Tire Size Calculator

Decode any tyre size and compare two sizes side by side — overall diameter, section width, sidewall, circumference and revolutions per mile — with the real inch-and-percent difference, the speedometer error and a ±3% fit check. Works across metric and flotation sizing.

Most tire-size tools either just spit out one diameter or hide the answer to the question you actually have: will it fit, and how far off will my speedo be? This one decodes the size code in plain language, compares two sizes at once — including a metric size against a flotation size — and surfaces the change in inches and percent, the ground-clearance change, the revs-per-mile change and the speedometer difference, with a ±3% “close to stock” indicator front and centre. Every figure is computed from standard tire-size geometry; nothing is looked up and nothing leaves your device.

All figures are computed from standard tire-size geometry — the published nominal dimensions. A real tire’s measured diameter varies a little with brand, tread, load and inflation, so treat these as the design values. Whether a size physically fits also depends on wheel offset, suspension travel and your vehicle’s bodywork, which a size calculator cannot see.

The rest of the sidewall This tool covers tire-size geometry and comparison. For the markings that set how much weight and speed a tire is rated for, see the tire load index & speed rating chart and the tire load range & ply rating chart. To see how a size change affects gearing and engine RPM — where overall diameter is the input — use the gear ratio calculator.

How to read a tire size

A tire size is a short code, and once you know which number is which it stops being mysterious. There are two common styles:

  • P-metric (e.g. 285/75R16) — the 285 is the section width in millimetres, the 75 is the aspect ratio (sidewall height as a percentage of the width), the R is radial construction, and the 16 is the rim diameter in inches. So the sidewall is 285 × 0.75 = 214 mm (8.42 in), and the overall diameter is 16 + 2 × 8.42 = about 32.83 in.
  • Flotation / inch (e.g. 33×12.50R15) — the numbers are already in inches: 33 is the overall diameter, 12.50 is the section width, and 15 is the rim. No maths needed; it states the height up front.

The decoder above accepts either style and breaks it into all of its dimensions in inches and millimetres, plus circumference and revolutions per mile, so you do not have to reach for a calculator yourself.

The tire-size geometry formulas

Everything here comes from these standard relationships — no copied tables, just the public geometry:

sidewall (mm) = width × (aspect / 100)
sidewall (in) = sidewall (mm) / 25.4
overall diameter (in) = rim + 2 × sidewall (in)
section width (in) = width (mm) / 25.4
circumference (in) = π × diameter (in)
revs per mile = 63360 / circumference (in) ≈ 20168 / diameter (in)
revs per km = 1000 / (circumference (in) × 0.0254)

For a flotation size the diameter, width and rim are already in inches, so the section-width and sidewall steps are skipped and the same circumference and revs formulas apply. Spot-check: 285/75R16 works out to a 8.42 in sidewall, 32.83 in overall diameter, 103.1 in circumference and about 615 revolutions per mile.

Compare two tire sizes side by side

This is the mode most people come for. Put your current size against the one you are weighing up and you get the change in overall diameter (in inches and as a percent), the change in section width and sidewall, the ground-clearance change, the revs-per-mile change and the speedometer difference — with a colour-coded delta and a ±3% fit badge. You can even cross-compare a metric size against a flotation size, because both are reduced to the same inch geometry first.

Δ diameter % = (new − old) / old × 100
ground-clearance change = Δ diameter / 2
(the axle rises by half the diameter change)

Worked example: going from 265/70R17 (31.61 in) to 285/75R17 (33.83 in) is +2.22 in, about +7%. That lifts the axle roughly 1.1 in and is outside the ±3% window, so expect noticeable speedometer error and a fitment check before buying.

Speedometer error from a tire-size change

Your speedometer is calibrated to the original tire’s circumference: it counts wheel revolutions and assumes the stock rolling distance per turn. Fit a bigger tire and each revolution covers more ground, so the speedometer under-counts — a bigger tire makes the speedo read low, meaning you are actually going faster than it shows. A smaller tire reads high. The odometer follows the same logic, accumulating distance more slowly on a bigger tire.

actual speed = indicated × (new diameter / old diameter)
error % = (new diameter − old diameter) / old diameter × 100

For the 31.61 in → 33.83 in example, at an indicated 60 you are actually doing about 64. Many regions allow up to roughly ±5% speedometer error, and the ±3% diameter window keeps you comfortably inside that.

Convert metric to inches (and back)

The convert mode turns a P-metric size into its flotation (inch) equivalent — useful when a forum post or an off-road build is quoted in inches — or takes a flotation size and finds the nearest standard metric size on the same rim. Because a “33-inch tire” is really just any tire about 33 in tall, several metric sizes can stand in for it depending on the rim, and the tool shows the computed diameter so you can see how close each match really is.

Plus sizing without breaking the speedometer

Plus sizing means going to a larger rim while keeping the overall tire diameter roughly the same, so steering feel sharpens without throwing off the speedometer or gearing. The trick is to drop the aspect ratio as the rim grows (and often widen the section a little). The plus-size helper above takes your starting size, a target rim and a target width, and suggests the aspect ratio that keeps the overall diameter closest to the original — all from standard tire-size geometry, so there is no copied fitment chart involved.

Frequently asked questions

How do I read a tire size like 285/75R16?

Read it as three parts. The 285 is the section width in millimetres, the 75 is the aspect ratio (the sidewall is 75% of the width, so about 214 mm / 8.42 in), the R is radial, and the 16 is the rim in inches. Add two sidewalls to the rim for the overall diameter: 16 + 2 × 8.42 ≈ 32.83 in. The decoder above does this and shows the width, sidewall, diameter, circumference and revolutions per mile in inches and mm.

How do I compare two tire sizes to see if a bigger tire will fit?

Put your current size on one side and the candidate on the other; the compare mode shows the change in overall diameter (inches and percent), the change in width and sidewall, the ground-clearance change and the revs-per-mile change. The common guide is to stay within about ±3% of the original diameter — roughly an inch on a typical tire — to avoid rubbing and big speedometer error, and the tool flags whether you are inside that window. Real clearance also depends on wheel offset and bodywork, which a size calculator cannot see.

How does tire size change my speedometer reading?

The speedometer is calibrated to the original tire’s circumference, so it counts revolutions and assumes the stock rolling distance. A bigger tire travels further per revolution, so the speedo reads low — you are going faster than it shows; a smaller tire reads high. Actual speed = indicated × (new diameter / old diameter). Going from a 31.6-in to a 33.8-in tire is about +7%, so an indicated 60 is really about 64. Compare mode shows this directly.

What is the difference between a metric size and a flotation size like 33x12.50R15?

They describe the same tire in two languages. A P-metric size like 285/75R16 gives width in millimetres and sidewall as a percentage, so the height needs a little maths. A flotation size like 33×12.50R15 states it in inches: about 33 in tall, 12.5 in wide, on a 15-in rim. The compare mode accepts one of each at once, so you can put a metric size against a flotation size and read both in the same inch-and-mm geometry.

What does revolutions per mile mean and why does it matter?

It is how many full turns a tire makes per mile: 63,360 inches in a mile divided by the tire’s circumference in inches. A bigger tire has a longer circumference, so it turns fewer times per mile. That number drives speedometer and odometer accuracy and is the input gearing and final-drive calculations use — fewer revs per mile on a bigger tire means the odometer reads fewer miles than you actually travel.

What is plus sizing and how do I keep the diameter the same?

Plus sizing is fitting a larger rim while keeping the overall diameter about the same, so handling sharpens without upsetting the speedometer or gearing. You go up in rim size and down in aspect ratio, often widening the section a touch. The plus-size helper takes a starting size and target rim and suggests an aspect ratio that holds the overall diameter close to the original, computed from standard tire-size geometry rather than a copied chart.

Is this tire size calculator free and does it send my data anywhere?

It is completely free with no signup, and every value is computed in your browser from the public tire-size geometry formulas. Nothing you type is sent to a server, and there is no manufacturer database or copyrighted lookup table involved. Open your browser’s developer tools and watch the network tab to confirm nothing leaves your device — and it keeps working offline once the page has loaded.