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Tire size conversion guide

# Tire size conversion guide

## How do I convert a tire size between metric and other formats?

The modern metric tire size format (e.g. 225/45R18) encodes section width in mm, aspect ratio as a percentage, construction type (R for radial), and rim diameter in inches. The P-metric format used in North America adds a "P" prefix and may use slightly different load capacity calculations. The LT (Light Truck) format uses the same structure but with a "LT" suffix and higher load ranges. Flotation tires (e.g. 31×10.50R15LT) express overall diameter and section width directly in inches. Classic inch-based tires (e.g. 6.00-16) give section width and rim diameter in inches without an aspect ratio. Converting between formats requires applying the appropriate formula for each measurement.

## How-to steps

- **Identify the source format** Determine whether the tire is metric, P-metric, LT-metric, flotation, classic inch, or alpha-numeric before converting.
- **Convert geometry** Compute overall diameter, section width, sidewall height, and rim diameter in a single unit system before searching equivalents.
- **Compare against fitment limits** Use diameter change, load index, speed rating, rim-width compatibility, and clearance checks before fitting a converted size.

## Worked examples

Metric and flotation markings look different, but the table is calculated from the same nominal diameter, width and rim-size rules.

| Size | Overall diameter | Section width | Sidewall | Circumference |
| --- | --- | --- | --- | --- |
| 225/45R17 | 24.97 in / 634 mm | 8.86 in / 225 mm | 3.99 in / 101.3 mm | 78.45 in / 1993 mm |
| 285/75R16 | 32.83 in / 834 mm | 11.22 in / 285 mm | 8.42 in / 213.8 mm | 103.14 in / 2620 mm |
| 33x12.50R15 | 33.00 in / 838 mm | 12.50 in / 318 mm | 9.00 in / 228.6 mm | 103.67 in / 2633 mm |

**What to do with it:** Round marketing sizes can hide several millimetres of difference.

## Tire size format reference

| Format | Example | Market | Structure | Decoded | Notes |
| --- | --- | --- | --- | --- | --- |
| **Metric (Euro-metric)** | **225/45R18** | Europe, Asia, worldwide standard since 1980s | Section width mm / aspect ratio % + construction + rim diameter in | 225 mm wide, 45% sidewall, Radial, 18" rim | Universal format for modern passenger cars worldwide. |
| **P-metric** | **P225/60R16** | North America (USA, Canada) | "P" prefix + section width mm / aspect ratio % + construction + rim diameter in | Passenger vehicle, 225 mm wide, 60% sidewall, Radial, 16" rim | "P" = passenger vehicle. LT prefix used for light truck. Load capacity calculated differently from Euro-metric (P-metric load index assumes 4-ply rating reduction when using single load tables). |
| **LT-metric** | **LT265/70R17** | North America — pickups, SUVs, light trucks | "LT" prefix + section width mm / aspect ratio % + construction + rim diameter in | Light truck, 265 mm wide, 70% sidewall, Radial, 17" rim | LT tires have stiffer construction and are rated for higher loads. They come in Load Ranges (C/D/E) rather than load index alone. LT load capacity exceeds equivalent P-metric size. |
| **Flotation (inch-based modern)** | **31×10.50R15LT** | North America — off-road, 4WD | Overall diameter in × section width in + construction + rim diameter in + truck class | 31 inch overall diameter, 10.50 inch section width, Radial, 15" rim, Light Truck | Expresses the actual overall diameter and section width directly in inches. Common on off-road tires where ground clearance (indicated by OD) is the key parameter. |
| **Classic inch (crossply era)** | **6.00-16** | Pre-1970s worldwide; still used for vintage vehicles | Section width in - rim diameter in | 6.00 inch section width, 16 inch rim | No aspect ratio is specified — typically 80–100% implied. No "R" because crossply construction was universal. Used for classic car, agricultural, and some light commercial fitment. |
| **Alpha-numeric (North America 1960s–70s)** | **GR78-15** | USA — classic American muscle cars and family cars | Load range letter + aspect ratio + rim diameter | G load range, 78 series, 15" rim | G, H, J, etc. indicate load/width class (not an alphabetic sequence — different manufacturers used different scales). Aspect ratio is the number (78 = 78% profile). Used by American manufacturers as a transitional format. |

## Conversion formulas

| Conversion | Formula | Worked example | Notes |
| --- | --- | --- | --- |
| **Metric to overall diameter** | `OD (mm) = (section width mm × aspect ratio / 100 × 2) + (rim diameter in × 25.4)` | 225/45R18 → (225 × 0.45 × 2) + (18 × 25.4) = 202.5 + 457.2 = 659.7 mm | This gives the calculated overall diameter. Actual measured OD may differ by ±2%. |
| **Flotation to metric (approximate)** | `Section width mm ≈ section width in × 25.4 \| Aspect ratio ≈ ((OD in − rim in) / 2 / (section in)) × 100` | 31×10.50R15 → section ≈ 267 mm, AR ≈ ((31-15)/2/10.5)×100 ≈ 76 → approx. 265/75R15 | Flotation-to-metric is approximate. The flotation format expresses actual OD; metric expresses nominal section width. |
| **Metric to flotation (approximate)** | `OD in = OD mm / 25.4 \| Section in = section width mm / 25.4` | 265/75R16 → OD ≈ 31.6 in, section ≈ 10.43 in → approx. 32×10.50R16 or 31×10.50R16 | Round OD to nearest common flotation size. Section width is usually rounded to nearest 0.50 in. |
| **Classic inch to metric (approximate)** | `Section width mm ≈ section width in × 25.4 \| Aspect ratio assumed 80–90%` | 6.00-16 → section ≈ 152 mm, assume AR 90 → approx. 155/90R16 or 155/85R16 | Classic inch tires do not have a defined aspect ratio. Modern equivalent sizes may not exist in identical overall diameter. Consult a tire fitment specialist for classic vehicle applications. |
| **P-metric to Euro-metric load capacity** | `P-metric capacity = Euro-metric capacity × 0.9 (for single fitment)` | P225/60R16 93H → load index 93 = 650 kg (Euro-metric) → 650 × 0.9 = 585 kg (P-metric single fitment) | The 10% reduction applies when using a P-metric tire on a metric-load-standard vehicle. TIA/RMA standard. Most modern vehicles are dimensionally identical — the difference matters primarily for load-critical applications. |

## P-metric vs Euro-metric: what actually differs

P-metric and Euro-metric tires with the same size designation (e.g. P225/60R16 vs 225/60R16) are physically identical. The same tire can carry both designations. The difference is in how load capacity is calculated at the point of use:

- **Euro-metric (single fitment):** load capacity = full load index value. A 93H tire carries 650 kg per corner.
- **P-metric (on a US-specification vehicle):** the TIA/RMA standard reduces the load capacity by 10% when used in single fitment applications on vehicles where the load table uses the Euro-metric standard. A P225/60R16 93H would be rated at 585 kg in single fitment per TIA.
- **In practice:** for the vast majority of passenger car applications, the 10% difference is irrelevant — vehicles are not typically loaded to the maximum axle rating per tire. The difference becomes relevant for heavy trucks, trailers, and high-load-rating applications.

Our [Load index chart](https://tirefitlab.com/learn/load-index/) and [Tire load capacity guide](https://tirefitlab.com/learn/tyre-load-capacity/) guides cover load index ratings and capacity calculations in full.

## Common metric ↔ flotation equivalents

| Inch / flotation size | Metric equivalent (approx.) | Overall diameter | Notes |
| --- | --- | --- | --- |
| **145R13** | 145/80R13 | 547 mm | Common on small European cars (Mini, Fiat 126). The "R" without explicit aspect ratio implies ~80–82%. |
| **155R13** | 155/80R13 | 566 mm | Older Renault, Citroën, Peugeot compact cars. |
| **175R14** | 175/80R14 | 616 mm | Light commercial and some older estate cars. |
| **P195/75R14** | 195/75R14 | 648 mm | P-metric and Euro-metric are dimensionally the same. Load capacity differs slightly. |
| **P235/75R15** | 235/75R15 | 729 mm | Classic North American half-ton pickup stock tire. |
| **31×10.50R15LT** | 265/75R15 | 777 mm (metric) / 31 in = 787 mm (flotation) | Close but not identical OD. Flotation 31" is slightly larger than metric 265/75. |
| **33×12.50R17LT** | 305/65R17 | 818 mm (metric) / 33 in = 838 mm (flotation) | Popular off-road upgrade size. Metric equivalent is approximate. |
| **35×12.50R17LT** | 315/70R17 | 855 mm (metric) / 35 in = 889 mm (flotation) | Common overland/expedition fitment. The metric equivalent OD is slightly smaller. |

## Classic inch ↔ modern metric equivalents

Classic crossply inch sizes do not have a direct modern equivalent because no aspect ratio was specified in the original format. The equivalents below are commonly used for classic vehicle restoration — they match section width approximately and use modern radial construction. Always verify the overall diameter matches the original fitment, and check that the tire fits within the vehicle's wheel arch without fouling.

| Classic size | Modern radial equivalent (approx.) | Overall diameter | Typical vehicles |
| --- | --- | --- | --- |
| **5.20-13** | 165/80R13 | 555 mm | Mini Cooper, early Fiat 500 |
| **5.60-13** | 175/80R13 | 571 mm | VW Beetle, Fiat 128 |
| **5.60-15** | 175/80R15 | 622 mm | Classic American compacts |
| **6.00-13** | 185/80R13 | 586 mm | Hillman Imp, Simca |
| **6.00-15** | 185/80R15 | 638 mm | Morris Minor, early Ford Popular |
| **6.40-15** | 205/75R15 | 669 mm | MGB, Triumph TR, early Jaguar E-type |
| **6.70-15** | 215/75R15 | 686 mm | Classic American muscle car baseline |
| **7.10-15** | 235/75R15 | 729 mm | Large American cars, 1950s–60s |

See the [Crossply vs radial tires explained](https://tirefitlab.com/learn/crossply-vs-radial/) guide for a full explanation of classic vs radial construction and the handling implications of mixing types on older vehicles.

## LT tires: Load Range vs Load Index

LT (Light Truck) tires use a dual load designation: they carry a standard load index number AND a Load Range letter that indicates ply rating equivalent:

- **Load Range C** — 6-ply rating equivalent. Maximum 50 psi (3.4 bar). Common on smaller LT tires and cargo van fitments.
- **Load Range D** — 8-ply rating equivalent. Maximum 65 psi (4.5 bar). Common on 3/4-ton pickup trucks.
- **Load Range E** — 10-ply rating equivalent. Maximum 80 psi (5.5 bar). Standard for 3/4-ton and 1-ton heavy-duty trucks and towing applications.
- **Load Range F** — 12-ply rating equivalent. Maximum 95 psi (6.5 bar). Used on heavy commercial and dual-rear-wheel fitments.

When using our [Tire size calculator](https://tirefitlab.com/tire-size-calculator/), enter the metric portion of any LT or P-metric size in standard format (e.g. for LT265/70R17, enter 265/70R17). The calculator computes geometry from the metric dimensions regardless of the prefix.

## More tools

- [Tire size calculator](https://tirefitlab.com/tire-size-calculator/)
- [How to read a tire size](https://tirefitlab.com/learn/how-to-read-tire-size/)
- [Tire sidewall markings decoder](https://tirefitlab.com/learn/tyre-sidewall-markings/)
- [Load index chart](https://tirefitlab.com/learn/load-index/)
- [Crossply vs radial tires explained](https://tirefitlab.com/learn/crossply-vs-radial/)
- [Tire upsizing and downsizing guide](https://tirefitlab.com/learn/tyre-upsizing-downsizing/)
- [Plus sizing guide](https://tirefitlab.com/learn/plus-sizing/)
- [Tire & wheel reference guides](https://tirefitlab.com/learn/)

## Detailed reference pages

Choose a common value or size to open its calculated data and explanation.

- [245/70 R16](https://tirefitlab.com/learn/equivalent/245-70-r16/)
- [265/70 R16](https://tirefitlab.com/learn/equivalent/265-70-r16/)
- [235/70 R17](https://tirefitlab.com/learn/equivalent/235-70-r17/)
- [265/70 R17](https://tirefitlab.com/learn/equivalent/265-70-r17/)
- [275/70 R18](https://tirefitlab.com/learn/equivalent/275-70-r18/)
- [285/65 R18](https://tirefitlab.com/learn/equivalent/285-65-r18/)
- [285/60 R18](https://tirefitlab.com/learn/equivalent/285-60-r18/)
- [305/40 R22](https://tirefitlab.com/learn/equivalent/305-40-r22/)
- [31x10.50 R15](https://tirefitlab.com/learn/equivalent/31x10.50-r15/)
- [33x12.50 R15](https://tirefitlab.com/learn/equivalent/33x12.50-r15/)
- [31x10.50 R16](https://tirefitlab.com/learn/equivalent/31x10.50-r16/)
- [33x12.50 R16](https://tirefitlab.com/learn/equivalent/33x12.50-r16/)
- [35x12.50 R17](https://tirefitlab.com/learn/equivalent/35x12.50-r17/)
- [33x12.50 R18](https://tirefitlab.com/learn/equivalent/33x12.50-r18/)
- [35x12.50 R18](https://tirefitlab.com/learn/equivalent/35x12.50-r18/)
- [35x12.50 R20](https://tirefitlab.com/learn/equivalent/35x12.50-r20/)
- [37x13.50 R20](https://tirefitlab.com/learn/equivalent/37x13.50-r20/)

[Last reviewed: 2026-06-22](https://tirefitlab.com/methodology/)

Seasonal check

## Planning a long summer drive?

Use the budget and running-cost tools before a trip, especially if the current tires are worn or the replacement size changes diameter.

[Estimate tire budget](https://tirefitlab.com/tire-budget-calculator/)

## More in this topic: Size validation

Decode markings, compare sizes, and understand diameter, load and speed changes.

- [**How to read a tire size** How to read a tire size: decode every number and letter in P205/55R16 91V — width, aspect ratio, rim diameter, load index, speed rating.](https://tirefitlab.com/learn/how-to-read-tire-size/)
- [**Tire type guide — P, LT, metric, radial & flotation** What do the letters in a tire size mean? P, LT, metric, radial (R) and flotation tire types explained.](https://tirefitlab.com/learn/tire-types/)
- [**Tire sidewall markings decoder** Complete guide to every marking on a tire sidewall: size string, construction code, load index, speed rating, DOT date, M+S, 3PMSF snowflake, UTQG, max pressure, country of manufacture, and brand-specific symbols.](https://tirefitlab.com/learn/tyre-sidewall-markings/)
- [**Tire load index chart** Use the tire load index chart to convert ratings 60–130 into kg and lb, then verify the vehicle placard and axle-load requirements.](https://tirefitlab.com/learn/load-index/)

### Useful tools

[Tire size converter (mm ↔ inches)](https://tirefitlab.com/tire-size-converter/)[Tire size calculator](https://tirefitlab.com/tire-size-calculator/)