Adjustable Wrenches: How to Use One Without Rounding Every Nut in Sight

You tighten a compression fitting with your fingers, then give the nut a turn with an adjustable wrench. Feels snug. You turn the water on, walk away, and come back to a puddle. The fitting body rotated when you tightened the nut, which means the olive never compressed evenly onto the pipe. Or worse, you grabbed the wrench the wrong way round, the jaw slipped, and now the nut has rounded corners that no tool will grip properly. A replacement fitting costs £2 – £4. The water damage from the leak costs considerably more. Two decent adjustable wrenches and five minutes reading this page prevent both problems.

What it is and when you need one

An adjustable wrench (called an adjustable spanner in most UK shops) has one fixed jaw and one movable jaw connected by a worm screw mechanism (a small thumbwheel that slides the movable jaw in and out). You turn the thumbwheel to open or close the jaws until they fit snugly around a nut or bolt head. One tool covers a range of fastener sizes instead of needing a separate fixed spanner for each.

The tool was invented by a Swedish engineer called Johan Petter Johansson, who patented the single-movable-jaw design in 1892. His company eventually became Bahco, which is still the name most plumbers reach for. Over 100 million adjustable wrenches have been manufactured since then, and the basic design hasn't changed because it doesn't need to.

In plumbing, you'll use adjustable wrenches constantly. Every compression fitting on copper pipe has a hexagonal nut that needs tightening. Every tap connector, isolation valve, and appliance connection uses one. They're a beginner-level tool with no learning curve beyond one critical technique: which direction to apply force. Get that right and it's the most useful tool in your plumbing kit. Get it wrong and you'll round nuts all day.

Types and what to look for

Not all adjustable wrenches are the same. The detail that matters most for plumbing is jaw width, and most homeowner guides skip it entirely.

Standard jaw vs wide jaw

A standard adjustable wrench opens to around 24-27mm on an 8-inch (200mm) model. That's fine for general DIY, small bolts, and non-plumbing tasks. But compression fitting nuts on 22mm copper pipe measure 32mm across the flats. A standard-jaw 8-inch wrench won't open wide enough.

A wide jaw model of the same length opens to 38mm or more. That's the one you want. Wide jaw wrenches have a slimmer head profile too, which helps in the tight spaces under sinks and behind baths where plumbing connections live.

Sizes and what each one does

For a kitchen extension or bathroom renovation, buy two: a 6-inch and an 8-inch. The 8-inch handles 90% of the work. The 6-inch gets into spaces where the 8-inch won't fit, like behind a pedestal basin or between closely spaced pipes under a kitchen sink. Every plumber carries at least these two sizes.

What about the steel?

Adjustable wrenches are made from chrome vanadium steel (often stamped "Cr-V" on the tool). This is harder than standard carbon steel and resists the jaws deforming when you apply force to flat fastener surfaces. Chrome molybdenum ("Cr-Mo") is an alternative that's tougher under impact but less common in wrenches. For adjustable wrenches, Cr-V is the standard and the right choice. You don't need to think about this further.

What actually matters is the tolerance of the worm gear. Cheap wrenches have excessive play in the adjustment mechanism, so the movable jaw wobbles even when tightened. That wobble is the gap between jaw and nut that rounds corners. Quality wrenches have minimal play. You can feel the difference in the shop: turn the thumbwheel on a Bahco and it's smooth with zero slop. Do the same on a budget wrench and the jaw rocks side to side.

How to use it properly

The one rule that prevents rounded nuts

Apply force toward the fixed jaw.

Click to enlarge

The fixed jaw (the one that doesn't move) is structurally part of the wrench body. It can take the full load. The movable jaw is held in position by the worm screw, which is the weakest part of the tool. If you apply force toward the movable jaw, the load tries to push the jaw open. The worm screw flexes, a gap appears between jaw and nut, and the wrench slips. That slip is what rounds the corners off brass and steel nuts.

Position the wrench so that when you pull or push, the reaction force drives into the fixed jaw. On most wrenches, the fixed jaw is on the side nearest the handle. Look at your wrench, find the jaw that doesn't move, and make sure your turning force pushes the nut into that jaw. This becomes instinctive after a few uses.

Tightening compression fittings

This is the task you'll perform most often during plumbing work on an extension. A compression fitting seals a copper pipe joint by squeezing a soft metal ring called an olive onto the pipe when you tighten the nut.

The technique requires two wrenches. One holds the fitting body stationary. The other tightens the nut.

Slide the nut onto the pipe, then the olive, then push the pipe into the fitting body. Hand-tighten the nut until it's finger-tight and you feel the olive contact the fitting. Now hold the fitting body with one wrench (to stop the whole assembly rotating) and tighten the nut with the second wrench.

For reference, 15mm compression fitting nuts measure 24mm across the flats. 22mm fittings measure 32mm across the flats, though this varies slightly between manufacturers. A wide jaw 8-inch wrench covers both sizes comfortably.

Why two wrenches, not one?

If you tighten the nut with one wrench while holding the fitting body with your other hand, the body will rotate. On a fitting that's already connected to a pipe run, that rotation stresses the pipe and the joint you just made further along. On a new installation, it means the olive compresses unevenly, and one side of the seal is tighter than the other. That uneven compression leaks.

Two wrenches. Always. This is why every plumber owns at least a pair of adjustable spanners, and why you should too.

Threaded connections

For threaded fittings (as opposed to compression), wrap the male thread with PTFE tape before tightening. The wrench tightens the connection; the PTFE provides the seal. Without tape, a threaded joint weeps. Without the wrench, the joint is finger-tight and will work loose under vibration.

How to check your wrench is working

There's no calibration test like a spirit level flip test. But there is one check worth doing.

Close the jaws fully by turning the thumbwheel until the movable jaw meets the fixed jaw. Look at the contact line. The jaws should meet evenly along their full width with no gap. Now try to rock the movable jaw side to side with your finger. On a quality wrench, there's no perceptible movement. On a worn or cheap wrench, the jaw rocks visibly.

That rock is the play that causes slipping. If your wrench has it, the tool is either worn out or was never good enough. Replace it. A wrench that rounds one brass compression nut has cost you more in replacement fittings than a decent wrench costs to buy.

What to buy

Budget: £8-12

At this tier, you're looking at own-brand wrenches from the big retailers. The Magnusson 6-inch from Screwfix (around £9) or the Minotaur range from Toolstation (6-inch at £8 8-inch at £11) are typical. The Forge Steel 3-piece set from Screwfix at £10 gets you three sizes in one go, which sounds appealing.

These work for occasional light use. Tightening a tap connector once or twice, adjusting a garden hose fitting. But the jaw play on budget wrenches is noticeable, and for compression fittings where grip precision matters, they frustrate more than they help. Community forums are blunt on this point: "cheap adjustables are awful."

If this is genuinely a one-off job (connecting a washing machine, replacing a tap), a budget wrench will get you through. For an extension project where you're making dozens of compression joints, spend more.

Mid-range: TBC

This is the sweet spot for homeowners managing a build. Two brands stand out.

Stanley makes a solid 8-inch at around £12.5 from Screwfix. It's half the price of a Bahco, the jaw tolerance is acceptable, and it'll last a full extension project without complaint. A good starter wrench.

Draper Professional offers similar quality. The 8-inch is £12.5 at Toolstation, the 10-inch is £16.5. Chrome vanadium steel, reasonable jaw tightness.

Monument Wide Jaw models sit at the top of this range (8-inch at £21 – £26 depending on retailer). Monument is a UK plumbing tool manufacturer, and their wide jaw adjustables are designed specifically for plumbing work. The phosphate finish gives better corrosion resistance than chrome, though it looks less shiny. One ergonomic gripe noted in trade reviews: the hole in the handle end can dig into your palm during extended use.

Premium: £23-39

Bahco. There's a reason eight out of nine trade forum threads recommend them. The worm gear tolerance is visibly tighter than anything else on the shelf. The jaw opens and closes smoothly with zero play. The Ergo range adds a rubber grip and reversible jaws (one side smooth for chrome fittings, one side micro-serrated for extra grip on corroded nuts).

Specific models worth knowing:

• Bahco 9070 (6-inch standard): around £27 at Screwfix. Your tight-access wrench.
• Bahco 9031 (8-inch wide jaw): the most recommended single adjustable wrench across UK trade forums. Around £23 – £33 depending on retailer and model variant. 38mm jaw opening. This is the one to buy if you're buying one.
• Bahco 9031 Ergo (8-inch wide jaw with rubber grip): around £29 – £34. The reversible jaw feature is genuinely useful if you're working on both new brass fittings and older corroded ones.

For an extension project, a Bahco 8-inch wide jaw and a 6-inch (either Bahco or a mid-range alternative for the less-used size) is the combination that makes trade professionals say "that'll do the job." Total outlay: £35 – £60 for the pair.

The two-wrench kit

For a kitchen extension, buy:

• One 8-inch wide jaw (primary wrench, compression fittings): £12 – £33
• One 6-inch (tight access, secondary grip): £8 – £27
• Total: £20 – £60 depending on brand

That pair covers every adjustable wrench task across first-fix and second-fix plumbing.

Adjustable wrench vs pipe wrench

These are different tools for different jobs. Don't use one where you need the other.

An adjustable wrench has smooth, parallel jaws. It grips flat-sided fasteners: hexagonal nuts, square bolt heads, compression fitting bodies. The smooth jaws don't mark the surface, which matters on chrome tap connectors and visible fittings.

A pipe wrench has serrated, angled jaws that bite into round surfaces. It grips cylindrical pipes and round fittings where an adjustable wrench would slip. The serrations will scar any polished or chrome surface.

Use the adjustable wrench for compression nuts, tap connectors, isolation valve bodies, and any fitting with flat sides. Use the pipe wrench for gripping round pipe, stubborn threaded iron pipe, and anything cylindrical that needs turning.

Where you'll need this

• First fix plumbing - tightening compression fittings, valves, and connectors on pipe runs
• Second fix plumbing - tightening compression fittings on tap connectors, isolation valves, and appliance connections

Adjustable wrenches appear at every plumbing stage of any extension or renovation project. They're used whenever copper pipe meets a fitting, which happens dozens of times across first-fix and second-fix work.

Common mistakes

Using it backwards. Force toward the movable jaw. The jaw slips, the nut rounds, the fitting is ruined. Position the wrench so pulling force drives into the fixed jaw. Every time.

Leaving a gap between jaw and nut. The jaws must be snug before you apply force. Tighten the thumbwheel until both jaws contact the nut with zero wobble, then turn. A gap of even 1mm lets the wrench slip under load.

Using one wrench on a compression fitting. The fitting body rotates, the olive seats unevenly, the joint leaks. Hold the body with one wrench, tighten the nut with the other.

Over-tightening compression fittings. Follow the tightening technique described above. A crushed olive leaks worse than a loose one because the damage is permanent. If a joint drips, try an eighth of a turn more before assuming it needs dismantling.

Buying standard jaw instead of wide jaw. A standard 8-inch wrench won't open to 32mm. You'll stand in the kitchen at 7pm on a Saturday with a wrench that won't fit the 22mm compression nut, and the nearest Screwfix closes in ten minutes. Buy wide jaw from the start.