Wheel Geometry & Fitment

Backspacing-to-Offset Converter

Convert wheel backspacing to offset and offset to backspacing for any wheel width. Inch and millimeter results in both directions.

Wheel Geometry & Fitment

Convert mounting measurements.

Open Live Calculator

Backspacing-to-Offset Converter: Tire Size Inputs

Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.

ISO/TRA Compliant

Baseline Tire (T1)Stock OEM

Width (mm)

Aspect (%)

Rim (in)

Comparison Tire (T2)Proposed Setup

Width (mm)

Aspect (%)

Rim (in)

Baseline OD634.3 mm24.97"

Comparison OD637.2 mm25.09"

Diameter Variance+2.9 mm+0.46%

Speed @ 60 MPH60.3 MPH-4 revs/mi

How Backspacing-to-Offset Converter Works?

The Backspacing to Offset Converter works by moving between the 2 mounting measurements using half of the wheel's overall width. The converter runs in either direction in 3 steps.

Choose the direction
Pick backspacing → offset or offset → backspacing.
Enter wheel width and value
Type the width (8.0 in) and the known value (5.0 in backspacing).
Read the result
The converter returns ET+13 for a 5.0 in backspacing on an 8.0 in wheel.

Backspacing-to-Offset Converter Formula

The Backspacing to Offset Converter formulas are ET = BS − (W + 1) × 12.7 and BS = ET + (W + 1) × 12.7.

Backspacing → offsetET = BS_mm − (W + 1) × 12.7

Offset → backspacingBS_mm = ET + (W + 1) × 12.7

Inches ↔ mmBS_mm = BS_in × 25.4

W: Nominal wheel width (in)
12.7: Half of 25.4 mm
BS: Backspacing
ET: Offset (mm)

Worked example

A 10.0 in wheel with 4.5 in (114.3 mm) of backspacing converts to 114.3 − 139.7 = ET−25. A 7.0 in wheel at ET+40 converts to 40 + 101.6 = 141.6 mm (5.57 in) of backspacing.

Backspacing-to-Offset Converter Conversion Table

The Backspacing to Offset Converter conversion table converts 7 backspacing values into offset for 4 wheel widths.

Offset for 7 backspacing values and 4 widths
Backspacing	7.0" wheel	8.0" wheel	9.0" wheel	10.0" wheel
4.0 in (101.6 mm)	ET+0	ET-13	ET-25	ET-38
4.5 in (114.3 mm)	ET+13	ET+0	ET-13	ET-25
5.0 in (127.0 mm)	ET+25	ET+13	ET+0	ET-13
5.5 in (139.7 mm)	ET+38	ET+25	ET+13	ET+0
6.0 in (152.4 mm)	ET+51	ET+38	ET+25	ET+13
6.5 in (165.1 mm)	ET+64	ET+51	ET+38	ET+25
7.0 in (177.8 mm)	ET+76	ET+63	ET+51	ET+38

What is Backspacing-to-Offset Converter?

A Backspacing to Offset Converter is a tool that translates between the 2 ways of describing where a wheel mounts. North American truck shops quote backspacing in inches. European and Asian wheel makers quote offset in millimeters.

The same backspacing gives a different offset on wheels of different widths. A 5.0 in backspacing equals ET+25 on a 7 in wheel and ET−13 on a 10 in wheel.

The converter removes the most common cause of wrong wheel orders: comparing a backspacing value with an offset value.

What are the uses of Backspacing-to-Offset Converter?

There are 4 main uses of the Backspacing to Offset Converter, including cross-catalog shopping and spec sheet checks.

Cross-catalog shopping
Compare a US truck wheel listing with a European listing.
Spec sheet checks
Confirm a stamped ET matches a measured backspacing.
Forum advice
Translate recommended backspacing into ET.
Width changes
Find the offset that keeps the same backspacing on a wider wheel.