Aircraft Tire Sizing Systems: How to Read Size Markings

An aircraft tire size marking describes the tire's dimensions in one of several standard formats. The format depends on the sizing system. A Type III marking such as 6.00-6 gives section width and rim diameter. A Type VII marking such as 26 x 6.6 gives overall diameter and section width. A three-part marking such as H40x14.5-19 gives overall diameter, section width and rim diameter. Metric markings such as 1400x530R23 follow the same order, but the first two numbers are in millimetres. The separator also carries meaning. An x separates overall diameter from width. A hyphen before the rim diameter indicates bias-ply construction, and an R indicates radial construction. Other sidewall numbers are performance ratings, not dimensions. These include ply rating, speed rating, rated load and rated inflation pressure.

What do the numbers on an aircraft tire sidewall mean?

The numbers on an aircraft tire sidewall fall into two groups.

• The size designation. This states dimensions in inches or millimetres.
• The ratings and identifiers. These state the tire's capability and identity, for example ply rating, speed in mph, load in pounds, pressure in psi, tube type and part number.

Reading a tire correctly starts with telling these two groups apart. You then need to identify which sizing system the size designation uses.

The size designation typically uses three dimensional terms:

• Overall diameter (OD): the outside diameter of the inflated, unloaded tire, measured across the tread.
• Section width (SW): the width of the inflated tire at its widest point, measured across the sidewalls.
• Rim diameter: the diameter of the wheel rim at the bead seat, where the tire beads sit. In most aircraft tire markings this value is given in inches.

A fourth value, section height, is rarely printed. Section height is the distance from the bead seat to the top of the tread. It can be calculated when both overall diameter and rim diameter are known.

What are the main aircraft tire sizing systems?

Aircraft tire size designations come from several historical sizing systems. Each system encodes a different combination of dimensions. The table below summarises the formats you are most likely to see.

Sizing system	Format	Example	What the numbers give	What is missing from the marking
Type III	Section width - Rim diameter	6.00-6, 8.50-10	Nominal section width (in) and rim diameter (in)	Overall diameter
Type VII	Overall diameter x Section width	26 x 6.6	Nominal overall diameter (in) and section width (in)	Rim diameter
Three-part	Overall diameter x Section width - Rim diameter	15x6.00-6, H40x14.5-19	Overall diameter (in), section width (in) and rim diameter (in)	Nothing dimensional; all three main values are present
Metric	Overall diameter x Section width - or R Rim diameter	1400x530R23	Overall diameter (mm), section width (mm) and rim diameter (in)	Nothing dimensional, but units are mixed
Type I (legacy)	Overall diameter only	A single nominal diameter figure	Nominal overall diameter (in)	Section width and rim diameter

Type III: section width and rim diameter

A Type III aircraft tire size, such as 6.00-6, shows nominal section width in inches, then a hyphen, then rim diameter in inches. A 6.00-6 is therefore a tire about 6 inches wide that mounts on a 6-inch rim. An 8.50-10 is about 8.5 inches wide and mounts on a 10-inch rim.

Type III tires are low-pressure designs and are widely used on light and general aviation aircraft. The marking does not include overall diameter. To find it, you must look up the size in the tire manufacturer's data book.

Type VII: overall diameter and section width

A Type VII aircraft tire size, such as 26 x 6.6, shows nominal overall diameter in inches, then section width in inches. A 26 x 6.6 tire is about 26 inches tall and 6.6 inches wide.

Type VII tires are high-pressure designs associated with jet and military aircraft. The marking does not state the rim diameter. The matching rim size is published in manufacturer data books and wheel documentation, so it must be checked there rather than read from the tire.

Three-part designations: overall diameter, section width and rim diameter

A three-part aircraft tire size, such as H40x14.5-19, contains all three main dimensions. It shows overall diameter (40 in), section width (14.5 in) and rim diameter (19 in). Three-part sizes are the most complete format, because they let you calculate section height and aspect ratio directly from the sidewall.

Some three-part sizes borrow the Type III style for the width figure, for example 15x6.00-6. This size is about 15 inches in overall diameter, about 6 inches in section width, and fits a 6-inch rim.

Metric designations: millimetres plus an inch rim

A metric aircraft tire size, such as 1400x530R23, shows overall diameter and section width in millimetres. The rim diameter is still in inches. In this example:

• 1400 mm is about 55.1 in of overall diameter.
• 530 mm is about 20.9 in of section width.
• 23 is the rim diameter in inches.

The conversion factor is 1 in = 25.4 mm. Always check which units each position uses before comparing a metric size with an inch-based size.

Type I and other legacy designations

Type I is an older smooth-contour system in which the tire is designated only by its nominal overall diameter in inches. You are most likely to meet it on vintage aircraft and in older documentation.

How do you read the letters and separators in an aircraft tire size?

The letters and punctuation in an aircraft tire size marking carry meaning and are part of the designation.

• x separates overall diameter from section width.
• Hyphen (-) before the rim diameter indicates bias-ply construction. In a bias-ply tire, the casing plies run diagonally across the tire.
• R in place of the hyphen indicates radial construction. In a radial tire, the casing plies run across the tire from bead to bead. For example, 1400x530R23 is a radial tire, while H40x14.5-19 is a bias-ply tire.
• B in the rim position is used in some designations to indicate bias-belted construction.
• Letter prefixes such as H identify a particular design family within the three-part system. Read the prefix as part of the size itself. A tire with a prefix is a different size designation from one without it, even when the numbers match.

Construction matters for interpretation. A radial tire and a bias-ply tire with similar numbers are different tires. They can differ in dimensions, weight, footprint and approval status.

How to read an aircraft tire size marking step by step

You can read most aircraft tire size markings with a short, fixed sequence.

1. Find the size designation on the sidewall. It is usually the most prominent string of numbers joined by x, a hyphen or R.
2. Count the numeric parts. Two numbers joined by a hyphen usually indicate Type III (width-rim). Two numbers joined by x usually indicate Type VII (diameter x width). Three numbers indicate a three-part or metric designation.
3. Check the units. Three- or four-digit figures in the first two positions, such as 670 or 1400, are millimetres. Smaller figures, such as 26 or 40, are inches. The rim figure is normally in inches in both cases.
4. Note any prefix letter. Treat it as part of the size.
5. Read the separator before the rim diameter. A hyphen means bias-ply, R means radial, and B, where used, means bias-belted.
6. Identify what the marking does not tell you. A Type III marking omits overall diameter, and a Type VII marking omits rim diameter. Look up the missing value in the manufacturer's data book.
7. Read the ratings separately. Ply rating, speed rating, load rating and inflation pressure are capability limits, not dimensions.

How do you calculate sidewall height, aspect ratio and circumference from an aircraft tire size?

When an aircraft tire marking gives overall diameter and rim diameter, you can calculate the tire's nominal geometry with simple formulas. Use the same unit for every value; convert millimetres to inches, or inches to millimetres, first.

• Section height (sidewall height) = (overall diameter - rim diameter) / 2
• Aspect ratio = section height / section width
• Circumference = π x overall diameter (π ≈ 3.1416)
• Revolutions per mile = 63,360 in / circumference in inches

Worked example: H40x14.5-19

• Section height = (40 - 19) / 2 = 10.5 in
• Aspect ratio = 10.5 / 14.5 ≈ 0.72. The sidewall is about 72% as tall as the tire is wide.
• Circumference = 3.1416 x 40 ≈ 125.7 in
• Revolutions per mile ≈ 63,360 / 125.7 ≈ 504

Worked example: 15x6.00-6

• Section height = (15 - 6) / 2 = 4.5 in
• Aspect ratio = 4.5 / 6.0 = 0.75
• Circumference = 3.1416 x 15 ≈ 47.1 in

Worked example: 1400x530R23 (metric)

• Rim diameter in millimetres = 23 x 25.4 = 584.2 mm
• Section height = (1400 - 584.2) / 2 ≈ 407.9 mm
• Aspect ratio = 407.9 / 530 ≈ 0.77
• Circumference = 3.1416 x 1400 ≈ 4398 mm, or about 4.40 m

Why some sizes cannot be fully calculated from the marking

Two common formats leave out a value you need for these calculations.

• Type III (for example 6.00-6): the marking omits overall diameter. Section width and rim diameter alone do not give section height, aspect ratio or circumference, because the sidewall height is not encoded.
• Type VII (for example 26 x 6.6): the marking omits rim diameter. You can calculate circumference from the 26-inch overall diameter (about 81.7 in), but section height and aspect ratio require the rim diameter from the data book.

How do width, aspect ratio and rim diameter change an aircraft tire's size?

Overall diameter equals rim diameter plus twice the section height. Because of that relationship, the three dimensions trade off against each other in predictable ways.

• Larger rim diameter, same overall diameter: the sidewall gets shorter. Each 1 in increase in rim diameter removes 0.5 in of section height.
• Wider section, same section height: the aspect ratio falls. The tire becomes lower-profile in relative terms, while overall diameter stays the same.
• Larger overall diameter: circumference grows by the same percentage. A tire with a 5% larger overall diameter has a 5% larger circumference and makes about 5% fewer revolutions per mile.

The table below compares three illustrative three-part sizes. They were chosen to show the arithmetic, not as product data or approved alternatives for any aircraft.

Illustrative size	Overall diameter (in)	Section width (in)	Rim diameter (in)	Section height (in)	Aspect ratio	Circumference (in)	Revolutions per mile
30x8.8-15	30	8.8	15	7.5	0.85	94.2	about 672
30x9.5-16	30	9.5	16	7.0	0.74	94.2	about 672
32x8.8-16	32	8.8	16	8.0	0.91	100.5	about 630

The table shows three effects:

• First and second rows: the overall diameter and circumference are identical. The larger rim and wider section on the second size give it a shorter sidewall and a lower aspect ratio.
• First and third rows: 2 extra inches of overall diameter on a 30-inch tire is about 6.7% more. Circumference rises by the same 6.7%.
• Second and third rows: the rim diameter is the same, but the third size has a narrower section and more sidewall height. That gives it a noticeably higher aspect ratio.

What do aircraft tire rim sizes in the marking tell you about the wheel?

The rim diameter in an aircraft tire size marking states the nominal bead-seat diameter of the wheel in inches. It tells you only that the tire and wheel share the same nominal diameter. It does not confirm that the tire fits that wheel.

Aircraft tire rim sizes involve more than diameter. Rim width, flange height, bead seat geometry and, for tubeless tires, sealing requirements are defined for each tire size. These specifications are published in manufacturer data books and industry standards, not on the tire sidewall. A Type VII marking does not show rim diameter at all.

For these reasons, the rim figure on the sidewall is a starting point for tire and rim compatibility, not proof of it.

Which aircraft tire sidewall markings are ratings rather than dimensions?

Several aircraft tire size markings that sit near the size designation describe performance limits or identity, not physical size.

• Ply rating (for example 6 PR or 8 PR): an index of the tire's strength and load capability. It is not a literal count of fabric plies. Two tires of the same size can carry different ply ratings and therefore different load and pressure limits.
• Speed rating: the maximum ground speed the tire is qualified for, usually marked in miles per hour (mph).
• Rated load: the maximum static load per tire, usually in pounds (lb), at the rated inflation pressure.
• Rated inflation pressure: the pressure, usually in pounds per square inch (psi), that corresponds to the rated load. The aircraft's maintenance documentation sets the actual service pressure.
• TT or TL: tube-type or tubeless. A tubeless tire requires a wheel designed for tubeless use.
• Part number, serial number and approval markings: identifiers used for traceability and to confirm that the tire meets the applicable technical standard order.
• Retread markings: on retreaded tires, these show how many times the tire has been retreaded.
• Balance mark: often a red dot that marks the light point of the tire. It is positioned on the wheel as the maintenance manual directs.
• Vent marks: on tubeless tires, coloured dots mark vent holes in the lower sidewall. These holes let gas that has seeped into the casing escape.

How is calculated geometry different from actual tire dimensions and fitment?

The numbers in an aircraft tire size designation are nominal. Calculations based on them describe idealised geometry, not the exact tire on the aircraft. Three separate layers should be kept apart.

Nominal, calculated geometry

Section height, aspect ratio and circumference worked out from the marking are useful for understanding a size and comparing designations. They do not account for manufacturing tolerance, construction type or tread depth.

Actual measured dimensions

Real tires depart from nominal values in several ways:

• Inflated dimensions vary by manufacturer and construction, even for the same designation.
• New tires grow after mounting and inflation.
• Under the aircraft's weight, a tire deflects. Its loaded radius is smaller than half its overall diameter.

Manufacturer data books list actual inflated dimensions, maximum grown dimensions and loaded radius for each size. Use these figures, not the nominal marking, when clearance matters.

Approved fitment compatibility

Whether a tire may be installed on a given aircraft is decided by the aircraft's approved data, not by matching numbers. This data includes the type certificate data, maintenance manual and illustrated parts catalogue. Approval typically covers:

• the size designation
• ply rating
• speed rating
• tube-type or tubeless use
• construction

Two tires with the same circumference, or the same rim diameter, are not interchangeable for that reason alone. Bias-ply and radial tires are likewise not interchangeable unless the approved data allows it.

Summary: reading aircraft tire size markings correctly

To interpret an aircraft tire size marking, work through four points:

1. Identify the sizing system. Type III is width-rim, Type VII is diameter x width, and three-part or metric markings are diameter x width - rim.
2. Check units and construction. Look for inches or millimetres, and read the hyphen, R or B before the rim diameter.
3. Calculate geometry where the marking allows it. Use the section height, aspect ratio and circumference formulas.
4. Read ratings separately from dimensions. Ply rating, speed, load and pressure are capability limits.

Calculated geometry helps you understand and compare sizes. Actual dimensions come from the manufacturer's data. Whether a tire can be fitted is determined by the aircraft's approved documentation.