Where the 20p test answers "safe or not", a tread depth gauge answers "how much exactly". It turns a rough check into a number, which is what is needed to track wear over time or judge a tyre sitting close to the limit.
The types
All tread gauges do the same job, measure the depth of a groove, in slightly different ways:
- Digital: a probe and a screen giving a precise millimetre reading; the easiest to read accurately
- Dial or needle: a sprung probe driving a needle around a dial; no battery to flatten
- Stick or depth-probe: a simple sliding pin with a printed scale; cheap, pocket-sized and perfectly adequate
Any of them is inexpensive, and any of them beats guessing. The accuracy that matters comes from how the gauge is used, not how much it cost.
Taking a reading
The method is the same whichever type is in hand:
- Sit the gauge's flat base across the tread, spanning a main groove
- Push the probe down into the groove until it reaches the bottom
- Hold it steady and read the depth in millimetres
- Lift, move to another spot, and repeat
A new tyre reads around 8mm. The legal minimum is 1.6mm, and the tyre's own wear indicator bars sit at exactly that depth, so a groove reading 1.6mm is one where the tread has reached those bars.
Measure in the right place
A gauge only gives a true answer if it is used in the right spot:
- Always in a main circumferential groove, never on a wear bar or in a sipe, those would read shallow and mislead
- At several points across the tread and around the tyre
- Recording the lowest reading, since that is the part the law and safety are judged on
If one side reads much lower than the other, the cause is worth chasing down through the wear pattern before a new tyre simply wears the same way.
What to do with the number
A reading is only useful if it prompts a decision. As a rough guide: around 3mm is the point many drivers start planning a replacement, because wet braking lengthens noticeably below it; 1.6mm is the legal and safety floor, not a target. Depth is only part of it, the right time to replace a tyre also takes in age and condition.
From the workshop: a stick gauge that lives in the door pocket gets used; a fancy one in a drawer at home doesn't. Whichever you buy, keep it in the car, and always read the worst groove, not the deepest one that makes the tyre look better than it is.
Sources and accuracy. The methods and depth guidance here reflect common practice and TyreSafe advice at the time of writing. The 1.6mm legal minimum is definitive; replacement points above it are judgement calls. If anything here looks wrong, get in touch and we will check it and put it right.
Common questions
How do you use a tread depth gauge?+
Sit the gauge's base flat on the tread, push the probe down into a main groove until it reaches the bottom, and read the depth in millimetres. Take readings at several points and go by the lowest one.
What is a good tread depth in mm?+
A new tyre starts at around 8mm. The legal minimum is 1.6mm, but wet grip drops off well before then, so many drivers plan to replace at around 3mm rather than running to the limit.
Where do I measure with the gauge?+
In the main circumferential grooves, not on the raised wear bars or in the fine sipes, which would give a false, shallow reading. Check inner, middle and outer grooves and around the tyre.
Are cheap tread depth gauges accurate enough?+
Yes. An inexpensive digital or stick gauge is plenty accurate for checking against the 1.6mm limit and tracking wear. The key is using it consistently in the main grooves rather than the precision of the gauge itself.