Rim Diameter Difference Calculator
See how the rim size changes between two sizes. Use the calculator below. Results update as you type.
Overall Diameter = (Rim Diameter × 25.4) + 2 × (Width × Aspect Ratio / 100)How the Rim Diameter Difference Calculator Works
This calculator works by fixing the overall tire diameter and showing how much sidewall each rim size leaves. It answers how a bigger or smaller wheel changes the tire.
- Enter the target overall diameter, for example 26.0 in (660 mm).
- Enter the current rim, for example 16 in.
- Read the sidewall each rim size needs: 5.00 in on 16 in, 4.00 in on 18 in.
- Read the aspect ratio that sidewall needs at your width.
The Formula
The formula is Sidewall Needed = (Overall Diameter − Rim Diameter) ÷ 2.
Sidewall (in) = (D_target − Rim) ÷ 2 Aspect Ratio = Sidewall (mm) ÷ Width (mm) × 100 Overall Diameter = (Rim Diameter × 25.4) + 2 × (Width × Aspect Ratio ÷ 100)Worked example: on a 26.0 in target, a 19 in rim leaves (26 − 19) ÷ 2 = 3.50 in (88.9 mm) of sidewall. On a 225 mm width, that is a 39.5% aspect ratio, so a 225/40R19 fits.
Conversion Table
The conversion table shows the sidewall needed on 8 rim sizes (15 in to 22 in) to hold a 26.0 in overall diameter. Each 1 in rim increase removes 0.50 in (12.7 mm) of sidewall.
The last column gives the aspect ratio at a 225 mm width. A 22 in rim needs a 22.6% ratio, below any tire sold in that width.
| Rim (in) | Target Dia (in) | Sidewall (in) | Sidewall (mm) | Rim Change vs 16 in | Aspect at 225 mm |
|---|---|---|---|---|---|
| 15 | 26.0 | 5.50 | 139.7 | -1 | 62.1% |
| 16 | 26.0 | 5.00 | 127.0 | 0 | 56.4% |
| 17 | 26.0 | 4.50 | 114.3 | +1 | 50.8% |
| 18 | 26.0 | 4.00 | 101.6 | +2 | 45.2% |
| 19 | 26.0 | 3.50 | 88.9 | +3 | 39.5% |
| 20 | 26.0 | 3.00 | 76.2 | +4 | 33.9% |
| 21 | 26.0 | 2.50 | 63.5 | +5 | 28.2% |
| 22 | 26.0 | 2.00 | 50.8 | +6 | 22.6% |
What Is the Rim Diameter Difference Calculator?
The Rim Diameter Difference Calculator is a tool that shows how changing wheel diameter changes the tire sidewall needed to keep the same overall height. It is the planning tool behind every plus-size and minus-size swap.
The rule is fixed geometry: every inch of extra rim takes half an inch of sidewall from the top and half an inch from the bottom.
Common Uses
This calculator has 4 uses: plus-size planning, winter minus-sizing, ride-quality checks and wheel shopping.
- Plan plus sizes: See the sidewall left on a larger wheel.
- Plan winter minus sizes: Regain sidewall with a smaller wheel.
- Check ride quality: Avoid sidewalls under 3 in (76 mm) on rough roads.
- Shop for wheels: Know the tire profile each wheel size needs.