Enter your tire's measurements to determine its remaining life and safety status.
32nds of an Inch
Millimeters (mm)
function updateTireLabels() {
var unit = document.getElementById("unit").value;
var newLabel = document.getElementById("newTreadDepthLabel");
var currentLabel = document.getElementById("currentTreadDepthLabel");
var newTreadInput = document.getElementById("newTreadDepth");
var currentTreadInput = document.getElementById("currentTreadDepth");
if (unit === 'mm') {
newLabel.innerHTML = "New Tire Tread Depth (mm)";
currentLabel.innerHTML = "Current Tread Depth (mm)";
newTreadInput.placeholder = "e.g., 8";
currentTreadInput.placeholder = "e.g., 4";
newTreadInput.value = "8";
} else {
newLabel.innerHTML = "New Tire Tread Depth (in 32nds)";
currentLabel.innerHTML = "Current Tread Depth (in 32nds)";
newTreadInput.placeholder = "e.g., 10";
currentTreadInput.placeholder = "e.g., 5";
newTreadInput.value = "10";
}
}
function calculateTreadDepth() {
var unit = document.getElementById("unit").value;
var newDepthRaw = parseFloat(document.getElementById("newTreadDepth").value);
var currentDepthRaw = parseFloat(document.getElementById("currentTreadDepth").value);
var resultDiv = document.getElementById("tireResult");
if (isNaN(newDepthRaw) || isNaN(currentDepthRaw) || newDepthRaw <= 0 || currentDepthRaw < 0) {
resultDiv.innerHTML = "Please enter valid, positive numbers for both fields.";
resultDiv.className = "result-replace";
return;
}
var MM_PER_32ND_INCH = 25.4 / 32; // Approx 0.79375
var LEGAL_LIMIT_32NDS = 2;
var LEGAL_LIMIT_MM = 1.6;
var newDepthIn32nds;
var currentDepthIn32nds;
var legalLimitIn32nds = LEGAL_LIMIT_32NDS;
if (unit === 'mm') {
newDepthIn32nds = newDepthRaw / MM_PER_32ND_INCH;
currentDepthIn32nds = currentDepthRaw / MM_PER_32ND_INCH;
if (currentDepthRaw newDepthIn32nds) {
resultDiv.innerHTML = "Current tread depth cannot be greater than the new tread depth.";
resultDiv.className = "result-replace";
return;
}
var totalUsableTread = newDepthIn32nds – legalLimitIn32nds;
if (totalUsableTread <= 0) {
resultDiv.innerHTML = "The 'New Tread Depth' must be greater than the legal limit (2/32\" or 1.6mm).";
resultDiv.className = "result-replace";
return;
}
var remainingUsableTread = currentDepthIn32nds – legalLimitIn32nds;
if (remainingUsableTread 100) percentageRemaining = 100;
if (percentageRemaining 6) {
recommendation = "Status: Good. Your tires have ample tread depth for safe driving in all conditions.";
resultClass = "result-good";
} else if (currentDepthIn32nds > 4) {
recommendation = "Status: Caution. Your tires are beginning to wear. Performance in heavy rain and snow is reduced. Start planning for replacement.";
resultClass = "result-caution";
} else if (currentDepthIn32nds > legalLimitIn32nds) {
recommendation = "Status: Replace Soon. Your tires have minimal tread remaining. Hydroplaning risk is high, and winter performance is poor. Replacement is recommended as soon as possible.";
resultClass = "result-replace";
} else {
recommendation = "Status: Replace Immediately. Your tires are below the legal minimum tread depth. They are unsafe and illegal to use on public roads.";
resultClass = "result-replace";
}
resultDiv.className = resultClass;
resultDiv.innerHTML = "Your current tread depth is " + currentDepthDisplay + "." +
"You have approximately " + percentageRemaining.toFixed(0) + "% of usable tread life remaining." +
"" + recommendation;
}
Understanding Tire Tread Depth
Tire tread depth is the vertical measurement from the top of the tire's rubber to the bottom of its deepest grooves. For new passenger car tires, this depth is typically between 8/32″ and 11/32″ (or about 6mm to 9mm). This tread is crucial for safety, as the grooves are designed to channel water, slush, and snow away from the tire's contact patch with the road. This ensures optimal grip and prevents hydroplaning.
Legal Tread Depth Requirements
In most U.S. states and many countries, the minimum legal tread depth is 2/32 of an inch (approximately 1.6 mm). Driving with tires below this limit is not only dangerous but can also result in fines. While 2/32″ is the legal limit, safety experts agree that tire performance, especially in wet or snowy conditions, diminishes significantly well before this point. It is recommended to replace tires when they reach 4/32″ for all-season use, or even earlier for winter driving.
How to Measure Your Tire Tread
You can measure your tread depth using a few simple methods:
Tread Depth Gauge: This is the most accurate tool. Simply insert the probe into a main tread groove and press the shoulders of the gauge flat against the tread block. Read the measurement.
The Penny Test (for U.S. users): Take a Lincoln penny and place it with Lincoln's head upside down in a tread groove. If you can see the top of Lincoln's head, your tread is at or below 2/32″ and needs immediate replacement.
Built-in Wear Bars: Most modern tires have indicator bars molded into the bottom of the tread grooves. When the tread wears down to be even with these bars, the tire is at the 2/32″ legal limit and must be replaced.
How to Use the Tire Tread Depth Calculator
Select Unit: Choose whether you are measuring in "32nds of an Inch" or "Millimeters". The calculator will adjust its defaults.
Enter New Tire Depth: Input the tread depth of your tires when they were new. If you don't know, the common default values (10/32″ or 8mm) are a good estimate.
Enter Current Depth: Input the tread depth you just measured from your tire.
Calculate: Click the "Calculate Remaining Life" button to see your results. The calculator will show the percentage of usable tread life left and provide a safety recommendation.
Example Calculation
Let's say your new all-season tires started with 10/32″ of tread. The legal limit is 2/32″. This means you have 8/32″ of usable tread (10 – 2 = 8).
You measure your current tread and find it is 5/32″. The remaining usable tread is 3/32″ (5 – 2 = 3).
To find the percentage of life remaining, the calculator does: (Remaining Usable Tread / Total Usable Tread) * 100 = (3 / 8) * 100 = 37.5%. At this level, your tires are still safe but are showing significant wear.
Frequently Asked Questions (FAQ)
When should I replace my tires?
For maximum safety, it's recommended to replace tires when they reach 4/32″ (about 3.2mm) of tread depth. For winter conditions, consider replacing them at 6/32″ (about 4.8mm) to ensure good traction on snow and ice. Never wait until they are at the 2/32″ legal limit.
What is hydroplaning?
Hydroplaning occurs when a layer of water builds up between your tires and the road surface, leading to a loss of traction and control. Tires with deep tread grooves are effective at channeling this water away. As tread wears down, the tire's ability to resist hydroplaning decreases dramatically.
Do all my tires wear out at the same rate?
Not usually. Front tires on a front-wheel-drive car often wear faster. Regular tire rotations (typically every 5,000-7,000 miles) help ensure they wear evenly, extending their overall life.