TireSizeCalc Tip

Tire size calculator

Type your current tire size and the size you are thinking of fitting. TireSizeCalc decodes both, works out the overall diameter, sidewall height, width, circumference and revolutions per mile, and shows the difference. It also tells you how far off the speedometer will read and lists other sizes within 3% of the original diameter. The example compares 225/45R17 with 245/40R18, which is 2.98% taller.

Metric (225/45R17) or flotation (33x12.50R15) sizes

    Similar sizes

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    Metric sizes (225/45R17, P215/65R15, LT265/70R17) and flotation sizes for trucks (33x12.50R15) are both understood.

    How the tire size calculator works

    For a metric size W/A R D, the sidewall height is W × A ÷ 100 millimetres and the overall diameter is D × 25.4 + 2 × sidewall. Circumference = π × diameter, revolutions per mile = 63,360 ÷ circumference in inches, and revolutions per km = 1,000,000 ÷ circumference in mm. With a new tire fitted, actual speed = speedometer reading × new diameter ÷ old diameter.

    Common tire sizes in inches

    SizeDiameterWidthSidewallRevs per mile
    195/65R1524.98″7.68″4.99″807
    205/55R1624.88″8.07″4.44″811
    215/55R1726.31″8.46″4.66″767
    225/45R1724.97″8.86″3.99″808
    225/65R1728.52″8.86″5.76″707
    235/55R1828.18″9.25″5.09″716
    245/40R1825.72″9.65″3.86″784
    265/70R1731.61″10.43″7.3″638
    275/55R2031.91″10.83″5.95″632
    285/75R1632.83″11.22″8.42″614

    TireSizeCalc calculates from the nominal size code (ISO / ETRTO metric and Tire and Rim Association P-metric and LT conventions). Actual dimensions differ by brand, tread depth, wheel width and load; manufacturers' spec sheets list the measured diameter and revolutions per mile. The ±3% guideline is a common rule of thumb, not a legal limit: check your vehicle's placard, load index, speed rating and clearance before changing size.

    Frequently asked questions

    How much bigger can I go on tire size?

    A common guideline is to stay within about 3% of the original overall diameter, and to check clearance, load index and the wheel width the tire is approved for.

    How is tire diameter calculated?

    Diameter = wheel diameter + 2 × sidewall height, where sidewall = width × aspect ratio ÷ 100. For 225/45R17: 17″ + 2 × 101.25 mm = 24.97″.

    Are these exact?

    They are calculated from the nominal size. Real tires vary by brand and model and are slightly shorter under load; the maker’s spec sheet lists the measured diameter.

    Is TireSizeCalc free?

    Yes, with no sign-up or ads.

    Are my sizes saved?

    Yes, in this browser only.

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