Quick Answer
To convert a metric tire size to inches: divide the width (mm) by 25.4 for section width in inches; multiply width × aspect ratio ÷ 100 ÷ 25.4 for sidewall height; then add rim diameter + 2 × sidewall height for overall diameter. Example: P225/65R17 → width 8.86", sidewall 5.76", OD 28.52". To convert inches to metric: multiply tread width in inches × 25.4 and round to the nearest 5 mm for section width; divide sidewall height by section width × 100 and round to the nearest 5 for aspect ratio.
| Metric Size | Outer Dia. | Sec. Width | Sidewall | Circumf. | Rev/Mi | Flotation Equiv. | Common Use |
|---|---|---|---|---|---|---|---|
| P205/55R16 | 24.9" | 8.07" | 4.45" | 78.2" | 810 | 24.9x8.07R16 | Economy sedan OE |
| P215/65R16 | 27.0" | 8.46" | 5.49" | 84.8" | 747 | 27.0x8.46R16 | Crossover / CUV |
| P225/65R17 | 28.5" | 8.86" | 5.76" | 89.5" | 708 | 28.5x8.86R17 | Mid-size SUV OE |
| P235/75R15 | 29.0" | 9.25" | 6.91" | 91.1" | 696 | 29.0x9.25R15 | Classic truck / Jeep |
| LT245/75R16 | 30.5" | 9.65" | 7.24" | 95.8" | 661 | 30.5x9.65R16 | Light truck all-terrain |
| P265/70R17 | 31.6" | 10.43" | 7.28" | 99.2" | 639 | 31.6x10.43R17 | Full-size truck / F-150 OE |
| LT265/70R17 | 31.6" | 10.43" | 7.28" | 99.2" | 639 | 31.6x10.43R17 | Truck AT4 / Raptor OE (E) |
| 275/55R20 | 31.9" | 10.83" | 5.97" | 100.2" | 633 | 31.9x10.83R20 | Full-size truck on 20s |
| LT285/75R16 | 32.8" | 11.22" | 8.39" | 103.0" | 615 | 32.8x11.22R16 | Off-road truck / Tacoma |
| LT315/70R17 | 34.4" | 12.40" | 8.70" | 108.1" | 586 | 34.4x12.40R17 | 35-equivalent lifted truck |
| LT325/65R18 | 34.6" | 12.80" | 8.28" | 108.7" | 583 | 34.6x12.80R18 | Lifted HD / Ram TRX |
| LT305/70R18 | 35.4" | 12.01" | 8.39" | 111.2" | 570 | 35.4x12.01R18 | Sierra 2500 AT4X OE |
All values calculated from ISO metric formula. Use the calculator below to convert any tire size instantly. Circumference = π × OD. Revs/mile = 63,360 ÷ circumference.
Metric Tire Size → Inches ConverterAll Prefixes Supported
Enter any metric tire size — P-metric, LT, Euro-metric, ST, or no prefix. Get outer diameter, section width, sidewall height, circumference, revs/mile, and the flotation inch equivalent instantly.
Quick Load — Common Sizes
Supports: P, LT, ST, C prefixes — XL, LL suffixes
Conversion Formulas
Given: 225/65R17 Section Width (in) = 225 ÷ 25.4 = 8.86" Sidewall Height = 225 × 0.65 ÷ 25.4 = 5.76" Outer Diameter = 17 + (2 × 5.76) = 28.52" Circumference = π × 28.52 = 89.62" Revs/Mile = 63,360 ÷ 89.62 = 707.0 Flotation Equiv. = 28.5x8.86R17
Metric Tire Size Dimension Reference Table
Pre-calculated dimensions for the most popular passenger, crossover, and truck metric tire sizes. Use this table for quick reference without entering the calculator. All values derived from the ISO metric formula: OD = rim + 2 × (width × AR ÷ 100 ÷ 25.4).
| Tire Size | Outer Dia. | Sec. Width | Sidewall | Circumf. | Rev/Mi | Common Use |
|---|---|---|---|---|---|---|
| 185/65R15 | 24.5" | 7.28" | 4.72" | 76.9" | 824 | Economy hatchback |
| 195/65R15 | 25.0" | 7.68" | 4.99" | 78.5" | 807 | Compact sedan (Golf, Civic) |
| 205/55R16 | 24.9" | 8.07" | 4.45" | 78.2" | 810 | Performance sedan |
| 225/45R17 | 25.0" | 8.86" | 3.98" | 78.5" | 807 | Sport car / +2 upgrade |
| 225/65R17 | 28.5" | 8.86" | 5.76" | 89.5" | 708 | Mid-size SUV crossover |
| 235/65R17 | 29.0" | 9.25" | 5.99" | 91.1" | 696 | Full-size crossover |
| 245/70R17 | 30.5" | 9.65" | 6.72" | 95.8" | 661 | Midsize truck / Tacoma |
| 265/70R17 | 31.6" | 10.43" | 7.28" | 99.2" | 639 | Full-size truck OE |
| 265/75R16 | 30.6" | 10.43" | 7.78" | 96.1" | 660 | Truck all-terrain 16" rim |
| 275/65R18 | 32.1" | 10.83" | 7.01" | 100.8" | 629 | Full-size truck SLT/AT4 |
| 285/70R17 | 33.7" | 11.22" | 7.78" | 105.8" | 599 | Canyon AT4X / Trail Boss |
| 305/70R18 | 35.4" | 12.01" | 8.39" | 111.1" | 571 | Sierra 2500 AT4X / Ram 2500 |
| 315/70R17 | 34.4" | 12.40" | 8.70" | 108.1" | 586 | 35" equivalent lifted truck |
| 325/65R18 | 34.6" | 12.80" | 8.28" | 108.7" | 583 | Lifted trucks on 18" wheels |
All values computed from the ISO metric tire size formula. Actual manufactured dimensions may vary ±2–3% by brand and model. Source: TRA (Tire and Rim Association) dimensional standards and ISO 4000-1.
Metric Tire Conversion Formulas
Metric → Inches
Core FormulaMetric → Inches Formula: Given P225/65R17 Section Width (in) = 225 ÷ 25.4 = 8.86" Sidewall Height = (225 × 0.65) ÷ 25.4 = 5.76" Outer Diameter = 17 + (2 × 5.76) = 28.52" Circumference = π × 28.52 = 89.62" Revolutions/Mile = 63,360 ÷ 89.62 = 707.0 rev/mi Flotation Equiv. = 28.5x8.86R17 Key constants: 25.4 mm = 1 inch 63,360 inches = 1 mile π = 3.14159
Inches → Metric
Reverse ConversionInches → Metric Formula: Given: 33x12.50R17 (Flotation / Inch format) Section Width (mm) = 12.50 × 25.4 = 317.5 → round to 315 mm Sidewall Height = (33 - 17) ÷ 2 = 8.0" = 203.2 mm Aspect Ratio = 203.2 ÷ 317.5 × 100 = 64.0 → round to 65 Metric Result ≈ 315/65R17 Note: Metric sizes use 5 mm width steps and 5-point AR steps. The closest standard metric may differ by 0.1–0.5" OD from the flotation original. Always verify final size against rim diameter and vehicle clearance.
Speedometer Correction
After Size ChangeSpeedometer Error After Tire Size Change: True Speed = Indicated Speed × (New OD ÷ Stock OD) Example: Stock P225/65R17 (28.52") → New 245/65R17 (30.57") At 60 mph indicated: True Speed = 60 × (30.57 ÷ 28.52) = 60 × 1.072 = 64.3 mph ⚠ You are traveling 4.3 mph FASTER than the dash shows. Calibration options: • Scan tool reprogramming (dealer or aftermarket OBDII) • HP Tuners / EFI Live for GM vehicles • DiabloSport inTune or Predator • Speedometer correction plug-in module (SpeedoHealer, Hypertech) Safe zone: Keep OD change within ±3% of stock to remain within most state speedometer tolerance laws (typically ±4 mph at 70 mph).
How to Use the Metric Tire Calculator
- 1
Choose the right conversion tab
Select 'Metric → Inches' to convert a standard metric tire size (P225/65R17, LT265/70R17, 205/55R16) into outer diameter, section width, sidewall height, circumference, revs per mile, and flotation inch equivalent. Select 'Inches → Metric' to do the reverse — enter a flotation string like 35x12.50R17 or enter diameter, width, and rim manually.
- 2
Enter your metric tire size or use a quick-load preset
In the Metric → Inches tab, type any valid metric tire size into the input field. Supported prefixes: P (passenger), LT (light truck), ST (trailer), C (commercial), or no prefix (Euro-metric). Supported suffixes: XL (extra load / reinforced), LL (light load). Press Enter or click Convert to Inches. Use the quick-load preset buttons for common sizes like P225/65R17 or LT265/70R17.
- 3
Convert inches or flotation sizes to metric
In the Inches → Metric tab, use Method 1 to enter a flotation size string (e.g. 33x12.50R17) and get the closest metric equivalent instantly. Use Method 2 to manually enter the overall diameter, tread width, and rim diameter as separate fields — useful when you have physical measurements but no size string.
- 4
Build any P-metric or LT size using the P-Metric Builder
Go to the P-Metric Builder tab and use the dropdown selectors to choose prefix, section width, aspect ratio, rim diameter, and suffix. The size string previews live. Click Calculate to see all dimensions. Use the Prefix Reference table below the builder to understand the differences between P, LT, Euro-metric, ST, and C type tires — including the P-metric 91% load reduction rule for light trucks.
- 5
Look up load index and verify your vehicle's GVWR
In the Load Index tab, enter the two or three digit load index number from your tire's sidewall (e.g. 97 from '225/65R17 97H'). Optionally enter your vehicle's GVWR in pounds. The calculator shows per-tire capacity, total 4-tire capacity, and a pass/fail GVWR adequacy check. It also shows the P-metric 91% reduction if your tires are P-metric on a truck or van.
- 6
Convert legacy alpha-numeric sizes using the Alpha-Numeric tab
If you have a vintage 1968–1982 American vehicle with an alpha-numeric tire size like H78-15 or L78-15, use the Alpha-Numeric tab to find the modern metric equivalent. Search by alpha-numeric code, metric equivalent, or vehicle type keyword. The in-page guide explains how to decode the letter-number system, including speed-rated variants like HR78-15.
P-Metric vs. LT vs. Euro-Metric — What the Prefix Really Means
The prefix (or absence of one) before a metric tire size is often misunderstood — it affects not just the construction standard but actual usable load capacity on your specific vehicle.
P-metric (prefix P) is the TRA North American standard for passenger vehicles. A P225/65R17 and a 225/65R17 (Euro-metric) are nominally the same size, but the Euro-metric carries slightly more load. Critically, P-metric tires used on light trucks, full-size SUVs, and vans have their load capacity reduced to 91% of the sidewall load index rating. This is not a tire defect — it is the official TRA standard, and it matters if your truck operates near its GVWR.
LT (Light Truck) tires are built to handle heavier loads and higher inflation pressures. Load Range E LT tires run at 80 PSI cold and carry significantly more weight per tire than a P-metric of similar size. If you tow a trailer, carry heavy payloads, or have a 3/4-ton or 1-ton truck, LT tires are the appropriate choice. LT tires also resist sidewall damage from off-road use better than P-metric tires due to thicker, multi-ply construction.
Euro-metric (no prefix) tires, common on European imports and many modern crossovers, follow ISO/ETRTO standards. They do not require the 91% reduction on trucks. If your door placard lists a P-metric size, a Euro-metric of the same nominal size is an acceptable replacement. If your placard lists LT, replacing with P-metric reduces your usable load capacity.
Flotation Tire Sizes vs. Metric — Why They Don't Match Perfectly
Flotation tire sizes (e.g. 35x12.50R17) directly state overall diameter as the first number — a 35-inch tire is literally 35 inches tall. Metric sizes (e.g. 315/70R17) encode width and aspect ratio, and you must calculate OD: 17 + 2×(315×0.70÷25.4) = 34.4 inches. So a 315/70R17 is not a 35-inch tire — it is 34.4 inches.
The mismatch exists because metric sizing uses standardized 5 mm width intervals and 5-point aspect ratio steps. There is no standard metric tire that produces exactly 35.0 inches OD on a 17-inch rim. The closest options are 315/70R17 at 34.4 inches (0.6” short) and 325/70R17 at 35.2 inches (0.2” over). Aftermarket tire manufacturers producing “35x12.50R17” flotation tires typically produce them at 34.6–34.8 inches actual OD — splitting the difference between the two metric standards.
For street-legal and ADAS-sensitive vehicles, use the metric equivalent from the Flotation Table tab and verify the actual OD falls within ±3% of your stock tire for accurate speedometer and ABS calibration.