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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsMeasure the fastener head across two opposite, parallel flat faces. That across-flats dimension is the width an adjustable wrench must grip. Choose a wrench whose maximum jaw opening is at least that wide, then close the movable jaw until both jaws sit firmly on the flats with almost no play.
This measurement tells you the wrench size, not the fastener’s thread diameter or pitch. If you are identifying a replacement fastener, measure the head and the threaded section separately.
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What “across flats” means
A conventional hexagonal nut or bolt head has six flat faces. Across-flats (AF) is the straight-line distance between two opposite, parallel faces. It is the dimension used for wrench openings and socket sizes. Gera Tools’ size reference, updated 6 June 2026, defines the dimension this way, and Portland Bolt explains that wrench openings are determined by it.
Do not measure from one corner of the hex to the opposite corner. That corner-to-corner distance is larger than the working width and will lead you to choose a jaw opening that is too large. An oversized or loose wrench can slip and round the corners of the fastener; U.S. Department of Education training material specifically warns about this damage risk.
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Tools you need
- A ruler or steel rule: adequate when the flats are clean, accessible and large enough to read clearly.
- A caliper: optional, but useful for a small, awkward, painted or worn head. Measure from flat face to flat face, not from the tips of the jaws over the corners.
- Cleaning material: a brush or rag can remove dirt, rust scale or burrs that would increase the apparent width.
- The wrench’s specification or jaw scale: use it to check maximum opening and to set an approximate position before making the final fit.
You do not need a specialized measuring instrument for every job. A straight rule is normally sufficient when the two faces are visible and undamaged.
Step-by-step: measure the head correctly
- Expose two opposite flats. Wipe the nut or bolt head and identify a pair of parallel faces. On a six-sided head, they are the faces directly opposite each other.
- Place the measuring tool across those faces. The rule or caliper should contact the flat surfaces, with the measurement line running from one face to the other. Keep the tool square to the faces; do not angle it across the head.
- Read the across-flats width. With a ruler, read the nearest marked increment. With a caliper, close the jaws gently until they touch the flats without squeezing or sliding onto the corners.
- Record the number and its unit. Write down millimetres or inches exactly as read. If your wrench specification uses the other unit, convert with a calculator or a reliable conversion chart rather than selecting a visually similar size.
- Check the result. Repeat the measurement on another pair of opposite flats. A substantial difference suggests dirt, damage, an angled tool or a non-hexagonal head.
Why the tool must span flats, not corners
The wrench jaws are intended to load the broad flat faces. Measuring corners describes the head’s maximum diagonal, not the opening needed to grip it. A wrench adjusted to that larger number leaves clearance at the flats, so force is concentrated on corners and the fastener is more likely to slip or deform.
Turn the measurement into a wrench choice
The measured AF width is the minimum jaw-opening capacity you need. Compare it with the manufacturer’s stated maximum jaw opening, usually given in millimetres or inches. The maximum must meet or exceed your measurement; it is not the same as the wrench’s overall length.
| Selection check | What to verify | Why it matters |
|---|---|---|
| Maximum opening | At least the measured across-flats width | A wrench that cannot open far enough will not reach both flats. |
| Units and scale | Metric, SAE (inch), or both; readable jaw markings | Markings help position the jaw, but the actual fit remains the final test. |
| Jaw condition | Jaws meet evenly, move smoothly and have no obvious damage | Excess clearance or a bent jaw can let the tool slip and damage the head. |
| Physical access | Head thickness, handle length and room to swing | A capacity that fits on paper may still be unusable in a confined location. |
Set the opening slightly wider than the measured head, place both jaws fully on the flats, and turn the knurl until the gap is reduced to the point where there is little perceptible movement. Do not force the jaws diagonally or grip only one edge.
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What product specifications can and cannot tell you
Manufacturer listings illustrate why capacity must be checked. Würth article 071522108 lists a metric scale and a maximum across-flats opening of 30 mm. That 30 mm figure applies to that particular article; it is not a universal capacity for adjustable wrenches.
Stanley model 87-473 is described as a 12-inch adjustable wrench with an adjustable jaw, knurl adjustment and laser-etched SAE and metric jaw scale. “12-inch” is the nominal wrench length, not the maximum opening. The listing’s scales can help you set a starting position, but you still need to seat the jaws on the fastener.
Fit the wrench before applying force
- Open the jaw enough to pass over the head. Keep the wrench aligned with the fastener rather than approaching at an angle.
- Seat both jaws on full flats. The fixed and movable jaws should contact broad, opposite faces. Neither jaw should rest on a corner, washer edge or nearby obstruction.
- Close the movable jaw. Turn the knurl until the jaws touch firmly. A tiny clearance that disappears when you apply light hand pressure is preferable to a visibly loose fit.
- Check for rocking. Try to move the wrench by hand without turning the fastener. If it rocks, readjust it or use a better-fitting tool.
- Apply force in the correct direction. Keep the wrench square to the head and pull in a controlled manner. If the jaw opens, slips or leaves marks, stop and reassess the fit.
Adjustable-wrench performance includes jaw-clearance requirements. BS ISO 6787:2018 specifies adjustable-wrench dimensions, admissible adjustable-jaw clearance and performance-test conditions. The standard does not make a worn tool safe; inspect the individual wrench and its engagement with the fastener.
Do not mistake head width for thread size
A 17 mm or 5/8-inch AF reading identifies the width of the head that the wrench must grip. It does not establish the threaded shank’s outside diameter, thread pitch or whether the thread is metric, UNC, UNF or another system. Different fasteners can share a head width while having different threads, and manufacturers may use different head dimensions for similar thread sizes.
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If you need to identify or replace the fastener, measure the threaded portion separately:
- Measure the outside diameter of the male thread (or the corresponding internal diameter of a nut) with a suitable rule or caliper.
- Determine pitch by measuring the distance between thread crests for a metric thread, or threads per inch for an inch thread. A thread-pitch gauge is useful when the distinction is uncertain.
- Record the head AF, thread diameter, pitch and any relevant length or grade marking as separate pieces of information.
Use the AF value only for selecting the gripping tool. Use the thread measurements and markings for fastener identification.
When the measurement is difficult
Paint, rust or debris on the flats
Remove loose coating or corrosion from the contact areas and measure again. Do not file away sound metal merely to make a number match a chart. If corrosion has changed the flats, measure the remaining parallel surfaces and choose a wrench that fits the actual head; a heavily damaged head may require a different removal method rather than more jaw pressure.
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Because AF is defined by the flats, rounded corners do not automatically change the nominal dimension. They do reduce the area available to a wrench. Close the jaws carefully on the least-damaged pair of flats and reject any wrench that rocks. If the flats themselves are rounded, an adjustable wrench may continue to slip; a purpose-designed extraction tool or replacement fastener may be safer.
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Very small heads
A ruler may be too coarse for a small fastener. Use a caliper, keep its jaws parallel to the flats and apply only light pressure. Confirm the reading by checking whether the selected wrench actually seats without play.
Limited access
Measure before entering the confined space if possible, then compare the required opening with the wrench’s head thickness and handle length. A longer handle is not automatically better: it may not fit or may encourage excessive force. Choose dimensions that allow the jaws to remain square while you turn.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Reading scales and converting units
Some adjustable wrenches have etched SAE and metric scales. Treat a scale as a setup aid, not as a substitute for checking the fit. Markings can be difficult to read, and the fastener may not conform exactly to a nominal increment.
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Keep the unit attached to every measurement. If the head measures in millimetres and the wrench is marked in inches, perform an explicit conversion and then verify the physical fit. Do not assume that a nearby metric and inch size are interchangeable; a small difference can create enough clearance to damage the corners.
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| Situation | Best practice | Reason |
|---|---|---|
| Known metric fastener | Measure in millimetres and use a metric-marked scale when available. | Matching units reduces setup errors. |
| Known inch fastener | Measure in inches or convert deliberately, then test the jaws. | Nominally close sizes may not fit tightly. |
| Unknown fastener | Measure the head first; identify thread diameter and pitch separately. | AF alone cannot identify the thread. |
Worked examples
Example: a clean metric nut
You measure opposite flats and obtain 24 mm. Select an adjustable wrench whose published maximum opening is at least 24 mm. Open the jaws, place them on the two 24 mm flats, and close the knurl until the nut cannot rock. The 24 mm value tells you the required jaw opening; it does not tell you the nut’s thread diameter.
Example: a head that exceeds the wrench capacity
Your caliper reads 31 mm, while the wrench specification lists a 30 mm maximum opening. Do not force the 30 mm tool over the head or rely on an angled grip. Choose a wrench with a stated capacity above 31 mm and enough physical access for the job. The Würth 30 mm listing is an example of a product limit, not a recommendation for this situation.
Example: an inch-marked tool on a metric head
A metric head measures 19 mm, but the available wrench has only an inch scale. Convert the measurement carefully, set the jaws near the result, and then close them on the flats. If the jaws do not seat without play, use a better-matched tool instead of accepting a close-looking mark.
Common mistakes and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| The wrench slips immediately | Measured corners, left the jaw too open or approached at an angle | Re-measure opposite flats and close the jaw until both faces are fully seated. |
| One corner becomes shiny or rounded | Excess jaw clearance or damaged wrench jaws | Stop, inspect the jaws and use a tighter-fitting, undamaged wrench. |
| The wrench will not pass over the head | Maximum opening is smaller than the AF width | Use a wrench with greater published capacity. |
| The measurement changes each time | Dirt, burrs, an angled tool or damaged flats | Clean the head, hold the tool square and measure another pair of flats. |
| The fastener replacement is wrong despite a matching wrench | Head width was mistaken for thread size | Measure thread diameter and pitch independently. |
How to evaluate an adjustable wrench before buying
When several tools appear suitable, compare the specifications in this order:
- Capacity: eliminate any model whose maximum opening is below your measured AF width.
- Jaw clearance and movement: turn the knurl and inspect whether the movable jaw tracks evenly. A nominal capacity is useful only if the jaws remain parallel and stable.
- Scale readability: metric and SAE markings can speed setup when their units match the fastener. They do not replace a fit check.
- Access dimensions: consider head thickness, handle length and the available swing arc. The longest wrench is not automatically the right one.
- Condition: reject jaws with chips, spreading, severe wear or visible misalignment, especially for a fastener that must not be damaged.
BS ISO 6787:2018 is a useful reference for the kinds of dimensions, jaw-clearance limits and performance tests addressed for adjustable wrenches. It is a standard for tool characteristics, not a guarantee that every tool in service remains within those limits.
Quick Recap
Quick checklist
- Locate two opposite, parallel flats.
- Measure across those flats, perpendicular to them.
- Record the value and unit; repeat if the result is uncertain.
- Keep head width separate from thread diameter and pitch.
- Choose a wrench whose maximum jaw opening meets or exceeds the AF measurement.
- Use the scale only to set a starting position.
- Seat both jaws fully on the flats and close the knurl until there is almost no play.
- Stop if the wrench rocks, slips or marks the corners; inspect the tool and choose a better fit.
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