A g5 tolerance on a shaft is an ISO 286 designation: the lowercase g sets where the tolerance zone sits relative to the nominal diameter (slightly below it, guaranteeing the shaft is always a touch undersize), and the 5 sets how wide that zone is (a tight IT5 grade — precision territory). Together, g5 describes a high-precision shaft meant for a small, controlled clearance fit.
The Two Parts of Any ISO 286 Code
Every ISO tolerance callout — g5, h7, H7, p6, whatever’s on the print — breaks into the same two pieces:
- The letter (fundamental deviation) — where the tolerance zone starts relative to the nominal size. Uppercase letters are for holes, lowercase letters are for shafts. Letters near the front of the alphabet (a, b, c… for shafts; A, B, C… for holes) sit toward clearance; letters near the end sit toward interference. H is the special case: for a hole, it puts the lower deviation at exactly zero, meaning the hole is never undersize.
- The number (IT grade) — how wide the tolerance zone is. Lower numbers mean tighter tolerances. IT01–IT6 cover gauges and high-precision work, IT7–IT13 cover general machining and assembly, IT14 and up cover casting and coarse fabrication.
This is also why G5 and g5 are not the same thing, and H7 and h7 are not the same thing — the case of the letter tells you whether you’re looking at a hole or a shaft spec, not just a stylistic choice.
g5 Shaft Tolerance Table
Deviations in micrometers (µm), upper/lower, from ISO 286-2:
| Nominal size (mm) | Upper deviation | Lower deviation |
|---|---|---|
| 3–6 | -4 | -9 |
| 6–10 | -5 | -11 |
| 10–18 | -6 | -14 |
| 18–30 | -7 | -16 |
| 30–50 | -9 | -20 |
| 50–80 | -10 | -23 |
| 80–120 | -12 | -27 |
Both deviations are negative — that’s what “g” means for a shaft: the entire tolerance zone sits below the nominal diameter, so the shaft is always at or under nominal, never over.
Worked Example: Ø20 g5
A 20 mm nominal diameter falls in the 18–30 mm band, where g5 gives -7 µm upper deviation and -16 µm lower deviation. That means:
- Maximum allowed diameter: 20.000 – 0.007 = 19.993 mm
- Minimum allowed diameter: 20.000 – 0.016 = 19.980 mm
So a callout of “Ø20 g5” means the finished shaft must measure between 19.980 mm and 19.993 mm — a total tolerance band of only 13 µm, which is why g5 typically requires precision grinding, not general turning.
h7 Hole Tolerance: The Other Half of the System
Where g5 is a common shaft callout, h7 deserves its own explanation because it works the opposite way and shows up constantly as the “preferred hole” in general engineering. (Note: lowercase h7 is also a shaft designation — see the note below on H7 vs. h7.)
H7 (uppercase — a hole spec) sets the lower deviation to exactly zero, so an H7 hole is never smaller than nominal, and the entire tolerance band sits above nominal, sized by the IT7 grade.
| Nominal size (mm) | H7 upper deviation | H7 lower deviation |
|---|---|---|
| Up to 3 | +10 | 0 |
| 3–6 | +12 | 0 |
| 6–10 | +15 | 0 |
| 10–18 | +18 | 0 |
| 18–30 | +21 | 0 |
| 30–50 | +25 | 0 |
| 50–80 | +30 | 0 |
| 80–120 | +35 | 0 |
| 120–180 | +40 | 0 |
| 180–250 | +46 | 0 |
| 250–315 | +52 | 0 |
| 315–400 | +57 | 0 |
Worked Example: Ø50 H7
A 50 mm nominal size falls in the 30–50 mm band, where H7 gives +25 µm upper deviation and 0 lower deviation. So a “Ø50 H7” hole must measure between 50.000 mm and 50.025 mm.
Why H7 Is the Default Hole in Most Shops
H7 is the preferred hole in the hole-basis system: hold the hole at H7 (cut with a fixed-size reamer or fine boring tool) and get every fit — from sliding to force — by changing only the shaft letter. That’s cheaper in tooling and gauging than trying to hold a variable hole size to match a fixed shaft. H7 is also a moderate-precision, machining-grade tolerance — tight enough to need reaming, fine boring, or grinding, but loose enough to avoid the extra cost of an H6 finishing pass.
H7 vs. h7: Don’t Confuse the Case
Uppercase H7 is a hole spec, entirely above nominal. Lowercase h7 is a shaft spec, entirely below nominal (h works the same way g does, but with the IT7 grade instead of IT5 — a wider, less precise band than g5). At 20 mm: H7 runs 20.000–20.021 mm, while h7 runs 19.979–20.000 mm. Same digits, opposite parts, opposite direction from nominal — always check the case before reading a print.
Common H7 Shaft Pairings and What They Produce
Holding the hole at H7 and varying only the shaft letter is how the hole-basis system produces every standard fit type:
| Fit | Type | What it’s for |
|---|---|---|
| H7/g6 | Clearance | Sliding fit — moves and turns freely, locates accurately |
| H7/h6 | Clearance | Locational clearance — snug, stationary, still hand-assembled |
| H7/k6 | Transition | Compromise between clearance and interference |
| H7/n6 | Transition | More accurate location, greater interference tolerated |
| H7/p6 | Interference | Rigid, aligned assembly without special press requirements |
| H7/s6 | Interference | Medium drive fit for ordinary steel parts |
| H7/u6 | Interference | Force fit for highly stressed parts or shrink fits |
For the underlying concepts behind the clearance and interference ends of this table, see What Is a Clearance Fit? and What Is Interference Fit?.
Where g5 and h7 Actually Get Used
- g5 — high-precision clearance shafts: machine tool spindles, precision guide shafts, anywhere you need accurate sliding or rotation with minimal play.
- h7 — the default “preferred hole” for general engineering fits, bearing housings, and dowel-pin locations, paired with whatever shaft letter produces the fit you need.
Reading Any Other Code on Your Print
The method is always the same: identify the letter (case tells you hole vs. shaft, position tells you clearance vs. interference), identify the number (width of the tolerance band), find your nominal size range in the ISO 286 tables, and read off the upper and lower deviations. Don’t guess at a tolerance from the callout alone — always confirm against the ISO 286 tables for your specific nominal size, since bands shift meaningfully between size ranges.
Related reading: What Is a Clearance Fit? · What Is Interference Fit?
