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How to use a depth gauge

Updated: 22 October 2026

The hole looks deeper than the rebate you set, and the gauge in your hand is the suspect. Most of the time the base lifted on a chip, the zero was taken in mid-air, or the vernier was read at an angle. This is how to use a depth gauge so one reading is a depth and not a slant. It assumes you already own one of the tools in the [depth gauge comparison](/hub/measuring-and-marking-tools/depth-gauge). Peter F. used the maker notes and the UK review notes in this batch, not a filmed workshop test. A qualifying purchase can earn a commission at no extra cost to you.

The hole looks deeper than the rebate you set, and the gauge in your hand is the suspect. Most of the time the base lifted on a chip, the zero was taken in mid-air, or the vernier was read at an angle.

Our top picks

Trend Trend GAUGE/D60

Best overall

Trend Trend GAUGE/D60
9.4/10about £19.49
View offer
PRAZTECH PRAZTECH 150 mm rule-head

Best value

PRAZTECH PRAZTECH 150 mm rule-head
7.7/10about £8.99
View offer
KATSU Tools KATSU 200 mm vernier

Alternative

KATSU Tools KATSU 200 mm vernier
7.3/10about £11.99
View offer
On this page
  1. 1.Seat the base before you trust the number
  2. 2.Zero, then read
  3. 3.Lock it, then move the gauge away
  4. 4.One seated reading, then stop

Seat the base before you trust the number

Wipe oil and chips off the face and the base first. Mitutoyo's manual puts that first, because a chip under one shoulder tips the head and the beam measures a diagonal. Press the whole base down until it does not rock. If the base is not on the face you chose, the distance from beam end to base is not the depth you wanted. A narrow caliper rod in a wide step misses the shelf and reads deep. UK reviewers use the KATSU's wide head when a caliper rod cannot span the cavity, and one review puts the shoulders at about 102 mm. The Trend's 60 mm jaw is a fence, not a hole. Clearance for it is on the router table depth gauge page. If two gauges disagree, reseat both. Do not average them.

Zero, then read

Zero on a flat you trust, base in firm contact and beam end on that same flat, so the gauge reads zero before it reads a hole. Do not slide the base while you zero, and do not zero in mid-air or on the lip of the hole. On a digital, press zero only after that contact. On a rule-head, the rule end should be flush with the head. On the KATSU, flush means the scales say zero. Read the main-scale millimetres at the vernier zero, then add the one division that lines up. Do not promote a neighbour because you looked from the side. If the scale is double-printed, stop and use a lamp. On a digital, confirm millimetres before you cut. A unit button left on inches turns a 12 mm rebate into a fraction. Zero anywhere is for a comparison from a shoulder, not for absolute depth from the base. Which listing is worth the cell is the vernier or digital depth gauge page.
Trend Trend GAUGE/D60

Testsieger

Trend Trend GAUGE/D60
9.4/10about £19.49
View offer

Lock it, then move the gauge away

Lock only after the base is seated and the beam touches the floor. On the PRAZTECH and the MachEl that is a nut. On the KATSU it is a screw, and one UK review says the thread failed almost immediately: a lock that spins is a return, not a technique. On the Trend the lock is plastic, and UK reviews say it strips if you tighten it hard. Snug is enough. Lift the gauge clear before you start the machine. Mitutoyo's manual says not to measure rotating work. Trend's sheet excludes high-vibration machines, and the magnets are for a stopped iron top, not a running motor. Set the height, lock lightly, read, remove the gauge, then switch on. A flat-plate zero shows that this gauge repeats today. It is not a UKAS calibration and it does not certify the seller's accuracy line.

One seated reading, then stop

If the base rocks, fix the seat before you recut. If the zero was taken in the air, take it again on a flat. If the vernier was read from the side, look straight on and add one lined-up mark. If two gauges still disagree after that, write down a stable offset or retire the bent base. Choosing a different shape is a buying question, not the next step while the cutter is still stopped.

Common questions

Where do I zero the gauge?
On a flat plate or a face you trust, with the whole base in contact and the beam end on that same surface, so the reading is zero before you measure a hole. Do not zero in mid-air, and do not slide the base while the display or the scale is being set. A KATSU that shows 0.12 mm or 0.3 mm on that flat is carrying that offset into the hole.
How do I read a vernier without adding the wrong line?
Read the main-scale millimetres at the vernier zero, then add the single vernier mark that lines up with a main-scale mark. Look straight at the scale so parallax does not promote the neighbouring line. If the graduations are smeared or double-printed, which shows up in KATSU's UK reviews, do not guess the lined-up mark.
Why is the hole deeper than I set?
Usually the base rocked, the beam stopped on a chip or on the side of a tight hole, or the zero was taken somewhere other than a flush face. A digital left on inches will also change the only units that matter. Seat it again before you recut. Two gauges that still disagree need a zero check, not an average.
Can I check the depth while the cutter is spinning?
No. Mitutoyo's manual forbids measuring rotating work, and Trend's sheet is not for a machine that is vibrating under power. Set the stopped cutter or blade, remove the gauge, then start the motor. A magnet on the base is not a permit to leave the jaw in the cut.
What if two gauges disagree?
Reseat both on the same face and the same floor. If they still differ, one of them has a zero offset or a base that is not flat. Write the offset down if it is stable. Do not split the difference and cut to the midpoint. A bent base is a reason to stop using that gauge.

Related pages

Depth gauge buying guide

Depth gauge

Best depth gauge: name the job, then keep one tool

Router table depth gauge for cutter height and fence

Vernier or digital depth gauge: parallax or a cell

Sources

Public UK sources and the amazon.co.uk listings compared on this page. Listings captured 20 October 2026.

  1. ISO 13385-2:2020, calliper depth gauges (retrieved 20 October 2026)
  2. BSI BS EN ISO 13385-2:2020 (retrieved 20 October 2026)
  3. ISO 13385-2:2020 preview (retrieved 20 October 2026)
  4. Trend GAUGE/D60, UK product page (retrieved 20 October 2026)
  5. Cutwel, depth gauges guide (retrieved 20 October 2026)
  6. Mitutoyo depth-gauge manual (retrieved 20 October 2026)
  7. GOV.UK, General Product Safety Regulations 2005 (retrieved 20 October 2026)
  8. Consumer Rights Act 2015, section 9 (retrieved 20 October 2026)