Tire Stretch Calculator
Enter your tire size and rim width to get the exact stretch stage, adjusted sidewall height, and a color-coded safety rating — instant, no signup.
Quick Answer
A 225/40R18 on a 9.5" rim has a measuring rim of 6.2", a recommended range of 5.2"–7.2", and sits 2.3" over the maximum — that is Extreme stretch, with the sidewall height reduced from 90 mm to roughly 87 mm and a real risk of bead separation under cornering load.
Common tire sizes:
Common rim widths:
Tire Stretch Stages by Rim Width Overage
| Stage | Overage | Safety | Notes |
|---|---|---|---|
| No Stretch | 0" (at or below max) | Safe | Bead fully seated. No handling changes. |
| Stage 1 — Mild | 0" – 0.5" | OK | Common in stance builds. Bead stays seated well. |
| Stage 2 — Moderate | 0.5" – 1.0" | Caution | Cornering grip affected. Not for track use. |
| Stage 3 — Aggressive | 1.0" – 1.5" | Warning | Blowout risk in hard cornering. Warranty voided. |
| Extreme | > 1.5" | Danger | Bead separation risk. Not safe for public roads. |
| Rim Too Narrow | Below min | Warning | Tire may not seat correctly. Use a wider rim. |
Formula
Measuring Rim (in) = (Section Width mm ÷ 25.4) × 0.70
Recommended Min = Measuring Rim − 1.0"
Recommended Max = Measuring Rim + 1.0"
Stretch = max(0, Rim Width − Recommended Max)
- Measuring rim is defined by TRA/ETRTO standards as ~70% of the tire's section width
- Recommended range is ±1 inch from the measuring rim — the industry standard tolerance
- Example: 225 mm ÷ 25.4 × 0.70 = 6.2" measuring rim; recommended range 5.2"–7.2"; a 9.0" rim stretches 1.8" (Extreme)
- Adjusted sidewall ≈ nominal sidewall − (stretch mm × 0.10) — each mm of stretch removes ~10% of itself from the sidewall height
How to Use This Calculator
- 1
Enter your tire size
Type your tire size in the standard metric format — for example, 225/40R18. The three numbers are section width in millimeters, aspect ratio as a percentage, and rim diameter in inches.
- 2
Enter your rim width in inches
Type the width of your rim in inches. Rim widths are typically measured from bead seat to bead seat. Common widths range from 7.0" to 10.5". Check your wheel's spec sheet or the back of the wheel if unsure.
- 3
Calculate and read the stretch stage
Click Calculate Stretch. The result shows your stretch stage (No Stretch, Stage 1–3, or Extreme), the amount of stretch in both inches and millimeters, and the adjusted sidewall height.
- 4
Review the safety note
Read the color-coded safety classification. Green means no concern; amber means moderate caution; orange or red means reduced safety margin. Factor this in before deciding on your fitment.