Woodruff Key Size Chart: How to Read and Select Key Numbers

A woodruff key size chart does one job: it connects the key’s four-digit number to the two dimensions that define the part — key width and the nominal diameter of its semicircular back. Once you can read that numbering system, a spec sheet that says “key no. 808” tells you the key is 1/4 in wide and fits a seat with a 1 in diameter arc, no chart lookup required. That is the difference between memorizing a table and understanding a system.

This guide explains how woodruff key sizes are encoded under ANSI B17.2, works through real key numbers digit by digit, shows how keyseat depth and key height relate to the numbered dimensions, and walks through measuring an unmarked key and selecting a size for a given shaft. It also covers where the inch-system keys meet their metric counterpart in DIN 6888, and where woodruff keys fit in a shop that cuts keyways. For the key’s geometry, self-aligning behavior, and strengths and weaknesses as a torque transmitter, see our companion article What Is a Woodruff Key?; this page stays with sizes and selection.

What a Woodruff Key Size Chart Actually Tells You

A typical woodruff key size chart lists, for each key number: the key width, the nominal diameter (the diameter of the semicircle, which is also the diameter of the shaft keyseat arc and of the cutter that produces it), the total key height, and the keyseat depth and height-above-shaft values the shop needs to machine the seat and the hub keyway. The chart is dense because every row carries a full machining recipe, not just a part size.

The good news is that you rarely need the chart to decode a key’s two headline dimensions. The American numbering system encodes them directly in the key number itself. Learn the rule once and the first two columns of any chart — width and nominal diameter — become self-checking: if a supplier’s chart lists a no. 1210 key at some width other than 3/8 in, the chart is describing a different series or contains a typo, and you have caught it before the parts ship. Everything else on the chart (seat depth, height above shaft, hub keyway dimensions) still comes from the standard: per ANSI B17.2, always verify those values against the standard itself for production work.

How to Read Woodruff Key Numbers: The Four-Digit Rule

The key number is a four-digit code with two halves:

• First two digits = key width in 1/32 inch units. Divide by 32 to get width in inches.
• Last two digits = nominal diameter in 1/8 inch units. Divide by 8 to get the diameter of the key’s semicircular back in inches — the same diameter as the keyseat arc in the shaft.

Read as two pairs, the number tells you the key’s shape outright. This decoding rule applies across the common series, from the small 200-series keys used on instrument-scale shafts up through the wide, large-diameter keys at the top of the range. Three worked examples show how mechanical the decoding is.

Example 1: Key No. 808

Split 808 into 08–08. The first pair, 08, gives width: 8/32 in = 1/4 in. The second pair, 08, gives nominal diameter: 8/8 in = 1 in. So a no. 808 woodruff key is 1/4 in wide with a 1 in nominal diameter — one of the most widely used sizes in general machinery, a common choice on shafts in the 1 in class.

Example 2: Key No. 204

Split 204 into 02–04. Width = 2/32 in = 1/16 in. Nominal diameter = 4/8 in = 1/2 in. The 204 is a small key: 1/16 in wide on a 1/2 in arc, typical of fractional-horsepower motors, small pumps, and automotive accessory shafts where torque is light and shaft diameter is modest.

Example 3: Key No. 1210

Split 1210 into 12–10. Width = 12/32 in = 3/8 in. Nominal diameter = 10/8 in = 1 1/4 in. Note that this example is why you read the number as digit pairs, not as “twelve hundred and ten”: the 12 belongs to width, the 10 to diameter. The 1210 is a medium-heavy key that shows up on larger motor and pump shafts.

One caution: the rule decodes the number, it does not replace the standard. Key height, keyseat depth, and tolerances are separate tabulated values — per ANSI B17.2, verify against the standard before releasing a design. A handful of special-purpose series also exist outside the plain four-digit scheme, so if a number does not decode to a sensible width and diameter, check the supplier’s drawing before ordering.

Woodruff Key Sizes: Representative Chart

The table below lists representative woodruff key sizes across the practical range, with each width and nominal diameter decoded by the four-digit rule above. The final column gives the order of magnitude of shaft size each family serves in common practice — a selection aid, not a specification. Per ANSI B17.2, verify shaft-size recommendations and all keyseat values against the standard’s own tables before finalizing a design.

*Order-of-magnitude guidance only — per ANSI B17.2, verify against the standard. Notice the pattern the table makes visible: for a given nominal diameter, a larger first pair means a wider (and proportionally stronger) key; for a given width, a larger second pair means a deeper seat and a key with more metal in the shaft. Width sets torque capacity; nominal diameter sets how deeply the key seats and how it distributes load.

Keyseat Depth, Key Height, and How the Dimensions Fit Together

Three machined values surround every woodruff key, and a size chart exists mainly to coordinate them:

• Keyseat diameter. The semicircular pocket in the shaft has the same diameter as the key’s nominal diameter — the number encoded in the last two digits. This is why the same number also identifies the woodruff keyseat cutter: a no. 808 key needs a cutter with a 1 in diameter arc.
• Keyseat depth. How deep the pocket is milled into the shaft. Deeper seats bury more of the key; shallower seats leave more key exposed. The standard fixes the depth for each key number so that keys and seats from any manufacturer interchange.
• Height above shaft. How much the key stands proud of the shaft surface once seated. The hub keyway is sized to engage this exposed portion, so its depth and width must match the chart values for the same key number — the hub side is a plain rectangular slot, sized with a small clearance over the key width.

The logic of the system is interchangeability by number. Buy any manufacturer’s no. 608 key, mill the shaft seat with a 3/4 in-arc cutter set to the standard depth, cut the hub keyway to the standard width and depth, and the assembly fits — with the key deep enough in the shaft to resist rolling and enough height above the shaft to carry torque into the hub. That coordination is exactly what a raw dimensions discussion of keys and keyways covers for square and rectangular keys in our related guide to keyway dimensions and standards; the woodruff system achieves the same coordination through its numbered series.

How to Measure a Woodruff Key

When you hold an unmarked key — a spare from a bin, a key salvaged during a repair — two measurements identify it:

• Width. Measure across the two parallel flat sides with a micrometer or calipers. Multiply the reading by 32 to get the first two digits of the key number (a 0.250 in width gives 32 × 0.250 = 8, so the number starts with 08).
• Height. Place the key on a surface plate with its curved back down and measure from the plate to the flat top. This total height is a tabulated value for each key number, and combined with the width it usually pins the number down to one row of the chart.

The nominal diameter is the one dimension you cannot read directly off the key with hand tools — it is the diameter of an arc, and the arc is less than a full circle. Three practical routes close the gap. First, and most common: match your measured width and height against the ANSI B17.2 tables, which list height for every key number. Second, measure the shaft it came from: the keyseat arc’s diameter equals the key’s nominal diameter, and a telescope (pin) gauge or small bore gauge across the semicircular pocket reads it directly. Third, on a shop with an optical comparator or profile projector, trace the arc and read the radius outright. Once the diameter is known, dividing by 1/8 in gives the last two digits (a 1 in diameter gives 8, finishing the 08-08 = 808 identification).

How to Select the Right Woodruff Key Size

Selecting follows a short, ordered sequence:

• 1. Fix the shaft diameter. The shaft is already sized by the torque and bending loads of the application; the key follows the shaft, never the reverse.
• 2. Classify the duty. Steady, light-to-moderate torque on a small shaft is woodruff country. High torque, shock loads, or frequent reversals push toward larger keys or square keys entirely — woodruff keys shine in the middle of the range and taper off on heavy drives, a limitation covered in the woodruff key fundamentals guide.
• 3. Look up the recommended key number for the shaft diameter. The standard’s tables pair each key number with a range of shaft sizes (per ANSI B17.2, verify against the standard). As a sanity check, the selected key’s width lands in the neighborhood of one quarter of the shaft diameter — the same rule of thumb familiar from square key practice.
• 4. Decode and double-check. Apply the four-digit rule to confirm width and nominal diameter match your shaft and hub envelope before parts are ordered.
• 5. Match the tooling. The keyseat cutter’s arc must equal the key’s nominal diameter, and cutter width equals key width. If the shop already owns a particular woodruff cutter, selecting the key number that matches it can save a tooling purchase on repair work.
• 6. Verify capacity. For anything beyond light duty, check bearing and shear stresses on the keyed cross-section using the tabulated height-above-shaft values — per ANSI B17.2, verify all input dimensions against the standard.

Inch vs. Metric: ANSI B17.2 and DIN 6888

Woodruff keys exist in two parallel systems. The inch system follows ANSI B17.2 and uses the four-digit numbering you now know how to decode. The metric system follows DIN 6888 and does not use the number code at all: metric woodruff keys are specified directly by their dimensions in millimeters — width and nominal diameter — along with matching keyseat values.

The two systems correspond in concept but not in catalog. Because 1 in equals 25.4 mm, any ANSI key can be expressed in metric terms (the 1/4 in × 1 in of a no. 808 is 6.35 × 25.4 mm), but the metric size series was laid out in round millimeter steps, not round fractions of an inch, so the catalogs walk past each other. A metric seat milled for its standard will not properly accept an inch key of “equivalent” size, and vice versa. When a drawing mixes systems, convert everything first, then select from the one system your tooling and stock actually support — do not straddle. The practical rule for global sourcing: US-origin machinery and older equipment everywhere tend to inch-series keys; current European and Asian machinery tends to DIN 6888.

How Woodruff Keyseats Are Machined — and Where Broaching Fits

To be precise about process: woodruff keyseats are milled, not broached. The semicircular pocket is cut with a woodruff keyseat cutter — a disc-shaped milling cutter whose diameter equals the key’s nominal diameter and whose width equals the key width — in a single plunge on a milling machine or in a machined slot on a production line. The cutter’s arbored disc form is what gives the seat its defining arc; nothing about the operation needs the progressive-tooth linear pass that defines broaching.

Broaching enters the picture on the other side of the key family. Square and rectangular parallel keys — the standard choice as shafts grow and torque climbs — ride in straight-sided keyways, and internal keyways are the single most broached feature in industry: a keyway broach pulled or pushed through the hub bore finishes the slot in one stroke, to size, at production rates. A shop can extend that capability from a single fixture by following our keyway broaching set guide, and the full dimension-and-process treatment lives in our keyway broaching complete guide. Many transmission-class parts carry both features: a broached keyway in the hub and a milled woodruff seat, or a design that migrated from woodruff to square keys as loads grew.

Frequently Asked Questions

How do you read woodruff key numbers?

Split the four-digit number into two pairs. The first two digits give the key width in 1/32 inch increments; the last two give the nominal diameter in 1/8 inch increments. Key no. 808 decodes as 8/32 = 1/4 in wide and 8/8 = 1 in nominal diameter; no. 204 decodes as 1/16 in wide and 1/2 in diameter.

How do I know what size woodruff key I need?

Start from the shaft diameter and the duty, then select the key number the standard’s tables recommend for that shaft size — per ANSI B17.2, verify against the standard. Confirm the decoded width and diameter fit the hub envelope, check that a keyseat cutter with a matching arc is available, and verify capacity by stress-checking the keyed section for anything beyond light duty.

How are woodruff keys measured?

Measure the width across the parallel sides with a micrometer and the total height from the curved back to the flat top on a surface plate, then match both values to the standard’s size table to identify the key number. The nominal diameter is read indirectly: from the matching table entry, from the shaft keyseat arc measured with a pin gauge, or by tracing the arc on an optical comparator.

Are woodruff keys metric or imperial?

Both systems exist. Inch-series keys follow ANSI B17.2 and use the four-digit numbering system; metric keys follow DIN 6888 and are specified directly in millimeters. The two size series do not line up, so a key must be replaced from the same system the machine was built in — converting dimensions does not make an inch key fit a metric seat.

When the Shaft Outgrows the Woodruff Key: The Square-Key Path

Woodruff keys earn their keep on small and mid-size shafts, in light-to-moderate duty, where their self-aligning seat and simple milling operation fit the economics of the machine. As torque rises, designers move to square and rectangular keys in broached keyways — deeper engagement, more bearing area, and a slot that scales with the shaft. If your design has reached that point, the economics change too: a broached keyway in each hub is a seconds-long, one-stroke operation on the right machine, and tooling amortizes quickly across a production run.

That is our ground. Browse broach tools for keyway broaches sized to standard key series, or go straight to the broaching machines range — vertical, horizontal and CNC builds for keyway production at volume. Tell us the hub bore, the key size and the material; we will match the broach and the machine to the part. And if the shaft still lives in woodruff country, take the numbering rule with you: first pair width in thirty-seconds, second pair diameter in eighths, verified against ANSI B17.2 every time.