SAE to Metric Tire Size Conversion: The Complete Guide

There are two ways to write down the same tire, and they look nothing alike. One is 315/70R17. The other is 35x12.50R17. Both describe a tire roughly 34 inches tall and 12.5 inches wide, and if you are shopping across both systems — which you will be, the moment you look at light-truck tires — you need to move between them without guessing.

Most conversion charts get this slightly wrong, in a way that matters. This guide shows you the actual arithmetic, gives you a converter that runs the same calculation as our fitment tools, and is honest about the one thing conversion cannot tell you.

The short version

• Metric sizes are written width/aspect × R × wheel — 315/70R17. Width is millimetres. Aspect is a percentage, not a measurement.
• Flotation (SAE) sizes are written height × width × R × wheel — 35x12.50R17. All three numbers are inches.
• A flotation size is a nominal name, not a measurement. A "35-inch" tire measures about 34.7 inches. Across our catalogue the shortfall is roughly one percent — less than the folklore says, but real.
• Matching the size does not mean matching the load rating. That part is separate, and it is the part that gets people hurt.

How to read a metric tire size

Take 315/70R17:

Yes — millimetres and inches in the same size. That is not a mistake in the guide; it is genuinely how the standard is written.

The number that trips people up is the aspect ratio, because it is a ratio, not a height. A 70 on a 315 tire is a 220 mm sidewall. A 70 on a 215 tire is a 150 mm sidewall. Same number, sidewalls 70 mm apart. This is why you cannot compare two tires by their middle number.

To get overall diameter:

diameter = wheel diameter + (2 × sidewall height)

315/70R17
  sidewall  = 315 × 0.70          = 220.5 mm
  wheel     = 17 × 25.4           = 431.8 mm
  diameter  = 431.8 + (2 × 220.5) = 872.8 mm = 34.4"

The sidewall is doubled because there is one above the wheel and one below it.

How to read a flotation (SAE) tire size

Take 35x12.50R17:

This system is far easier to read — every number is inches, and the first one is the number you actually care about when you are worried about clearance. That is exactly why the off-road and light-truck world uses it.

There is no aspect ratio anywhere in a flotation size. If you want it, you calculate it:

aspect ratio = sidewall ÷ section width × 100
             = ((35 − 17) ÷ 2) ÷ 12.5 × 100
             = 9 ÷ 12.5 × 100
             = 72%

Why the arithmetic gives you the wrong answer

You can convert a flotation size to metric with the maths above, and the result will be geometrically correct and commercially useless.

Run 33x12.50R17 through the formula and it returns 315/65R17. That is a legitimate size on paper. Searched against a catalogue of roughly 36,000 tires, it returns zero products. Nobody makes it.

Tire sizes are not a continuous space you can round into. Manufacturers build a specific, fairly small set of sizes, and the metric size that the market treats as the equivalent of a given flotation size is a matter of fact, not a calculation. The real answer for 33x12.50R17 is 285/70R17 — 63 tires from 23 brands, and its measured diameter is within four hundredths of an inch.

Everything below is built the other way round: from measured diameters of sizes we actually stock.

Two things flotation sizes get wrong at once

The diameter runs small. A tire sold as a 35 does not measure 35 inches. Nothing requires it to, and manufacturing, tread depth and load deflection all push the same direction. The trade rule of thumb is 2–3% under, and the sizes the market cross-shops bear that out: the metric tires treated as equivalent to a 35 measure 34.1"–34.4", not 34.8".

The width runs wide. This one gets missed. A "12.50" flotation tire is 12.5 inches — 317 mm — by name only. Look at what it is actually equivalent to and the metric width is consistently narrower:

So converting a flotation size by matching its width to the nearest metric width — the obvious approach, and what our own calculator used to do — lands you on a tire that is both too wide and, to compensate, too short.

Which metric size to actually shop for

Every row below comes from measured diameters of tires in stock. Bold is the closest match; the second row for a size is the practical alternative, usually with far more choice available. "Available" is how many we carry, because a perfect match nobody sells is not an answer.

16-inch wheels

17-inch wheels

18-inch wheels

20-inch wheels

22-inch wheels

Above 35 inches, there is no metric equivalent

Worth knowing before you go looking. The tallest metric tire available with meaningful choice, by wheel size:

A 37 or a 40 has no metric counterpart you can buy on a normal light-truck wheel. This is not an oversight in the tables — it is why the off-road world stayed on flotation sizing in the first place. Past about 35 inches, flotation is the only system that describes what is being sold.

Converting metric to flotation

Same arithmetic, run backwards:

315/70R17 → flotation

  section width = 315 ÷ 25.4                    = 12.4"  → 12.50
  sidewall      = 315 × 0.70 = 220.5 mm ÷ 25.4  = 8.68"
  diameter      = 17 + (2 × 8.68)               = 34.4"
  nominal       = 34.4 ÷ 0.975                  ≈ 35"

  → 35x12.50R17

That last step is the correction running in reverse. Because flotation sizes only exist in whole-inch diameters and half-inch widths, the result rounds hard — several metric sizes map onto the same flotation name.

Why there are two systems at all

Metric sizing came out of European passenger-car standardisation and is built for precision within a narrow range. Passenger tires vary a lot in width and very little in overall height, so describing them by width and a sidewall percentage is efficient.

Flotation sizing came from agricultural and light-truck use, where the question is not "how wide" but "will it fit under the truck, and will it float on soft ground rather than cut into it." Overall height is the number that answers both, so it goes first.

Neither is more correct. They are optimised for different questions, which is why off-road tires are still sold in both, often on the same shelf.

What conversion does not tell you

This is the part to read twice.

Matching physical dimensions does not mean two tires are interchangeable on your vehicle. Size is one of four things that have to line up:

Load index and load range. A 315/70R17 might be a Standard Load passenger tire or an E-rated light-truck tire, and they carry very different weights at very different pressures. If you tow, haul, or drive a three-quarter-ton truck, dropping to a lower load rating is unsafe even at identical dimensions. Check the load index number after the size, and check your door jamb for the minimum.

Speed rating. The letter after the load index. Winter tires frequently carry a lower speed rating than the summer tires they replace, which is legal and normal — but know before you buy.

Actual width versus wheel width. Section width is measured on a specified rim. Mount the same tire on a narrower wheel and it gets taller and narrower; on a wider wheel, shorter and wider. Every tire has a recommended rim width range, and conversion arithmetic ignores it entirely.

Clearance under load. Static diameter is not what rubs. A tire at full compression with the wheel turned needs meaningfully more room than a tape measure suggests.

What a size change does to your speedometer

Your speedometer counts wheel rotations and multiplies by an assumed rolling distance. Change the tire's diameter and that assumption is wrong by exactly the same proportion:

actual speed = indicated speed × (new diameter ÷ original diameter)

Fit a tire 3% taller and at an indicated 100 km/h you are doing 103. Your odometer under-counts by the same 3%, which quietly stretches your service intervals and, on a lease, matters financially.

The converter above shows this for whatever pair you select. As a rule of thumb, a difference under about 0.6 inches is what most fitment guides treat as interchangeable; beyond that, expect to verify clearance and consider a speedometer recalibration.

Common mistakes

Treating the aspect ratio as a height. It is a percentage of that tire's own width. 70 means nothing until you know the first number.

Comparing section width to tread width. Section width is the widest point of the tire — the sidewall bulge — not the part touching the road. Tread is typically 20–25 mm narrower.

Trusting the number on the sidewall of a flotation tire. Covered above, and worth repeating: a 35 is a 34.

Changing the wheel diameter by accident. The R number is the one part of a size that is a hard physical constraint. Everything else is a trade-off; this one is a yes or no.

Converting a size but not the load rating. The most common way a dimensionally perfect conversion turns into an unsafe one.

Frequently asked questions

Is a 33-inch tire actually 33 inches?

No. Expect roughly 32.2–32.8 inches, depending on whose spec you trust and whether the tire is loaded.

There are two numbers in circulation and both are honest about different things. Manufacturer spec sheets in our catalogue put a 33 at about 32.8" — unloaded, full tread. The trade rule of 2–3% under puts it nearer 32.2", and that is what tracks real behaviour on a vehicle. The metric sizes the market actually treats as equivalent land in between.

For shopping, none of this matters much: use the tables above. For clearance on a lift, measure the specific tire.

What does the aspect ratio actually mean?

It is the sidewall height expressed as a percentage of the section width — not a measurement in its own right. On a 315/70R17, the 70 means the sidewall is 70% of 315 mm, or 220.5 mm. The same 70 on a 215 tire gives a 150 mm sidewall. Lower numbers mean shorter sidewalls, which usually means sharper handling and a firmer ride.

Can I convert metric to SAE as well as the other way?

Yes — the converter above runs in both directions. Expect the metric-to-flotation direction to be coarser, because flotation sizes only exist in whole-inch diameters and half-inch widths, so several metric sizes collapse onto one flotation name.

Does the wheel size change when I convert?

Never. 33x12.50R15 converts to a R15 metric size. If a chart or a salesperson hands you a different R number, the conversion is wrong and the tire will not mount on your wheel.

Two tires are both sold as "35" — why are they different sizes?

Because the number is a name, and manufacturers interpret it differently. Even in published specs our catalogue shows a half-inch spread between brands for the same nominal size, and real measurements spread wider still. It is the single most common reason a "same size" replacement does not fit the way the old one did.

Why doesn't the calculator just do the maths?

It does, and for metric-to-metric that is exactly right. But converting to metric by arithmetic produces sizes nobody manufactures — 315/65R17 is geometrically valid and has zero products behind it. Equivalence between the two systems is a fact about what gets built and sold, so the tables above are drawn from measured diameters of tires actually in stock.

How much size difference is safe?

Most fitment guidance treats a change within about 3% of overall diameter — roughly 0.6 inches on a typical light-truck tire — as interchangeable without further work. Past that you should check clearance at full lock and full compression, and consider whether your speedometer needs recalibrating. Load rating has to be checked regardless of how small the size change is.

Where can I get help with a specific fitment?

Browse tires by size or vehicle, or contact Autrex with your vehicle and what you are trying to fit. We would rather answer a question than process a return.