Quick Tire Size Conversion Calculator — Convert Inches, mm & % ChangeA tire size conversion calculator helps drivers, mechanics, and vehicle enthusiasts compare tire dimensions across different sizing systems, check compatibility with wheels and suspension, and estimate the effect of tire changes on clearance and speedometer readings. This article explains how tire sizes are written, how to convert between common formats (inch and metric), how to calculate diameter, circumference, rim-to-road height, and percent change between two sizes — and provides worked examples and practical guidance for choosing replacement tires.
Why a tire size conversion calculator matters
Changing tire size affects ride height, speedometer accuracy, gearing, and handling. Using the wrong replacement can cause rubbing, ABS/traction-control errors, or incorrect gear ratios. A calculator takes the math out of this by converting widths, aspect ratios, and rim diameters into a common basis (usually overall diameter and circumference), and computing percent differences so you can assess whether a new size is acceptable.
Common tire size formats
Tire sizes appear in several formats. The most common:
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Metric (P/Metric): 205/55R16
- 205 = section width in millimeters
- 55 = aspect ratio (sidewall height as percentage of width)
- R16 = rim diameter in inches (radial construction)
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Imperial/inch: 26×2.1 (often used for bicycles/ATVs)
- 26 = overall tire diameter in inches (approximate)
- 2.1 = section width in inches
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Decimal/overall diameter: 27.5”, 29” — often for mountain bikes or hybrid tires.
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LT and load-range modifiers (e.g., LT245/75R16) indicate light-truck construction; these follow same geometry for size calculations but different load ratings.
Key measurements and definitions
- Section width (W): the width of the tire from sidewall to sidewall. Metric: millimeters. Imperial: inches.
- Aspect ratio (AR): sidewall height expressed as a percentage of section width. Sidewall height (H) = W × AR. For metric, H in mm; for imperial, convert inches ↔ mm.
- Rim diameter (D_rim): the diameter of the wheel bead-seat where the tire mounts, measured in inches.
- Overall/tire diameter (D_overall): D_overall = D_rim + 2 × sidewall height (converted to same units). Note rim diameter must be converted to mm when adding to mm sidewall heights.
- Circumference ©: C = π × D_overall.
- Radius ®: half of D_overall.
- Percent change: ((New − Old) / Old) × 100%. Used to evaluate acceptable differences for speedometer, ABS, and clearance.
Conversions you need
- Inches to millimeters: 1 in = 25.4 mm
- Millimeters to inches: 1 mm ≈ 0.0393701 in
- π ≈ 3.14159265
Step-by-step calculation method
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Parse sizes:
- Metric example: 205/55R16 → W = 205 mm, AR = 55%, D_rim = 16 in.
- Inch example: 26×2.1 → D_overall ≈ 26 in, W = 2.1 in (if you need section width).
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Convert rim diameter to mm for metric calculations:
- D_rim_mm = D_rim_in × 25.4
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Calculate sidewall height (H):
- H_mm = W_mm × (AR / 100)
-
Calculate overall diameter:
- D_overall_mm = D_rim_mm + 2 × H_mm
Or in inches:
- D_overall_in = D_rim_in + 2 × H_in
-
Calculate circumference:
- C_mm = π × D_overall_mm
- Convert to inches if needed: C_in = C_mm / 25.4
-
Percent change between old and new tires:
- % change diameter = (D_new − D_old) / D_old × 100%
- % change circumference similarly.
Worked examples
Example 1 — Metric to inches
Old: 205/55R16
- W = 205 mm
- AR = 55% → H = 205 × 0.55 = 112.75 mm
- D_rim = 16 in → D_rim_mm = 16 × 25.4 = 406.4 mm
- D_overall = 406.4 + 2 × 112.75 = 631.9 mm
- D_overall_in = 631.9 / 25.4 ≈ 24.88 in
- Circumference = π × 631.9 ≈ 1,984.6 mm ≈ 78.13 in
Example 2 — Comparing two sizes and percent change
Old: 205/55R16 (D_old ≈ 631.9 mm)
New: 215/50R16
- W = 215 mm; AR = 50% → H = 107.5 mm
- D_overall_new = 406.4 + 2 × 107.5 = 621.4 mm
- % change diameter = (621.4 − 631.9) / 631.9 × 100% ≈ −1.66%
- This is within typical acceptable ranges (many manufacturers recommend ±3% max).
Example 3 — Imperial to metric conversion (bicycle-style)
Tire listed 27.5 × 2.2
- D_overall_in = 27.5 in → D_overall_mm = 27.5 × 25.4 = 698.5 mm
- Width = 2.2 in → W_mm = 2.2 × 25.4 = 55.88 mm
- If you need aspect ratio: AR = H / W. For many bike tires the rim diameter is not explicit; bikes use ISO bead seat diameter—use manufacturer specs if converting precisely.
Acceptable percent change guidelines
- Speedometer/odometer: Manufacturers often accept up to ±3% change before recalibration is recommended.
- Clearance and rubbing: Keep changes small to avoid rubbing; consider suspension lift or wheel offset if diameter increases significantly.
- ABS/traction systems: Large percent changes can alter calibration—consult vehicle service manual.
Practical tips when choosing replacement tires
- Match rim bead diameter exactly (16 in rim requires 16 in tires).
- Keep percent diameter change within ±3% where possible.
- If you increase diameter, you reduce effective gearing (lower RPM at a given speed) and under-report speed; decreasing diameter does the opposite.
- Consider width changes impact on wheel clearance and fender coverage. Wider tires may rub; narrower tires can affect handling and load capacity.
- Check load index and speed rating for replacements. LT tires require matching construction and load range.
- When in doubt, use a tire conversion calculator and consult a tire professional.
Example quick calculator (formulas summary)
- H(mm) = W(mm) × (AR / 100)
- D_overall(mm) = (D_rim(in) × 25.4) + 2 × H(mm)
- D_overall(in) = D_overall(mm) / 25.4
- Circumference = π × D_overall
- % change = (New − Old) / Old × 100%
Limitations and cautions
- Some tires (especially specialty or high-profile tires) have actual measurements that differ slightly from nominal sizes due to tread depth and manufacturer tolerances.
- Bicycle and ATV tire naming can be ambiguous; use ISO/ETRTO bead-seat diameter where precise fit is required.
- Always verify load rating, speed rating, and vehicle manufacturer recommendations before changing tire sizes.
If you want, I can:
- provide a ready-to-use spreadsheet with these formulas, or
- build a short interactive calculator script (JavaScript) you can drop into a webpage.
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