources
- UN Regulation No. 124 - Uniform provisions concerning the approval of wheels for passenger cars and their trailers. UNECE; Official Journal of the European Union. 42006X1227(08), OJ L 375, 27.12.2006. 2006-12-27. Para 2.6: definition of inset (offset), positive, zero or negative. Retrieved 2026-11-01.
- UN Regulation No. 30 - Uniform provisions concerning the approval of pneumatic tyres for motor vehicles and their trailers (Revision 3). United Nations Economic Commission for Europe (UNECE). E/ECE/324 E/ECE/TRANS/505 Rev.1/Add.29/Rev.3. 2007-03-29. Para 6.1.2.1: tire outer diameter and section width. Retrieved 2026-11-01.
- 49 CFR 571.139 - Standard No. 139; New pneumatic radial tires for light vehicles. National Highway Traffic Safety Administration (NHTSA), Code of Federal Regulations (eCFR). 49 CFR 571.139. eCFR up to date as of 2026-10-15. S6.1.2: passenger tire width up to 7 percent (or 10 mm) over the specified width. Retrieved 2026-11-01.
- Computed by tiresizecalc.net from the cited formula. Sri Murugan Systems. Backspacing = rim width / 2 + offset; front spacing = rim width / 2 - offset; tire sidewall positions use the nominal section width centred on the rim; 1 in = 25.4 mm. Retrieved 2026-11-01.
- Computed by tiresizecalc.net from the cited formula. Sri Murugan Systems. Clearance left = measured gap - movement; the gap above the tire changes by half the diameter change. Retrieved 2026-11-01.
Last reviewed 2026-11-01. The full source list, with dates and file checksums, is in How the numbers are calculated. To report an error, email contactus@tiresizecalc.net.