A tire size conversion calculator takes the code printed on a tire's sidewall, something like 225/65R17, and turns it into measurements you can actually work with: section width, sidewall height, overall diameter, and circumference. It also moves that same size between the metric notation used on most passenger cars and the inch-based flotation notation common on trucks and off-road tires.
The tool does three jobs. It decodes a tire size into its individual measurements, it converts that size into another format, and it lines two sizes up side by side so the difference between them is easy to see. If you already have your tire code, you can type it directly into the calculator above. If you don't, the sidewall itself, your owner's manual, or the sticker on the driver's door jamb will have it, and where exactly to look is covered a little further down this page.
How Do You Read a Tire Size Code?
Every tire sidewall carries a tire size code made up of numbers and letters, and each position in that code has a fixed meaning. Take 225/65R17 as an example. Reading left to right: 225 is the section width in millimeters, 65 is the aspect ratio expressed as a percentage of that width, R marks radial construction, and 17 is the rim diameter in inches. A load index and a speed rating usually follow the size itself, describing how much weight the tire can carry and how fast it's rated to run.
That's the code at a glance. The next two sections break down what each segment actually measures and why the format changes depending on the vehicle a tire is built for.
What Does Each Number and Letter in a Tire Size Mean?
Code Segment | Example Value | What It Means |
Vehicle class letter | P (in P225/65R17) | Identifies the tire's intended use: P for passenger car, LT for light truck, ST for special trailer, T for temporary spare. |
Section width | 225 | The tire's width in millimeters, measured sidewall to sidewall at its widest point. |
Aspect ratio | 65 | Sidewall height expressed as a percentage of the section width. |
Internal construction | R | Radial (R) is standard on almost all modern tires. Diagonal or bias-ply (D) and bias-belted (B) construction are older methods that flex differently and generally wear faster and ride harsher. |
Rim diameter | 17 | The diameter, in inches, of the wheel the tire is designed to mount on. |
Load index | 95 | A numeric code tied to a load capacity table. A load index of 95 carries about 1,520 lb at 42 psi. |
Speed rating | H | A letter tied to a maximum sustained speed. H corresponds to a maximum of 130 mph. |
What Is the Difference Between Metric, P-Metric, and Flotation Tire Sizing?
Tires are sized using a few different notations, and which one you'll see depends mostly on the vehicle the tire was built for.
Format | Example Code | What Each Number Means | Typical Use Case |
Metric / P-Metric | 225/65R17 | Width and aspect ratio in millimeters and percent, rim diameter in inches | Passenger cars and most crossovers |
Flotation (inch) | 33x12.50R17 | Overall diameter and section width given directly in inches, rim diameter in inches | Trucks, SUVs, and off-road tires |
Alpha-numeric (older) | L78-15 | A letter tied to a width and load class, followed by an aspect ratio and rim diameter | Some classic and older vehicles |
These formats exist separately mostly because passenger cars and light trucks or off-road vehicles developed different sizing conventions over time, one built around millimeters and one built around inches. All three still describe the same physical measurements, width, sidewall height, and diameter, just written differently. How to move a size from one format to the other is covered in the conversion section below.
What If You Do Not Know Your Tire Size Code?
If you don't have a tire code in hand, you have a couple of options. If you can physically get to the tire, the size is printed directly on the sidewall. If not, check your vehicle's owner's manual or the sticker on the driver's side door jamb, which usually lists the factory recommended size. Some tools also let you look up a size by entering your vehicle's year, make, and model instead of typing a code, which is worth using if you'd rather not track down the physical tire. Either way, once you have the code, you can enter it directly into the calculator above.
How Do You Convert Metric Tire Sizes to Inches?
Converting a metric tire size to inches comes down to three steps: multiply width by aspect ratio to get sidewall height, divide that figure by 25.4 to move from millimeters to inches, then add the rim diameter. That number, 25.4 millimeters per inch, is the one detail people most often get wrong or leave out entirely, so it's worth keeping in mind on its own before looking at the full formula.
What Is the Formula for Converting Tire Width, Aspect Ratio, and Diameter?
Sidewall height (mm) = Width x Aspect Ratio / 100
Sidewall height (in) = Sidewall height (mm) / 25.4
Overall diameter (in) = Rim diameter + (2 x Sidewall height in inches)
The sidewall height gets multiplied by two because it appears twice in the tire's overall diameter, once above the rim and once below it. A single sidewall measurement only accounts for half of what actually makes up the tire's total height.
What Is a Worked Example of a Metric to Inch Tire Conversion?
Using 225/65R17 as the example size: sidewall height in millimeters is 225 x 65 / 100, which comes to 146.3 mm. Divide that by 25.4 and the sidewall height in inches is about 5.8 in. Add that twice to the 17-inch rim diameter and the overall diameter works out to roughly 28.5 in. That's the full size converted from start to finish, and you could reproduce it with nothing more than a basic calculator. Keep in mind the actual mounted size can vary slightly by tire brand and mold, based on manufacturer tolerance data.
What Is a Quick Reference Chart for Common Metric to Inch Tire Conversions?
For sizes people search for often, especially in the 30 to 35-inch range where truck and off-road tires get named in both metric and flotation format, here's a quick lookup so you don't have to run the formula by hand every time.
Tire Code | Format | Converted Value |
225/65R17 | Metric | ~28.5 x 8.9R17 |
235/75R15 | Metric | ~28.9 x 9.3R15 |
265/70R17 | Metric | ~31.6 x 10.4R17 |
315/70R17 | Metric | ~34.4 x 12.4R17 |
33x12.50R17 | Flotation | ~33 x 12.5R17 (already in inches) |
These are calculated values, and actual mounted size varies slightly by tire brand and mold, so treat this chart as a starting point and confirm your exact size against the calculator above.
How Do You Calculate Overall Tire Diameter and Circumference?
Once a size is converted, the two numbers that matter most for fitment and speed-related questions are overall diameter and circumference, not the raw code itself. Diameter tells you how tall the tire actually sits, and circumference tells you how much ground it covers in a single rotation, which is what drives everything from speedometer accuracy to gear ratio effects covered later on this page.
What Is the Formula for Overall Tire Diameter?
Overall diameter = Rim diameter + (2 x Sidewall height). This holds regardless of whether the source size is written in metric or flotation format; only the method for finding sidewall height changes, since flotation sizes already give you sidewall height in a form closer to inches. As a standalone reference, this is the same formula walked through in the conversion section above.
How Do You Calculate Revolutions per Mile?
Circumference equals overall diameter multiplied by pi. Revolutions per mile equals the number of inches in a mile, 63,360, divided by circumference in inches. This number matters because it's what a vehicle's speedometer and odometer use internally to translate wheel rotation into distance and speed. Using the 225/65R17 diameter calculated earlier, about 28.5 in, circumference comes to roughly 89.5 in, which puts the tire at about 708 revolutions per mile.
How Do You Compare Two Different Tire Sizes?
Comparing two tire sizes usually comes up when you have a current size and a size you're considering switching to. The calculator lines up width, sidewall height, overall diameter, and circumference for both sizes at once, so you can see exactly where they diverge instead of guessing from the codes alone.
What Is the Percentage Difference Formula for Tire Diameter?
Diameter difference (%) = (New diameter - Original diameter) / Original diameter x 100. A positive result means the new tire is larger, a negative result means it's smaller. For example, moving from a 28.5-inch diameter to a 29.5-inch diameter works out to a difference of about 3.5 percent.
What Is the Safe Tolerance Range When Changing Tire Size?
A widely used rule of thumb is that keeping overall diameter within about plus or minus 3 percent of the stock size generally keeps speedometer, ABS, and stability control systems within their intended range. Changes beyond roughly 5 to 7 percent can require recalibration or gearing changes. This is a general guideline used across tire and fitment sources, not a guaranteed rule, so it's worth confirming against your vehicle manufacturer's own specifications before committing to a size.
Diameter Difference | Rating | What It Means |
Under 3% | Generally safe | Speedometer, ABS, and stability control typically stay within their intended range |
3% to 5% | Use caution | May introduce a noticeable speedometer offset; check with your vehicle manufacturer |
Above 5% | High difference | Often requires recalibration or a gearing change before it's a safe swap |
Can You Use a Different Aspect Ratio, Like 60 Instead of 65?
Yes, but the aspect ratio number directly changes sidewall height, and sidewall height directly changes overall diameter. A lower aspect ratio number means a shorter sidewall relative to the same width, so a 60-series tire has a shorter sidewall than a 65-series tire of the same width. As an example, a 225/60R17 has a sidewall height of about 5.3 in, against roughly 5.8 in for a 225/65R17 at the same width and rim size, which puts the overall diameter about 3 percent lower. That's close enough to the tolerance range above that it's worth running the exact size through the calculator and checking it against that tolerance rule before assuming the swap is safe.
How Does Changing Tire Size Affect Your Speedometer and Odometer?
Speed and distance readings are calculated from how many times the tire rotates, and that calculation is calibrated for the factory tire size. Changing the size changes how much ground each rotation covers, which throws off both readings. The two sections below cover the formula behind that error and which direction it runs.
What Is the Formula for Speedometer Error After a Tire Size Change?
Actual speed = Indicated speed x (New diameter / Original diameter). For example, going from an original diameter of 31.6 in to a new diameter of 33.8 in, a 60 mph indicated reading is actually closer to 64 mph. Odometer error follows the same ratio, so distance readings shift by the same percentage as speed readings.
Why Do Larger Tires Make the Speedometer Read Slower Than Actual Speed?
A taller tire has a larger circumference, so it covers more ground per rotation than the factory tire did, which makes the speedometer under-report actual speed. A shorter tire does the opposite and makes the speedometer read faster than actual speed. As a real-world example, a tire that's about 3 percent taller than stock can turn a 60 mph indicated reading into an actual speed closer to 62 mph, which is worth keeping in mind in speed-limited zones.
What Is Plus Sizing and How Do You Calculate It?
Plus sizing means moving to a larger wheel diameter while lowering the tire's aspect ratio, so the overall tire diameter stays close to the original. People do this mainly for appearance, brake caliper clearance, or handling response, and when it's done within the tolerance range covered earlier, it's meant to leave gearing and speedometer accuracy unaffected.
How Do You Keep Overall Diameter the Same When Upsizing Wheels?
The practical method is to increase wheel diameter by a set amount, then lower the aspect ratio using the diameter formula from earlier in this page to solve for the sidewall height that keeps total diameter constant. As an example, a 215/65R16 has an overall diameter of about 27 in. Moving to an 18-inch wheel while keeping that same 27-inch diameter means the sidewall height needs to drop to about 4.5 in, which works out to roughly a 215/53R18, rounded to the nearest size actually available from tire manufacturers. Diameter is only one part of the fitment check, though. A wider or taller tire on a larger wheel can also reduce fender and suspension clearance, which is covered in the wheel offset section further down this page.
What Common Mistakes Do People Make When Converting Tire Sizes?
A handful of errors come up repeatedly when people work through tire size conversions by hand. Here's what tends to go wrong and how to think about each one correctly.
Common Mistake | Why It Happens | Correct Approach |
Confusing wheel diameter with overall tire diameter | Both are expressed in inches, and the rim diameter is part of the tire code itself, so it's easy to treat them as the same number | Wheel diameter is only the rim size; overall tire diameter also includes sidewall height on both sides of the rim |
Forgetting to double the sidewall height | The formula only shows one sidewall height value, so it's easy to add it just once | Sidewall height needs to be counted twice, since it appears above and below the rim |
Treating aspect ratio as an absolute number instead of a percentage | The number looks like a standalone measurement rather than a percentage of width | Aspect ratio only means anything relative to the tire's section width, not on its own |
Assuming the calculated result exactly matches the manufacturer's spec | A formula produces a precise-looking number, which reads as more exact than it is | Treat calculated results as a close approximation; actual size varies slightly by brand and mold |
Misjudging which direction the speedometer error goes | It isn't intuitive that a larger tire makes the speedometer read slower, not faster | A taller tire under-reports speed; a shorter tire over-reports it |
Mixing tire sizes or tread patterns across only one or two tires | Replacing tires one or two at a time feels cheaper and more convenient than replacing a full set | Mismatched sizes or tread patterns can strain the drivetrain and confuse traction or stability control systems, particularly on all-wheel-drive and four-wheel-drive vehicles |
What Other Tire and Wheel Factors Should You Check Before Buying New Tires?
Tire size is only one part of a safe fitment decision. Load index, speed rating, wheel offset, gear ratio, tire pressure, and tire age all interact with whatever size you've just worked out. The sections below each cover one of those factors in more depth.
How Does Tire Size Affect Load Index and Speed Rating?
Changing tire size can also change the load index and speed rating printed at the end of the code, and a replacement tire needs to meet or exceed your vehicle's original load and speed requirements, not just match the diameter you calculated.
If the size change nudges either number, the load index to speed rating calculator will show whether the replacement still meets your vehicle's original ratings.
How Do You Check Wheel Offset and Backspacing Before a Size Change?
A converted tire size still needs a wheel with the correct offset and backspacing to clear the fender and suspension components; overall diameter alone doesn't guarantee fitment. A size change can affect fender clearance, wheel well clearance, suspension clearance, and scrub radius, all factors a full fitment check has to account for beyond diameter.
Once diameter and tolerance are confirmed here, the wheel offset to backspacing calculator handles the full offset, backspacing, and clearance check.
How Does Tire Size Affect Gear Ratio and Engine RPM?
A larger overall tire diameter changes the effective gear ratio, which changes engine RPM at a given speed and can affect acceleration and fuel economy, tying directly back to the circumference and diameter values already calculated on this page. The exact RPM shift also depends on your vehicle's specific axle gear ratio, so it can't be pinned down from tire size alone.
For the full RPM lookup across different tire and gear ratio combinations, the tire size to RPM calculator will work it out.
How Do You Read Tire Pressure and Load Ratings on the Sidewall?
Alongside the size code, the sidewall also lists maximum pressure and load information. A tire size change doesn't change the correct inflation pressure for your vehicle, which should still follow the door jamb placard rather than the tire's maximum rating printed on the sidewall.
For full PSI guidance based on the size you just worked out, use the tire load to PSI calculator.
How Do You Check Rim and Tire Fitment Compatibility?
A converted or plus-sized tire also needs to match an acceptable rim width range for its section width, not just the rim diameter used in the size code. Mounting a tire on too narrow or too wide a rim affects handling and tire wear even when the diameter checks out.
Using the width value already calculated on this page, the tire width to rim width calculator will confirm the acceptable rim range for your exact size.
How Do You Convert Tire Pressure Between PSI and kPa?
Readers converting a tire size between metric and inch formats often also need to convert pressure units, especially when comparing a vehicle's specification between regions that use different pressure units.
The PSI to kPa converter handles that conversion directly.
How Do You Check Tire Age From the DOT Code?
When buying a replacement tire in a newly converted size, checking tire age matters as much as checking size, since tires degrade over time regardless of how much tread is left. The DOT code on the sidewall shows the week and year of manufacture.
As a purchase checklist item, the DOT code to tire age calculator will read that date for you from the code itself.
What Is the Difference Between All-Season, Winter, and All-Terrain Tires in Your Size?
Once a compatible size is confirmed, that same size is usually available across several tread types, and the choice between them comes down to climate and use case rather than dimensions.
You can browse all-season, winter, and all-terrain tires in your size once you've settled on the measurements.