Tire Tread Void Ratio: Meaning and Calculation

Tire tread void ratio is the share of the tread pattern area taken up by open space, mainly tread grooves, instead of rubber. It is a dimensionless percentage: void ratio (%) = void area ÷ total tread pattern area × 100. A tread with 30% void has 70% rubber in the measured area. The ratio is not printed on the sidewall. To understand it, you first need to know which sidewall numbers describe the tire's size and ratings and which describe the tread design. Only the tread design sets the void ratio.

Does the tire sidewall show the tread void ratio?

No. No standard sidewall marking gives the tread void ratio. The sidewall lists dimensions, construction, load and speed ratings and, in some markets, comparative grades. Void ratio comes from the tread pattern geometry. Manufacturers decide it during design and seldom publish it. Even so, the sidewall numbers matter. They tell you the section width, the sidewall height and the overall size, and those set the space the tread pattern has to work in.

Reading a typical size marking: 225/45R17 94W

Each part of a metric tire size has its own meaning and unit:

Marking	Type of value	Meaning	Unit
225	Dimension	Nominal section width, measured from sidewall to sidewall	Millimetres
45	Ratio	Aspect ratio: sidewall height as a percentage of section width	Percent (no unit printed)
R	Construction code	Radial construction	None
17	Dimension	Rim diameter the tire is designed to fit	Inches
94	Performance rating (code)	Load index: a code for the maximum load per tire, looked up in a load index table	Code, not kilograms
W	Performance rating (code)	Speed rating: a code for the maximum rated speed under specified conditions	Code, not km/h

A single size marking mixes millimetres, a percentage and inches. The load index and speed rating are codes, not measurements. None of these values describes how much of the tread is groove and how much is rubber.

Other sidewall markings that hint at tread design

A few markings relate to the tread pattern, but none is a void ratio figure:

• M+S (mud and snow): a manufacturer declaration tied to tread pattern features such as groove layout. It is not a measured performance result.
• Three-peak mountain snowflake symbol: means the tire passed a standardised snow traction test. It reports test performance, not a geometric ratio.
• UTQG grades (treadwear, traction, temperature): used in the United States. These are comparative grades from standardised tests. They are not dimensions.
• TWI (tread wear indicator) marks: show where the wear bars sit in the tread grooves. They help you judge remaining tread depth, which is a separate measurement from void ratio.

What does tread void ratio mean in tread pattern geometry?

In tread pattern geometry, void ratio compares the open area of the pattern to the whole tread pattern area within the tread edges. The pattern has two kinds of surface:

• Land: the tread blocks and ribs, the raised rubber that touches the road.
• Void: the tread grooves, both circumferential and lateral, plus other recesses that sit below the road surface.

Engineers also use the land-to-void ratio, which states the same information the other way round. A pattern with 70% land and 30% void has a land-to-void ratio of 70:30, or about 2.33:1. A void ratio of 30%, a land ratio of 70% and a land-to-void ratio of 70:30 all describe the same tread.

Void ratio versus tread depth and void volume

Void ratio is an area measure. It says nothing about how deep the grooves are. Tread depth is a length, measured in millimetres or in 32nds of an inch. Groove depth combined with void area gives void volume, the total space available to hold and channel water, slush or mud. Two treads can have the same void ratio and still have very different void volumes if one has deeper grooves.

Tread pattern area versus contact patch area

Void ratio can be measured over two different areas, and the results are not always the same:

• Tread pattern area: the flat, unloaded layout of the tread design, often taken over one or more pattern pitches across the full tread width.
• Contact patch (footprint) area: the part of the tread actually touching the road under a given load and inflation pressure. Its shape and length change with load, pressure and tire construction.

When you compare void ratio figures, check which area each one is based on. A footprint value applies only to the load and pressure used to make the footprint.

How is tire tread void ratio calculated?

To calculate tread void ratio, measure the total tread pattern area and the void area inside it, then divide the void area by the total area and multiply by 100.

Formula: Void ratio (%) = Void area (mm²) ÷ Total tread pattern area (mm²) × 100

Land-to-void ratio = Land area ÷ Void area, where Land area = Total area − Void area.

Step-by-step footprint method

1. Inflate the tire to a known pressure and choose a known load. The footprint depends on both.
2. Make a footprint, for example an ink print on paper under the loaded tire. You can also photograph a section of tread laid flat for a pattern-area measurement.
3. Mark the boundary of the measured area: the outer tread edges at the sides, and either the footprint length or a whole number of pattern pitches along the tire.
4. Calculate the total area inside the boundary in mm², for example width × length for a rectangular region.
5. Measure the void area. For straight grooves, multiply groove width by groove length. For complex shapes, count grid squares or use image analysis on the print.
6. Divide the void area by the total area and multiply by 100 to get the void ratio in percent.
7. If needed, subtract the void area from the total area to get the land area, then work out the land-to-void ratio.

Worked example with illustrative numbers

The figures below are hypothetical. They show how the calculation works and do not describe any real tire model.

• Measured tread width between tread edges: 180 mm. This is narrower than the 225 mm section width, because section width runs from sidewall to sidewall.
• Measured region length: 140 mm.
• Total tread pattern area: 180 × 140 = 25,200 mm².
• Four circumferential grooves, each 9 mm wide and running the full 140 mm: 4 × 9 × 140 = 5,040 mm².
• Lateral grooves between the tread blocks, measured on the print: 2,520 mm².
• Total void area: 5,040 + 2,520 = 7,560 mm².
• Void ratio: 7,560 ÷ 25,200 × 100 = 30%.
• Land area: 25,200 − 7,560 = 17,640 mm². Land-to-void ratio: 17,640 ÷ 7,560 ≈ 2.33:1 (70:30).

Thin sipes cut into the tread blocks add very little to void area, even though they can noticeably affect how the blocks behave. Most of the void ratio comes from the main grooves.

Why measured void ratio changes over a tire's life

Tread grooves are often narrower at the bottom than at the top, and some patterns change shape with depth. As the tread wears, the groove openings at the surface can shrink, so the void ratio of a worn tire may differ from the new-tire value. Measurements also vary with inflation pressure and load, and between pitches on patterns that use varied pitch lengths to reduce noise.

What does a higher or lower void ratio mean for performance?

A higher void ratio gives more room to move water, snow or mud out of the contact area. A lower void ratio puts more rubber on the road. These are general design tendencies, not guaranteed results. Rubber compound, siping, groove depth, groove orientation and tire construction all change real-world performance.

Characteristic	Lower void ratio (more land)	Higher void ratio (more void)
Rubber in contact with the road	More	Less
Space to evacuate water, slush or mud	Less	More
Tread block stiffness	Usually stiffer, larger blocks or ribs	Usually smaller or more separated blocks, which may flex more
Typical design intent	Dry road grip and steering precision	Wet, snow or loose-surface traction
Tread noise tendency	Often lower	Often higher, depending on groove design

Void ratio is one variable in tread design. Do not use it on its own to rank tires for safety or grip. Where published test results or sidewall symbols for specific conditions exist, they say more about actual performance.

How do tire size numbers affect sidewall height, diameter and circumference?

Section width and aspect ratio together set sidewall height. Sidewall height and rim diameter together set overall diameter, and overall diameter sets circumference. These calculations describe the tire's outer geometry. They do not change the void ratio of the tread pattern, but they explain why tires of different sizes put different tread areas on the road.

Calculation steps

1. Sidewall height (mm) = section width (mm) × aspect ratio ÷ 100.
2. Rim diameter (mm) = rim diameter (inches) × 25.4.
3. Overall diameter (mm) = rim diameter (mm) + 2 × sidewall height (mm).
4. Circumference (mm) = π × overall diameter (mm).
5. Revolutions per kilometre = 1,000,000 ÷ circumference (mm).

Example comparison: 225/45R17 versus 245/40R17

Value	225/45R17	245/40R17
Section width	225 mm	245 mm
Sidewall height	225 × 0.45 = 101.3 mm	245 × 0.40 = 98.0 mm
Rim diameter	17 × 25.4 = 431.8 mm	431.8 mm
Overall diameter	431.8 + 2 × 101.3 ≈ 634.3 mm	431.8 + 2 × 98.0 = 627.8 mm
Circumference	≈ 1,992.7 mm	≈ 1,972.3 mm
Revolutions per km	≈ 501.8	≈ 507.0

The 245/40R17 is 20 mm wider in nominal section width. Its calculated overall diameter is about 6.5 mm smaller, roughly 1.0% less. Over the same distance it turns about 1% more times, which can affect speedometer readings relative to the original size.

How size relates to the tread area and void ratio

A wider tire usually has a wider tread, but void ratio is a percentage, so a wider tread does not automatically mean a different void ratio. Within one tire model, different sizes may use different numbers of ribs, grooves or blocks, so the ratio can vary from size to size. At a given load and pressure, a wider tire tends to make a wider, shorter contact patch rather than a much larger one. The amount of void that actually reaches the road therefore depends on both the pattern and the footprint shape.

What are the limits of calculated tire geometry?

Calculated sizes and void ratios are nominal or condition-specific values, not exact descriptions of a mounted tire on a vehicle. Keep these limits in mind:

• Nominal vs actual dimensions: real section width and overall diameter vary with rim width, inflation pressure, manufacturer tolerances and tread depth. A new tire with full tread is slightly larger in diameter than the same tire worn down.
• Section width vs tread width: the first number on the sidewall is not the width of the tread pattern. Void ratio is measured across the tread, not across the full section.
• Footprint conditions: a footprint-based void ratio applies only to the load and pressure used when the footprint was made.
• Fitment compatibility: a matching calculated diameter does not mean a size fits a vehicle. Clearance to suspension and bodywork, approved rim widths, load index, speed rating and the vehicle maker's approved sizes must all be checked separately.
• Performance: neither void ratio nor calculated size predicts grip, wear or noise on its own. These depend on the whole tire design and the conditions of use.

Key points to remember

• Tire tread void ratio is void area divided by total tread pattern area, expressed as a percentage. It is not printed on the sidewall.
• Land-to-void ratio describes the same tread from the rubber side. For example, 70:30 corresponds to a 30% void ratio.
• Sidewall size markings combine millimetres (width), a percentage (aspect ratio) and inches (rim diameter). Load index and speed rating are codes.
• Sidewall height, overall diameter and circumference can be calculated from the size marking. They describe outer geometry, not the void ratio of the tread pattern.
• Calculated values are nominal. Actual dimensions, footprint behaviour and vehicle fitment must be checked separately.