Getting keyway depth wrong is one of the fastest ways to scrap a part. Cut too shallow and the key sits proud, so the hub won’t seat. Cut too deep and the key loses bearing area and can roll under load. This page gives you standard shaft and hub keyseat depths for inch (ANSI B17.1) and metric (DIN 6885 / ISO 773) keys, the formulas behind them, and a quick calculator for measuring depth across the shaft or bore.
Need the full width and height tables first? See our keyway dimensions and size chart. For width and depth tolerances by fit class, see the keyway tolerance chart.
How Keyway Depth Is Measured
There are two common ways to call out keyseat depth, and mixing them up causes most depth errors:
- Nominal depth from the edge of the keyseat: measured from where the keyseat meets the shaft surface (or bore surface) down to the bottom of the slot. For a standard square inch key, the nominal depth is half the key height in both the shaft and the hub.
- Depth across the shaft or bore (S and T dimensions): measured from the bottom of the keyseat to the opposite side of the shaft or bore. This is how most shops actually check depth with a micrometer or caliper, and how it’s usually called out on drawings.
The two differ by the chordal height (Y), the small arc of material between the keyseat edge and the true top of the round. On small shafts with wide keys, ignoring chordal height can put you several thousandths off.
Keyway Depth Formulas
For a shaft or bore diameter D, key width W, and key height H:
- Chordal height: Y = (D − √(D² − W²)) ÷ 2
- Shaft depth across the shaft: S = D − Y − H/2
- Hub depth across the bore: T = D − Y + H/2 + C
C is the hub clearance allowance. For standard parallel keys it’s typically +0.005″, which leaves room at the top of the key so the hub slides on without binding.
Example: a 1″ shaft with a 1/4″ square key. Y = (1 − √(1 − 0.0625)) ÷ 2 = 0.0159″. Shaft depth S = 1 − 0.0159 − 0.125 = 0.859″. Hub depth T = 1 − 0.0159 + 0.125 + 0.005 = 1.114″.
Inch Keyway Depth Chart (ANSI B17.1, Square Keys)
Depths below use the standard square key for each shaft size and a +0.005″ hub clearance. Values are rounded to three decimals.
| Shaft Dia. (in) | Square Key (in) | Nominal Depth, Shaft & Hub (in) | Chordal Height Y (in) | Shaft Depth S (in) | Hub Depth T (in) |
|---|---|---|---|---|---|
| 1/2 | 1/8 | 0.0625 | 0.0079 | 0.430 | 0.560 |
| 5/8 | 3/16 | 0.0938 | 0.0144 | 0.517 | 0.709 |
| 3/4 | 3/16 | 0.0938 | 0.0119 | 0.644 | 0.837 |
| 7/8 | 3/16 | 0.0938 | 0.0102 | 0.771 | 0.964 |
| 1 | 1/4 | 0.1250 | 0.0159 | 0.859 | 1.114 |
| 1-1/8 | 1/4 | 0.1250 | 0.0141 | 0.986 | 1.241 |
| 1-1/4 | 1/4 | 0.1250 | 0.0126 | 1.112 | 1.367 |
| 1-3/8 | 5/16 | 0.1562 | 0.0180 | 1.201 | 1.518 |
| 1-1/2 | 3/8 | 0.1875 | 0.0238 | 1.289 | 1.669 |
| 1-3/4 | 3/8 | 0.1875 | 0.0203 | 1.542 | 1.922 |
| 2 | 1/2 | 0.2500 | 0.0318 | 1.718 | 2.223 |
| 2-1/4 | 1/2 | 0.2500 | 0.0281 | 1.972 | 2.477 |
| 2-1/2 | 5/8 | 0.3125 | 0.0397 | 2.148 | 2.778 |
| 3 | 3/4 | 0.3750 | 0.0476 | 2.577 | 3.332 |
| 3-1/2 | 7/8 | 0.4375 | 0.0556 | 3.007 | 3.887 |
| 4 | 1 | 0.5000 | 0.0635 | 3.436 | 4.441 |
Quick reference: a 1/4″ keyway is 1/8″ (0.125″) deep nominal in both shaft and hub. A 3/8″ keyway is 3/16″ (0.1875″) deep. A 3/16″ keyway is 3/32″ (0.094″) deep. For rectangular (flat) keys, use half the key height instead of half the width.
Metric Keyway Depth Chart (DIN 6885-1 / ISO 773)
Metric standards call out depth directly: t1 is the keyseat depth in the shaft and t2 is the keyway depth in the hub, both measured from the shaft or bore surface at the keyseat edge.
| Shaft Dia. (mm) | Key b × h (mm) | Shaft Depth t1 (mm) | Hub Depth t2 (mm) |
|---|---|---|---|
| 6 to 8 | 2 × 2 | 1.2 | 1.0 |
| over 8 to 10 | 3 × 3 | 1.8 | 1.4 |
| over 10 to 12 | 4 × 4 | 2.5 | 1.8 |
| over 12 to 17 | 5 × 5 | 3.0 | 2.3 |
| over 17 to 22 | 6 × 6 | 3.5 | 2.8 |
| over 22 to 30 | 8 × 7 | 4.0 | 3.3 |
| over 30 to 38 | 10 × 8 | 5.0 | 3.3 |
| over 38 to 44 | 12 × 8 | 5.0 | 3.3 |
| over 44 to 50 | 14 × 9 | 5.5 | 3.8 |
| over 50 to 58 | 16 × 10 | 6.0 | 4.3 |
| over 58 to 65 | 18 × 11 | 7.0 | 4.4 |
| over 65 to 75 | 20 × 12 | 7.5 | 4.9 |
| over 75 to 85 | 22 × 14 | 9.0 | 5.4 |
| over 85 to 95 | 25 × 14 | 9.0 | 5.4 |
| over 95 to 110 | 28 × 16 | 10.0 | 6.4 |
| over 110 to 130 | 32 × 18 | 11.0 | 7.4 |
| over 130 to 150 | 36 × 20 | 12.0 | 8.4 |
Common lookups: an 8 mm keyway (8 × 7 key) is 4.0 mm deep in the shaft and 3.3 mm in the hub. A 40 mm shaft takes a 12 × 8 key at 5.0 / 3.3 mm. A 50 mm shaft takes a 14 × 9 key at 5.5 / 3.8 mm. A 60 mm shaft takes an 18 × 11 key at 7.0 / 4.4 mm.
To check metric depth across the shaft, use D − t1. To check across the bore, use D + t2.
Keyway Depth Calculator
Enter your shaft or bore diameter and key size (any consistent unit) to get chordal height and depth across the shaft and bore.
Common Keyway Depth Mistakes
- Ignoring chordal height. Measuring depth from the top of the round instead of the keyseat edge makes the slot shallow by the chordal height.
- Using shaft depth for the hub. Metric hub depths (t2) are shallower than shaft depths (t1). They aren’t interchangeable.
- Forgetting hub clearance. Without the top clearance, the key bottoms out and the hub won’t slide on.
- Measuring a worn or tapered bore. Check the bore at several points before you set depth off it, especially on rework.
- Depth drift on long internal keyways. Tool deflection on long or blind bores can leave the far end shallow. Rigid tooling and a controlled feed matter more than speed.
Cutting Accurate Internal Keyway Depth
Hub keyways are where depth control gets hard. Broaching depends on shims and push-through access, and the depth you get is the depth the broach and shim stack allow. A keyseat miller feeds the cutter to depth under control, so you can hit a T dimension, cut blind keyways without relief holes, and check depth mid-cut.
Learn more in our step-by-step guide to cutting keyways, or see how our internal keyway cutters handle custom depths. Working with Woodruff keys? See the Woodruff key size chart for keyseat depths by key number.
Keyway Depth FAQs
How deep should a keyway be?
For a standard square inch key, the keyseat is half the key height deep in both the shaft and the hub, measured from the keyseat edge. For metric keys, use the t1 (shaft) and t2 (hub) values from DIN 6885 or ISO 773.
How deep is a 1/4″ keyway?
1/8″ (0.125″) nominal in both shaft and hub. On a 1″ shaft, that’s 0.859″ across the shaft and 1.114″ across the bore.
How deep is an 8 mm keyway?
An 8 × 7 mm key (shafts over 22 to 30 mm) has a shaft keyseat depth of 4.0 mm and a hub keyway depth of 3.3 mm.
How do I measure keyway depth in a bore?
Measure from the bottom of the keyway straight across to the opposite wall of the bore. That’s the T dimension. Compare it to D − Y + H/2 + C for inch keys, or D + t2 for metric keys.
What is the keyway depth formula?
Shaft depth S = D − Y − H/2 and hub depth T = D − Y + H/2 + C, where Y = (D − √(D² − W²)) ÷ 2 is the chordal height.
Is hub keyway depth the same as shaft keyseat depth?
For inch square keys, the nominal depth is the same (H/2), with a small hub clearance added. For metric keys, the hub depth t2 is noticeably shallower than the shaft depth t1.
Need Precise Internal Keyways?
National Machine Tool has built keyseat millers in Cincinnati for decades, used by shops cutting internal keyways to exact depth in gears, pulleys, impellers, and couplings. Request a quote or contact us to talk through your application.