Tire Diameter Calculator

How to use this tire diameter calculator

1. Type your tire size into the single-tire box. The format is 265/70R17 for metric (passenger and light-truck) or 33x12.50R15 for flotation (off-road and lifted-truck sizes). It's stamped on the sidewall, on the driver's door-jamb sticker, and in the owner's manual.
2. Read the overall diameter. The hero number is the tire's full height from the ground to the top of the tread — the figure tire sellers, lift builders, and fitment charts all use. Section height (one sidewall), circumference, and revs per mile come with it.
3. Compare two sizes if you're upsizing. Hit the Compare tab to drop in a stock size and a new size side by side — you'll get the diameter delta in inches and percent, speedometer error, and a true-scale visual of how much bigger the new tire runs.
4. Save or share it. "Share" copies a link with your size baked in; "PDF" prints a one-page spec sheet for the tire shop or alignment bay.

Why this diameter calculator is different

Almost every "tire diameter calculator" in the search results is really a tire-size calculator that makes you compare two sizes before it shows a single number, or a bare estimator with no context. We built this around the one thing you actually typed in for — the diameter of a tire — and then added the context the anonymous calc sites leave out:

• Overall diameter is the hero output. One size in, the true overall diameter out, in inches, no compare step required. Section height, circumference, and revs per mile sit right beside it.
• A real tire diameter chart on the page. Common sizes and their measured diameters are tabled below — handy when you just want to eyeball "is a 285/75R17 a 34-inch tire?" (it's 33.8") without typing anything.
• Metric and flotation in the same field. A 33x12.50R15 reads its diameter straight off the first number (33.0"); a 265/70R17 gets the sidewall math done for you. No "tire type" dropdown.
• Reviewed by an ASE Master Tech. The top results are anonymous calculator aggregators. Mike Reeves has 15 years in the bay and signs the math — cross-checked against Tire Rack and Discount Tire methodology and 20 test fixtures that run on every release.
• Branded PDF + free embed. Download a one-page spec sheet, or drop the same calculator on your forum thread or shop site with a one-line snippet.

How to calculate tire diameter (the math)

Overall diameter is the rim diameter plus two sidewalls — one stacked on top, one on the bottom. The only piece you compute is the sidewall, called section height.

Step 1 — section height (the sidewall)

Aspect ratio (the middle number) is the sidewall expressed as a percentage of the tire's width. So a 265/70R17 has a sidewall of 265 × 0.70 = 185.5 mm, which is about 7.30 inches.

section_height_mm = width_mm × (aspect_ratio / 100)

Step 2 — overall diameter

Add two sidewalls to the rim diameter. Convert the sidewall from millimeters to inches (÷ 25.4) because rim diameter is already in inches:

overall_diameter_in = rim_diameter_in + (2 × section_height_mm / 25.4)

For the 265/70R17: 17 + (2 × 185.5 / 25.4) = 31.6 inches. That matches what the calculator above returns and what's stamped in the fitment guides.

Flotation sizes skip the math. On a 33x12.50R15, the first number is the overall diameter in inches — 33.0". The 12.50 is the tread width and the 15 is the rim. That's why off-roaders talk in "33s," "35s," and "37s": the size already tells you the height.

Step 3 — circumference and revs per mile

Once you have diameter, circumference is just π × diameter, and revs per mile is how many times the tire turns to cover a mile (63,360 inches):

circumference_in = π × overall_diameter_in
revs_per_mile = 63360 / circumference_in

Bigger diameter means fewer revs per mile, which is exactly why a taller tire makes the speedometer read slow — the wheel-speed sensor counts rotations and the computer still assumes the stock circumference.

Tire diameter chart (common sizes)

Measured overall diameters for popular metric and flotation sizes, straight from this tool's test fixtures. Use it as a quick lookup, then run your exact size through the calculator above for section height and circumference.

Diameters are nominal (the molded "design" size); a mounted tire measures a touch smaller loaded, and tread wear shrinks it over its life. Treat these as the spec-sheet baseline.

Metric vs flotation: two ways to read diameter

The reason a diameter calculator needs to handle two formats is that the industry never standardized on one. Metric sizing (265/70R17) gives width in millimeters, aspect ratio as a percent, and rim in inches — diameter has to be computed. Flotation sizing (33x12.50R15) hands you the overall diameter in inches up front. A 275/70R18 and a 33x11.00R18 are nearly the same tire; one makes you do arithmetic and one doesn't. Type whichever is printed on your sidewall — the calculator reads both.

Worked examples

How tall is a 265/70R17, really?

A common factory truck and SUV size. Sidewall is 265 × 0.70 = 185.5 mm (7.30"), so overall diameter is 17 + 2 × 7.30 = 31.6 inches — call it a "32-inch tire" in shop talk. Circumference is 99.3", and it turns 638 times per mile. That's the baseline a lot of half-ton trucks leave the lot on.

Is a 285/75R17 a 34-inch tire?

Close, but no. Sidewall is 285 × 0.75 = 213.75 mm (8.42"), so diameter is 17 + 2 × 8.42 = 33.8 inches. People round it to "34s," but it's measurably shorter than a true 34, which matters when you're checking lift clearance to the tenth of an inch. The calculator's compare mode will show this is a +2.2" jump (+7%) over the stock 265/70R17 above — enough to throw the speedometer 7% slow.

What diameter is a 33x12.50R15?

No math needed — the flotation code states it: 33.0 inches overall. Circumference 103.7", 611 revs per mile. This is why off-road builds are spec'd in plain inches: the size you order is the height you get.

What overall diameter actually changes

Speedometer and odometer accuracy

The car computes speed from wheel rotations × stock circumference. A bigger-diameter tire covers more ground per rotation, so the speedometer under-reports and the odometer logs fewer miles than you actually drove. A +7% diameter change reads about 7% slow — indicated 60 mph is really 64 mph, and your maintenance intervals creep late.

Effective gearing, MPG, and towing

Diameter is final-drive gearing you can't see on the spec sheet. A taller tire raises effective gearing (fewer revs per mile), which softens acceleration, can drop highway RPM slightly, and cuts towing grunt. Trucks that go up a lot in diameter often re-gear the axle (say 3.55 → 4.10) to claw it back.

Fitment and clearance

Half of any diameter increase happens at the top of the tire, so a +2" diameter jump needs roughly +1" of clearance up top before the fender lip catches at full stuff. That's the number lift kits and fender flares are sized around — compute the diameter first, then size the clearance.

Frequently asked questions

How do I calculate my tire's diameter by hand?

Multiply the width by the aspect ratio as a decimal to get the sidewall in millimeters, double it, divide by 25.4 to get inches, and add the rim diameter. For 265/70R17: 265 × 0.70 = 185.5 mm sidewall, ×2 ÷ 25.4 = 14.6", + 17" rim = 31.6". Flotation sizes (33x12.50R15) give the diameter as the first number, so there's nothing to compute.

What is overall diameter versus section height?

Section height is one sidewall — the rubber between rim and tread. Overall diameter is the whole tire's height: the rim plus two section heights. People mix them up because tire sellers say "diameter" and sidewall stampings imply "height." This tool shows both so there's no ambiguity.

How do I convert a metric tire size to inches?

Run it through the calculator — it outputs the diameter in inches directly. By hand, the only conversion is the sidewall: millimeters ÷ 25.4 = inches. The rim is already an inch measurement, so you add it as-is.

Are calculated diameters exactly what I'll measure on the tire?

Nominal diameter is the molded design size. A real mounted tire usually measures a little smaller, more so under load (the contact patch flattens) and as the tread wears. For fitment and gearing math the nominal number is the right one to use; for a hard clearance check, measure the actual mounted tire.

Can I embed this tire diameter calculator on my site?

Yes — copy the snippet at the bottom of the page. It's free, no signup, and works on forums, build threads, and shop sites. Attribution to RevRated and Mike Reeves is included in the embed.

Related tools

• Tire Height & Diameter Calculator — the full compare-and-upsize version with vehicle lift presets.
• Plus-Size Tire Calculator — find every alternate size within ±3% diameter.
• Wheel Offset Calculator + Visualizer — pair diameter changes with offset/poke math.
• Browse all free tools by Mike Reeves →

Mike's recommendations for a tire change

Once you've settled on a diameter, these are the categories that make the swap go right:

• Best All-Season Tires — match your target diameter with a load index and speed rating that fits your vehicle. Spec mismatch is the most common reason a new size feels wrong.
• Best Winter Tires — winter setups usually go narrower on a smaller wheel while keeping overall diameter near stock to protect speedometer and ABS calibration.
• Best Tire Pressure Gauges — a bigger-diameter tire needs exact PSI to set the contact patch right. A digital gauge to ±0.5 PSI is the floor.
• Best Portable Tire Inflators — top off cold-morning pressure before sidewall flex steals MPG and ruins handling.

Sources & methodology

• Tire Rack — Tire Tech — canonical sidewall and overall-diameter math.
• Discount Tire — Tire Size Calculator Methodology — overall-diameter formula, cross-validation.
• SAE J1252 — Tire Selection for Light-Duty Vehicles — rolling-circumference baseline.

This calculator shares its tested engine with the Tire Height & Diameter Calculator; the 20 fixture cases it's validated against live in fixtures.json alongside the source. Tool outputs are reference values — always verify against your tire's sidewall and your vehicle's owner's manual. About Mike Reeves · Last reviewed June 25, 2026.

Embed this tool on your site

Free for forum mods, build-thread pinned posts, off-road YouTubers, tire shops, and personal blogs. Required attribution is included in the snippet. No fee, no signup, no analytics attached to the embed.

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  &middot; Reviewed by Mike Reeves, ASE Master Technician
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